Is Convergence More Than an Analogy - Final First Draftand stochastic processes play a de minimus role in shaping macroevolution over deep geological time, stating that such mechanisms

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Russell Powell Duke University

IS CONVERGENCE MORE THAN AN ANALOGY? APPEALING TO THE FREQUENCY OF HOMOPLASY AS EVIDENCE FOR A NON-CONTINGENTLY CONSTRAINED ADAPTATIONAL DESIGN SPACE

ABSTRACT

It is widely accepted among contemporary biologists and philosophers of biology that replaying the proverbial ‘tape of life’ would result in wildly unpredictable and radically different evolutionary outcomes. This position arises primarily from the empirical assumption that the invariant laws of nature carve out exceedingly broad channels that only loosely constrain the evolution of organismic design. A number of authors, however, have touted examples of ‘convergent evolution’ as evidence par excellence for the central role of adaptation in shaping predictable trajectories of macroevolution. Of these individuals, Simon Conway Morris (“SCM”) is arguably the most prominent champion of homoplasy and its purported implications for a non-contingent, counter-factually stable view of life. However, there are numerous conceptual and empirical difficulties that arise in appealing to the frequency of homoplasy as evidence for a non-contingently constrained adaptational design space. These problems are best illustrated by critically evaluating SCM’s use of the homoplasy literature in an attempt to bolster a strong adaptationist view of life, and thus his two major and most recent works dedicated to this end will be the focal point of my critique. Nonetheless, this paper is not intended to be a negative excoriation of SCM’s work; rather, it is designed to transform a less systematized approach to the philosophical implications of homoplasy into a positive and rigorous research program in counterfactual biology within which the relative significances of contingency and selection in shaping the history of life can be assessed. Specifically, my critique will be based primarily on five grounds: (1) SCM’s analytically and methodologically inadequate treatment of the role of homoplasy in phylogenetic reconstruction, including the fact that homoplastic pervasiveness confounds the very process by which it is identified; (2) SCM’s express failure to make the crucial distinction between convergent evolution (due to external, non-contingent constraint) and parallel evolution (due to internal, contingent constraint), and to consider how the respective frequencies of these importantly different sources of homoplasy affect his proffered view of life; (3) the presence of deep homology and its implications for macroevolutionary / exo-biological extrapolation; (4) the general failure to specify hierarchical levels of analysis in assessing homology and homoplasy, which only exacerbates an already confused and cross-talking discourse on evolutionary convergence, and (5) difficulties in quantifying or otherwise operationalizing the degree of homoplasy so as to assess its relative significance in and between given lineages. Of these five major points of criticism, the second looms most philosophically large and will thus form the heart of the critique.

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INTRODUCTION

It is widely accepted among contemporary biologists and philosophers of biology that

replaying the proverbial ‘tape of life’ would result in wildly unpredictable and radically different

evolutionary outcomes. Some biologists even espouse the more radical notion that virtually

every interesting event in the history of life, including the evolution of the most robust complex

adaptive solutions, falls into the realm of historical contingency. This position arises primarily

from the empirical assumption that the invariant laws of nature carve out exceedingly broad

channels that only loosely constrain the evolution of organismic design. On this view, observed

evolutionary products are sensitively dependent on stochastic initial conditions, a feature of

macroevolutionary processes which undermines attempts to formulate robust generalizations

regarding the evolution of organismic form. Because even the most successful biological

generalizations are undermined by multiply realizable solutions to contingent design problems

(Beatty, 1995) that are destined for obsolescence in the unrelenting arms race of natural selection

(Rosenberg, 2001; Van Valen, 1973), they tend to lack the nomic necessity, counterfactual

stability, and qualitative predicates canonically characteristic of natural laws.1

With this nomological vacuum as the backdrop, historicists tend to explain the

inhomogenous distribution of organismic form in morphospace (at all levels of the genealogical

hierarchy) as the result of internal developmental constraints that restrict the set of possible

variants on which selection can operate. Strong adaptationists, on the other hand, tend to view

the clumping of actualized morphology not as a contingent consequence of developmental

canalization, but rather as an ecologically optimal set of solutions to relatively stable functional

1 There are two notable exceptions to this rule: the Principle of Natural Selection (Rosenberg, 2001) and the Principle of Drift (Brandon, 2005, in press), neither of which, however, allow for macroevolutionary predictions regarding the evolution of form. In fact, both selection and drift may be seen as undermining macroevolutionary extrapolation.

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problems. In defending this view, a number of authors have touted examples of ‘convergent

evolution’ as evidence par excellence for the central role of adaptation in shaping predictable

trajectories of macroevolution.2 Of these individuals, Simon Conway Morris (“SCM”) is

arguably the most prominent champion of homoplasy and its purported implications for a non-

contingent, counter-factually stable view of life. In essence, his view is this: The ubiquity of

‘convergence,’ or the independent origination of similar forms in distantly related organisms, is

prima facie evidence for an adaptational design space that is so severely constrained by the

invariant chemico-physical laws that organismic form will, through the optimizing forces of

natural selection, repeatedly and inevitably converge on certain identifiable functional attractors

or biological properties. Thus, according to SCM, the program of universal biology should be

primarily aimed at identifying these elusive ‘laws of convergence’ which operate beneath the

surface of a superficially stochastic history.

However, there are numerous conceptual and empirical problems that arise in appealing

to the frequency of homoplasy as evidence for a non-contingently constrained adaptational

design space. These problems are best illustrated by critically evaluating SCM’s use of the

homoplasy literature in an attempt to bolster a strong adaptationist view of life, and thus his two

major and most recent works dedicated to this end (2003 / 1998) will be the focal point of my

critique. Nonetheless, this paper is not intended to be a negative excoriation of SCM’s work;

rather, it is designed to transform a less systematized approach to the philosophical implications

of homoplasy into a positive and rigorous research program in counterfactual biology within

which the relative significances of contingency and selection in shaping the tree of life can be

assessed.

2 See e.g. Conway Morris (2003); Foley (1999); Dennett (1995); Patterson (1988).

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Specifically, my critique is based primarily on five grounds: (1) SCM’s analytically and

methodologically inadequate treatment of the role of homoplasy in phylogenetic reconstruction,

including the fact that homoplastic pervasiveness confounds the very process by which it is

identified; (2) SCM’s express failure to make the crucial distinction between convergent

evolution (due to external, non-contingent constraint) and parallel evolution (due to internal,

contingent constraint), and to consider how the respective frequencies of these importantly

different sources of homoplasy affect his proffered view of life; (3) the presence of deep

homology and its implications for macroevolutionary / exo-biological extrapolation; (4) the

general failure to specify hierarchical levels of analysis in assessing homology and homoplasy,

which only exacerbates an already confused and cross-talking discourse on evolutionary

convergence, and (5) difficulties in quantifying or otherwise operationalizing the degree of

homoplasy so as to assess its relative significance in and between given lineages. Of these five

major points of criticism, the second looms most philosophically large and will thus form the

heart of the critique.

SIMON CONWAY MORRIS’ CORE CONTENTION: PERVASIVE HOMOPLASY IMPLIES THE INEVITABILITY OF EVOLUTIONARY ENDPOINTS

Biologists and philosophers have described the evolutionary phenomena of convergence

as nature’s way of re-winding the proverbial tape of life (Dennett, 1995), biology’s closest

analog to independent experimental replication (Gould, 1976; but see Gould, 2002). As one

preeminent comparative physiologist suggests, the project of identifying convergence offers

more than just evidence for adaptation (Endler, 1986), for it enables biologists to distinguish

functional complexes that strongly determine structural form from those that are less important

in design, increasing the predictive power of generalizations regarding the characters that matter

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most (Vogel, 1996 / 1998). In The Crucible of Creation: The Burgess Shale and the Rise of

Animals (1998) and more recently in Life’s Solution: Inevitable Humans in a Lonely universe

(2003), SCM sketches a view of life that is somewhat unusual from a modern philosophical and

biological standpoint—one that in several unique respects breaks rank with traditional

interpretations of macroevolutionary patterns. Pointing to the putative pervasiveness of

‘convergence,’ SCM defends the strong adaptationist view that life inexorably navigates toward

certain pre-ordained “endpoints,” to which “the routes are many, but the destinations few, and

the landscapes across which all organisms must travel are adaptive” (2003, p. 297). Thus, in

contrast to the contingency-flavored philosophical paradigms of biological lawlessness (e.g.

Beatty, 1995), SCM holds that if only biologists could identify these elusive ‘laws of

convergence’ operating beneath the surface of stochastic chaos, they would be able to divine

robust, counterfactually stable predictions regarding the evolution of life on Earth and

throughout the Universe.3

Pit against such a radical functionalist weltbild is the ‘historicist’ (sensu Gould, 2002)

view of the history of life, which views the inhomogeneous distribution or ‘clumping’ of

organisms across morphospace (noted by Dobzhansky, 1951) not as an optimal set of solutions

(courtesy of natural selection and the invariant physical laws) to functional problems, but as the

result of internal, contingent constraints restricting the realm of the possible.4 Rather than a

3 Ironically, while Conway Morris holds that the emergence of sentience is essentially inevitable once life has begun, he argues that there are extremely few places in the universe with conditions suitable for life (hence his titular construct, ‘Inevitable Humans in a Lonely Universe.’ There are, of course, numerous critiques of this unusual view, including a direct challenge by Gould (1998). Additionally, many structuralists believe that the universe is ‘pregnant’ with life (Kaufmann, 1995) and quasi-life (Salthe, 1994) which emerge from complexity dynamics ‘for free’ without the operation of natural selection. However, these structuralist contentions apply more to simpler forms of life, and they may have less (or nothing) to say about the inevitability of robust sentience or self-awareness. 4 A third macroevolutionary world view is ‘structuralism’ (see note above) which like adaptationism is enamored of homoplastic phenomena, but attributes its ubiquity not to the supremacy of selection but to non-functional (indeed, non-biological) laws of complexity (see infra note 15). Of course, these views are not mutually exclusive, as the

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predictable march of organismic form toward identifiable optimality or equilibrium, the

historicist’s history of life is (as Henry Ford was fond of saying) simply ‘one damn thing after

another’—an accumulation of accidents, perhaps intelligible in hindsight but wildly

unpredictable in prospect, fluctuating in rhythm with a stochastic ecology and exhibiting no bias

toward any particular functional solution.5 Of course, historicists agree with adaptationists that

natural selection is the only known mechanism for producing function, although they downplay

the role of selection in directing macroevolutionary patterns.6

While biologists and philosophers of biology quibble over the respective significance of

their favored evolutionary phenomena and attempt to relegate their opponent’s preferred

processes to a relevant but less significant role, SCM goes even further to declare that contingent

and stochastic processes play a de minimus role in shaping macroevolution over deep geological

time, stating that such mechanisms are completely “irrelevant…in so far as the history of life is

concerned” (1998, p. 205). This is a strong and iconoclastic claim, but in fact his assertions are

even more radical, as he refers to the notion that contingent forces drive evolutionary processes

as a biological fundamentalist “myth” (2003, p. 322). These views alone provide sufficient

grounds for labeling SCM as a hard adaptationist (sensu Amundson, 1994), as they entail that

geometrical, developmental, and selective constraints on bivalve / brachiopod accretionary growth make clear—instead, the debate centers on their respective significance in the history of life. 5 Thus, the historicist maintains, the vertebrate body plan is the result of a frozen accident—namely, a series of quirky stochastic events that permitted the competitively weak vertebrate ancestral lineage Pikaia to survive the early Paleozoic extinctions and ultimately radiate into 45,000 extant species. This is of course the upshot of the popular bible of contemporary historicism, Gould’s Wonderful Life (1989), which attributes the dearth of law-like generalizations in functional biology to the fact that evolutionary endpoints, rather than inevitable, are sensitively dependent on initial conditions and thus counterfactually instable. That is, negligible stochastic differences in early conditions result in substantially different macroevolutionary trajectories through deep geological time. According to Gould, these trajectories are prospectively unpredictable, either in principle or due to insurmountable epistemic limitations, but can be adequately re-constructed and fully appreciated in hindsight. 6 For instance, historicists tend to attribute phylogenetic trajectories to stochastic models rather than competitive interactions (see e.g. Gould & Calloway, 1980). Nevertheless, the crux of the historicist dispute with SCM is not the relative significance of selection per se but rather that the former attributes all macroevolutionary change—whether functionally or stochastically driven—to contingent rather than ineluctable events.

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(virtually) all scientifically interesting features of life are of adaptive provenance, with selection

overpowering any developmental constraints or stochastic tendencies such as genetic drift.7

Hard adaptationism assumes that adaptational design space is highly constrained externally, that

is by directional and subsequent stabilizing selection, the latter containing genetic drift once a

(local) optimality has been achieved. However, it also presupposes that morphospace is virtually

unconstrained internally, namely by postulating the inexorable tendency of natural selection to

overcome developmental constraints that would otherwise lead to the ubiquity of adaptive sub-

optimality. For the purposes of argument, I will simply assume the existence of strong external

(ecological) constraints on the evolution of form. It is primarily the latter assumption regarding

the flexibility of developmental channels that will be the basis of my challenge. Before

launching into the substantive critique, however, I will briefly address a methodological concern

regarding phylogenetic reconstruction in the context of concerted homoplasy.

1. THE ROLE OF HOMOPLASY IN PHYLOGENETIC RECONSTRUCTION: WHY HOMOPLASTIC PERVASIVENESS CONFOUNDS THE VERY PROCESS BY WHICH IT IS IDENTIFIED

Since all organisms presumably share a common ancestor, and the differences between

species are the result (in part) of descent with modification, biologists endeavor to reconstruct a

branching tree of life, first envisioned by Darwin (1859), which reflects the recency of common

ancestry. Generally, the ability to infer past states from present states depends on the present

7 According to Amundson’s (1994) terminology, ‘hard adaptationists’ may be distinguished from ‘soft adaptationists’ who agree that (virtually) all scientifically interesting features of organismic form are subject to natural selection, but disagree with the notion that selection can break through any and all developmental constraints to achieve (non-local) optimality. ‘Neutralists,’ on the other hand, contend that many interesting traits are not operated on by natural selection (such as the mammalian ear bone shape), and thus not all constraints on form are constraints on adaptation.

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states’ degree of ‘contingency,’ or sensitive dependence on initial conditions. If the present state

is highly multiply realizable, that is, could have been effected by an unmanageably large set of

distinct initial (past) conditions, historical scientists will be unable to map the present state onto

relevant features of a discrete and identifiable past state. Sober (1988) makes the following

helpful analogy: consider a ball released on the rim of an even bowl; as it moves either down or

up the bowl’s side, we may infer from those various states something about its starting position.

However, once the ball comes to rest (‘equilibrates’) in the middle of the bowl, the information

regarding its initial conditions is effectively destroyed, and nothing can be inferred about its past

states. This example is useful because it demonstrates that difficulties in extrapolating from

present to past states need not necessarily be due to epistemological limitations in handling the

complexity of information.

With respect to non-accidental macroevolutionary generalizations, there is an asymmetry

or inverse correlation between our ability to reconstruct the past and our capacity to project into

the future, assuming the relative significances of adaptation and contingent processes are

unchanging through time. This is because the recognition of a dominant role for contingency, or

counterfactual instability, undermines the potential for robust future extrapolation. At the same

time, however, contingent (non-equilibrial) phenomena tend to preserve historical signatures and

thereby make past-states inferable from present ones. The greater the sensitivity to initial

conditions—that is, the more intricately grooved the bowl of morphospace—the greater our

precision in pinpointing previous evolutionary states. In fact, it is the quirky trace of history that

forms the epistemological base of Darwin’s central homological argument—namely, that nested

sets of shared of traits (identified by Richard Owen in the pre-Darwinian era) reflect genealogical

relationships. To the contrary, homoplastic events tend to wipe out evolutionary memory by

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erasing the signature of history. Although radical contingency need not necessarily preserve

historical information, an intricately carved bowl is more likely to retain evolutionary memory

than one with sleek contours and a strong attractor at the center of the bowl base.

Thus, natural selection is not only the principle engine of evolution, but also the major

source of phylogenetic distortion (Vogel, 1996; Endler, 1986). If SCM is correct, and many or

most of the observed evolutionary ‘endpoints’ are highly multiply realizable, the degree of

certainty with which we can abduct propinquity of descent declines dramatically.8 That is, if

homoplasy is not only robust and concerted on occasion, but in fact ubiquitous at all hierarchical

levels as SCM would have us believe, then this would call into question core methodological

assumptions underlying modern phylogenetic reconstruction.9 Darwin found such a view

implausible, stating (1860) that

[I]t is incredible that the descendents of two organisms, which had originally differed in a marked manner, should ever afterwards converge so closely as to lead to a near approach to identity throughout their whole organization. If this had occurred, we should meet with the same form, independently of genetic connection, recurring in widely separated geological formations; and the balance of evidence is opposed to any such admission.

The abductive precept of ‘parsimony’ holds that entities should not be postulated without

necessity. In the context of phylogenetic reconstruction, parsimony generally dictates that the

8 It is true that the signature of history can be maintained by continuity (or lack there of) in genetic sequences that are orthogonal to morphological evolution or upon which organism-level homoplastic events multiply supervene. Most genetic mutations are neutral, and organismic change need not track genetic alteration (Vogel, 1996). Nevertheless, phylogenetic reconstructions based solely on molecular-genetic comparisons run into difficulties of their own, and inferring evolutionary relationships from shared derived macroscopic character traits (synapomorphies) remains the most effective and widespread means of cladistic reconstruction. In fact, SCM (1994) is himself somewhat skeptical of molecular methods of phylogenetic reconstruction, and thus relies heavily on morphological-level analysis to identify putative instances of homoplasy. Furthermore, he dedicates portions of his 2003 treatise to arguing for molecular-level convergence (e.g. pgs. 295-98), a phenomenon if as widespread as he claims would further undermine our ability to reconstruct the past and consequently impede the detection of homoplastic events. 9 There are, however, other methods of phylogenetic reconstruction which do not depend on parsimony, such as ‘maximum likelihood estimation’ (Fisher, 1912), which involves statistically based inferences about the probability distribution of a given data set.

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best phylogenetic hypothesis, consistent with a relevant data set, is the one that assumes the

fewest homoplasious (convergent or parallel) events.10 This follows from the notion that

postulating a single cause (e.g. homology) is more empirically plausible than relying on the

existence of multiple causes (e.g. convergences) for the presence of an identified similarity, or

shared derived trait (Hennig, 1966).11 Thus, the crux of cladistic reconstruction hinges on

identifying synapomorphies as derived homologies, thereby ruling out homoplasy (independent

origin). This methodological preference for derived characteristics is based on the evolutionary

assumption that derived character traits are usually produced only once and are not lost (Sober,

1988). Even if evolution adheres to the above two assumptions, however, we must reserve a

certain probability for homoplasy and thus we cannot deduce phylogenetic relationships based on

any set of similarities.12 As such, faulty assumptions regarding the frequency and degree of

homoplasy could profoundly affect the systematist’s ability to construct an accurate phylogeny.

For these reasons, “the ubiquity of homoplasy is a major concern in phylogenetic

analysis, which ironically is its primary means of detection” (Wake, 1991, p. 544). For as Wake

(Ibid) notes, “if homoplasy is rampant, existing cladistic methods fail.” This puts SCM into a 10 In order to determine the relatedness of various species within a given monophyletic group, cladists determine the distribution of relevant ancestral (plesiomorphic) and derived (apomorphic) characteristics. They then identify symplesiomorphic and synapomorphic traits, or those ancestral and derived character states shared by the various species within the monophyletic group (comprised of a single species and all of its descendent species). Because of its single origin, the entire tree of life forms the largest conceivable monophyletic grouping—as a result, no traits are ancestral or derived in the absolute (non-relational) sense, but only with respect to a specified level of genealogical hierarchy. 11 This intuition may be rooted in the precept of likelihood similar to that offered by Reichenbach (1956) and Salmon (1984) for the postulation of common causes, the operating supposition of which is that the existence of separate causes for similar phenomena is less likely than the existence of a single cause (Sober, 1988). Nevertheless, the less parsimonious reconstruction cannot be deductively ruled out. However, in the context of phylogenetic reconstruction, the preference for homology over homoplasy seems to be grounded more in the likelihoods attached to multiple origins rather than causes, for in theory there could be unified law-like causes of homoplasy that produce similarity more often than inter-generational transmission (and by implication propinquity of descent). 12 The potential for reversions and parallelisms may be far more common between groups that share certain developmental pathways—a crucial fact that will be addressed at length in the following Section.

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serious methodological bind, for his claims about the inevitability of certain evolutionary

endpoints hinge on our ability to identify homoplasy, which in turn depends on our capacity to

construct an accurate phylogeny (see e.g. Vogel, 1996). As noted above, however, our ability to

recognize homology and distinguish it from homoplasy turns in large part on our ability to detect

the contingent signature of history, the very phenomena that SCM endeavors to minimize. Thus,

his wholesale denial of the sensitive dependence of present evolutionary states on initial

conditions calls into question some of the most fundamental conclusions of evolutionary biology,

including the very notion of common ancestry itself.

Nevertheless, even if homoplasy is far more ubiquitous than most cladists would prefer

(an assertion made by Wake, 1991), the precept of cladistic parsimony can addresses this

problem by placing a lower but not upper bound on the amount of homoplasy recognized by a

given reconstruction—in other words, the most parsimonious tree is one that minimizes

assumptions (homoplasies), although it does not assume that such assumptions will be minimal

(Sober, 1988). I submit, however, that there is a threshold frequency of homoplasy after which

central cladistic assumptions regarding the evolutionary process are no longer justified.

Alternatively, distance measures which determine the overall genetic dissimilarity of

compared taxa may be employed in cladistic reconstruction, and could perhaps allow for

concerted macroscopic homoplasy while preserving the trace of evolutionary history through

molecular homology. Because natural selection tends to act at the organismic level, homoplasy

will be most frequently encountered in morphology, rather than sub-cellular biochemical

pathways, the vast majority of which represent ancestral similarities (Vogel, 1996 / 1998;

Sterelny, 2006). For similar reasons, the degenerate nature of the genetic code makes gene-level

convergences (where the same base sequences generate the same product) extraordinarily

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improbable.13 Nevertheless, if there is in fact widespread molecular convergence with respect to

either the genetic code or the message—such as at the level of protein configurations as SCM

contends (2003, p. 287-298), then these alternative molecular-based methods of phylogenetic

reconstruction might be vitiated as well.14 In sum, SCM recognizes (p. 297) that homoplasy is

“viewed by most cladists as [] profoundly disruptive to their neat schemes,” but he does not seem

to realize that he cannot have his cake of ubiquitous convergence and eat it too.

2. CONVERGENT VS. PARALLEL EVOLUTION:

HOW THE RESPECTIVE FREQUENCIES OF THESE DIFFERENT SOURCES OF HOMOPLASTIC CONSTRAINT AFFECT HISTORICIST AND ADAPTATIONIST VIEWS OF LIFE

The overarching if unarticulated assumption of hard adaptationist views of life is that the

channels of adaptational design space are highly constrained. It is this constraint which restricts

the universe of potential solutions to putatively pervasive design problems to a manageable

handful that admits of prediction without a burdensome litany of ceteris paribus qualifications.

In pointing to instances of homoplasy as evidence for counterfactually stable limitations on

organismic form, however, it is imperative to recognize that there are two major classes of

constraint that could alone or jointly be responsible for the observed morphological regularities.

The first class is external to the organism and non-contingent—namely, the constraints imposed

on formal design solutions by the chemico-physical laws and its interaction with the optimizing

agency of natural selection. For instance, because of the laws of optics and the properties of

light, there may be only a handful of ways to macroscopically arrange an image-forming organ 13 As Mayr noted (1963), because functional demands supervene on a diverse molecular and biochemical base, functional constraints only limit the end-product which can be realized via many genetic and biochemical routes. 14 Specifically, Conway Morris suggests homoplastic evolution in numerous occupied areas of protein ‘hyperspace’, including (inter alia) proteases, peptidases, cytokinases, aminoacyl-tRNA synthetases, light-harvesting proteins, elastic proteins, chitin-binding proteins, and steroid signaling proteins (2002, p. 295). But see Szathmary’s review (2005) of Conway Morris (2003), in which he argues for a highly contingent peptide space.

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of given acuity—hence its ‘convergent’ evolution up to 15 times within and between distant

phyla (Land, 1992). Similar biomechanical constraints are probably responsible for the

‘convergent’ evolution of the fusiform shape in various marine-vertebrates, and numerous other

iteratively evolved forms whose viable morphospace is constrained by and particularly sensitive

to the invariant physical laws.

A second class of constraints, well known to the structuralists and historicists but often

overlooked by post-synthesis adaptationists, are those that are internal to the organism and

contingent—namely, developmental strictures on the potential universe of formal solutions due

to the canalization or entrenchment of developmental pathways and subsequent stabilizing

selection.15 Gene regulatory networks are hierarchical, with earlier linkages having more

pleiotropic effects than the more distal fine-grained terminal processes. The former upstream

sub-circuits, which Davidson and Erwin (2006) have termed ‘kernels,’ specify the more general

domain of the developing organism. Because kernels are ‘recursively’ wired, interference with

any single kernel gene “will destroy kernel function altogether and is likely to produce the

catastrophic phenotype of lack of [a] body part” (Ibid, p. 796). The result—massive

conservation in upstream regulatory sub-circuits—best explains why so little change has

occurred at the phylum level since the establishment of body plans in the base of the Cambrian.

Adaptive change has been largely relegated to lower levels in the network, offering an internal-

constraints perspective on the decimation-diversification model championed by Gould (1989).

Moreover, the progressive entrenchment of gene regulatory networks maps well onto the clumpy

15 In discussing explanations of the robust conservation of homologues, Sterelny & Griffiths (1999, p. 228) note that stabilizing selection need not be the driving causal force, since the persistence of such traits could equally result from chance, assuming change is less frequent than stasis. Dan McShea (2005) has shown, however, that modifications (and by implication complexity) accumulate rapidly in the absence of stabilizing selection. Viewed in this way, it is (ironically) the phylogenetic preservation of organismic structure—rather than its transformation—that lies at the heart of evolutionary explanation.

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distribution of form at all levels of the taxonomic hierarchy. In fact, it seems reasonable to

assume that the lability of homologous downstream sub-circuits probably underwrites the lion’s

share of parallel evolutionary events.

The ‘consensus paper’ of Maynard Smith et al. (1985, p. 281) suggests that development

can “serve as a directing force, accounting in part for oriented features of various trends and

patterns” (see also Alberch, 1982). While some external constraints may represent the pressures

of the physical laws on formal solutions to relatively universal design problems, internal

(developmental) constraints probably reflect not invariance but frozen contingency, or the radical

conservation of kernels which arose in response to local, stochastically fluctuating ecological

pressures in the pre-Cambrian. Unfortunately, SCM systematically conflates internal

(developmental) and external (abiotic, ecological) constraints on design space in arguing for a

fundamentally non-contingent view of life.

The most effective way of categorizing independently evolved similarities so as to reflect

the above distinction is by recognizing two causally differentiated sub-categories within the

larger rubric of homoplasy. These are convergent and parallel evolution. SCM’s only mention

of this “vital” (Simpson, 1961) distinction in his singular work on the importance of convergence

is in a brief footnote, in which he states: “Now is the time to avoid that old chestnut of whether it

is convergent evolution as against parallel evolution” (2003, p. 435 n. 1). One of the main

purposes of this paper, however, is to convince the reader that the distinction between convergent

and parallel evolution is no small chestnut. Expressly failing to acknowledge the importance of

this distinction, SCM reasons that the difference between convergence and parallel evolution is

“obviously” one of degree rather than kind (Ibid). But this conclusion is far from obvious,

particularly given the importance to many preeminent evolutionists of distinguishing these two

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importantly different phenomena (e.g. Diogo, 2005; Sterelny, 2005; Gould, 2002; Wake, 1991;

Patterson, 1988; Saether, 1983; Simpson, 1961; Haas, 1946). In fact, SCM’s failure to address

the implications of this distinction for his hard adaptationist view of life is perhaps the most

severe weakness in his appeal to the literature on homoplasy in order to bolster the project of a

universal biology.

To clear up perceived conceptual confusion in the literature regarding the contrast

between convergent and parallel evolution, Haas and Simpson (1946) published a seminal

review of the topic. They defined homology as similarity based on common ancestry, in

contradistinction to homoplasy, which simply refers to similarity that is not due to common

descent. The latter concept was introduced by E.R. Lankester (1870) in recognition of the

epistemic ambiguities surrounding causal origins, and thus does not make any positive claims

about common selective pressures or functional origins of the identified similarity. Use of the

word analogy, however, often used interchangeably with convergence, has come in the literature

to imply a common selected function as the underlying basis for a perceived similarity (Ibid).

Where analogy ultimately falls in the evolutionary lexicon is not terribly relevant for the purpose

of this paper, which rather is primarily concerned with whether convergence is more than an

analogy in the philosophical (rather than definitional) sense.16 Of greatest relevance here will be

the contraposition of convergence with parallel evolution, each a type of homoplasy but with

importantly different causal origins.

16 There are several possibilities concerning the definition, use, and status of the term ‘analogy.’ First, one might view analogy as a subset of parallelism or convergence, making positive claims about the selective causes of origin in either case. Alternatively, analogy may be considered a subset only of convergence, in which it makes selective-origin claims in the absence of or by factoring out underlying developmental homology. Finally, analogy may be equivalent to and synonymous with the notion of convergence, remaining neutral with regard to its underling causal mechanism.

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Haas preferred to distinguish convergence and parallelism geometrically (rather than

causally)—with the former entailing that two lineages resemble one another more than their

ancestors did, and the latter referring to cases in which two lineages evolve in the same direction

without resembling one another any more than their ancestors did. Simpson, however, advocated

a causal differentiation of the two concepts, with convergence resulting solely from common

selective pressures, and parallelism tied to underlying homologous developmental pathways, or

what he termed a “community of common ancestry” (1961, p. 103). Thus, while Haas might

view concurrent increasing body size in Orthoptera (grasshoppers, locusts and crickets) and

Pinnipeds (seals, sea lions and walruses) as an example of parallel evolution, Simpson would

presumably decline to use the label of parallelism—or analogy for that matter—since the causal

factors involve neither shared developmental generators nor common selective pressures.

Whereas the distinction between homology and homoplasy is relatively crisp, the concept

of parallelism occupies a ‘gray zone’ between definitional homology (retention by common

descent) and convergent homoplasy (similar design and function with entirely different

developmental-structural origins) (Patterson, 1988). Unlike convergence, parallelism contains

too much ‘homology-ness’ to be considered solely the result of similar ecological (extra-

organismic) pressures. As Gould (2002, p. 1078) remarks, “parallelism contains aspects of both

constraint and individual selection—not as a wishy-washy mixture in one grand pluralistic glop

of all-things-for-all-people, but in rigorously different parsings for different levels of

consideration” (emphasis in the original). The distinction between parallel and convergent

evolution is in some sense one of degree (Diogo, 2005), although the decision to categorize a

homoplastic event as one or the other is not arbitrary. It is true that all organisms share

important homologues (such as the general structure of the DNA code) as they descend from a

17

single common ancestor, and thus at certain grains of analysis the distinction between parallelism

and convergence disappears (Wilmer, 2003). However, it is the underlying homology with

respect to the generators directly causally responsible for the homoplastic event that defines

parallelism and non-arbitrarily distinguishes it from convergence. Nevertheless, as Diogo (2005

p. 739) adequately points out, there are in fact significant epistemic difficulties in differentiating

the two types of homoplasy in the practical phylogenetic context.

Theoretically, parallelism exists because ‘organismic branching’ occurs before ‘genetic

branching’ in cladistic reconstructions—that is, underlying developmental generators are

conserved, while changes in developmental timing, etc. produce recognizably differentiated

morphology befitting of taxonomic partition. In essence, then, parallel evolution is a form of

homoplasy that depends in a somewhat ironic sense on the more general domain of homology

not only for its recognition (see above) but its raison d’etre.17

Developmental biologists have often stressed the idea that homologues, by definition

non-functional categories (Amundson & Lauder, 1994), are not mere evolutionary

epiphenomena, but rather are tied to a different but equally viable notion of function—namely

the casual roles of various biological processes, such as embryogenesis and developmental

anatomy. As Amundson notes (2001, p. 3), “they see homologues as indicators of the

commonness of the developmental processes that give rise to them.” Gunter Wagner (1989), for

example, emphasizes shared developmental constraint as the conceptual and definitional basis of

homology. The problem with the developmental definition of homology, however, particularly

in relation to the present discussion, is that it appears broad enough to include parallel

evolutionary events, which are also entrenched in common underlying pathways of development

17 Both functionalist and structuralist views of the evolution of form predict the occurrence of parallel evolution (Wake, 1991), although the former do so for reasons external and the latter for reasons internal to the lineages in question.

18

that constrain the ultimate set of forms on which selection can act. As such, both causal role

functions (e.g. embryogenesis) and selective effects functions must be invoked to explain the

general frequency and specific instances of parallel evolution. Indeed, the category of

parallelism not only vindicates Amundson and Lauder’s argument (1994) that causal role

functions have a role to play in the ‘proximate’ sciences of anatomy and physiology—but it goes

even further to demonstrate that causal role function is a requisite explanatory device in

evolutionary biology as well. Additionally, causal role functions must be taken into account in

delineating the set of potential morphologies that can underwrite a given function, particularly

within the confines of a specified Bauplan.18 For these reasons, causal role functions will be

integral to any explanation of the inhomogenous clumping of organisms in morphospace, which

results in large part from internal developmental constraints on the evolution of adaptive

optimality.19

For sure, it is analytically possible (though empirically intractable) to parse the

homological components of parallelism from those which result solely from factors external to

the organism—namely selective pressures constrained by the physical laws.20 Nevertheless,

assuming a single origin of life, true convergence conceptually requires the replacement of all

underlying developmental-genetic architecture over time through processes of selection and drift.

Defined in this way, convergence becomes a negligible or even non-existent phenomenon. This 18 Generally, ‘Bauplan’ refers to a deep and robustly persistent homology that restricts the potential set of variation on which selection can act, or as we shall see, makes the realization of certain forms more likely. 19 Incidentally, by explaining the uneven distribution of forms in morphospace, developmental canalization can refute one of the major critiques marshaled by Creationists against Darwinian theory—namely, the lack of extant transitional forms between clustered Bauplans at various levels of the genealogical hierarchy. 20 I would venture a guess that this intractability in parsing the relative significance of internal and external constraints on the evolution of organismic form lies at the root of the conflation of parallel and convergent evolution in the literature. However, as the nascent science of ‘evo devo’ comes of age and the epistemic problems begin to evaporate, these terminological distinctions will become vital to our understanding of the evolution of organismic form.

19

is because in light of recent findings concerning ‘deep homology’ (see below), even our best case

scenarios of convergent evolution may in fact be re-defined under the internally (and

contingently) constrained rubric of parallelism.

Whereas external constraints may be viewed as limiting the universe of viable solutions

to ecological design problems, internal developmental constraints may positively influence the

evolution of local optimality by establishing preferred internal channels that make good solutions

more likely to be hit upon (e.g. Diogo, 2005; Gould, 2002).21 As such, developmental

constraints on variation may limit the so-called ‘evolvability’ of lineages by reducing the overall

isotropic variation on which the optimizing agency of natural selection may act, but they may

also positively shape the evolution of organismic form by rendering accessible certain ‘good

tricks in design space,’ to use Dennett’s (1995) terminology.22

Such a positive reading of internal constraint implies that developmental processes may

bias generative mechanics so as to enhance the production of certain adaptive features,

particularly ‘key innovations’ or traits causally associated with macroevolutionary diversification

and the origins of higher taxa.23 For example, Hunter and Jernvall (1995) have shown that the

‘hypocone,’ the extra cusp characteristic of the quadritubercular (rather than triangular) molars

21 The canonical definition of ‘developmental constraint’ was introduced by Maynard Smith et al. (1985), who held it to be “a bias on the production of variant phenotypes or a limitation on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system” (p. 266). As Amundson (1994) notes, constraints on form are only constraints on adaptation when they prohibit the production of phenotypic variants that would have been favored by natural selection over variants permitted by the formal constraints. To this extent, the discourse on constraint is entrenched (forgive the pun) in a counter-factual biological analysis. 22 In fact, canalization may be more than an incidental limitation on isotropic variation; it could also be (as Waddington, 1959, suggests) an adaptation in its own right designed to ensure developmental outcomes by maintaining the robustness of ontogenetic trajectories in the face of developmental disturbances. 23 To the extent that key innovations or ‘good moves’ in design space are connected to the origins of higher taxa, one way of testing SCM’s hypothesis regarding the inevitability of certain adaptive complexes is to determine whether or not they are correlated with speciel or functional diversity (via neontological and paleontological comparison with sister taxa). Many promising candidates, such as the image-forming eye (see below), yield mixed results (see De Queiroz, 1999). This is largely because key innovations are contingent on numerous spatio-temporal and ecological factors (De Queiroz, 2002), and thus can only be recognized retrospectively (Sterelny, 2006).

20

of most therian mammals, evolved “convergently” (read: in parallel) within this order of

mammals on more than twenty occasions and is associated with marked diversification. It is

almost certain that this highly iterative and presumably useful adaptation, which greatly

increases occlusal area for crushing herbivorous material, owes its repeated evolution to a

common developmental pathway underlying the generation of the therian dental arcade. Thus,

while mammalian developmental constraints may limit the range of solutions available to

members of that class, it may also positively bias the production of key innovations such as the

hypocone. At the same time, however, it would be unreasonable to argue that the mammalian

hypocone, because it evolved numerous times independently in one order of mammals, is an

inevitable evolutionary outcome insensitive to developmental constraints—but as we shall see,

this is precisely the type of argument put forward by SCM.

Importantly, by underwriting the parallel evolution of similar forms, internal constraint

may allow for a degree of macroevolutionary predictability, at least within specified taxonomic

(Bauplanian) parameters. Viewed in this way, internal constraints are not merely the cause of

global adaptive imperfection, but also the basis for local optimality. Thus, while particular

conserved developmental pathways (e.g. Pax-6/eyeless) may represent inextrapolatable frozen

contingencies, such generative entrenchment (Wimsatt & Schank, 1988) may facilitate the

repeated parallel evolution of predictable formal solutions which are optimal given the

developmental parameters.24 Consequently, we can make at least as good a case for

developmental attractors which preserve homology as SCM can make for functional attractors

which break it.

24 The notion of developmental entrenchment may be considered analogous to (but perhaps less coarse-grained than) the concept of ‘contingent irreversibility’ with respect to the major transitions of life (Maynard Smith & Szathmary, 1995; but see McShea (2001) on reversals in major transitions).

21

Ironically, developmental constraint may be responsible for much of predictable

functionalism, while at its core embodying a form of radical contingency. This conclusion may

be unsatisfying to those with a penchant for neat dichotomies, as it provides yet another instance

of noisy biology defying our hopes for a mathematical symphony of parading forms. Yet, the

parameters of biological evolution remain not classical but improvisational—quirky, historical

movements, unintelligible in prospect but brilliant in retrospect, that would make Newton and

Mozart cringe but Armstrong and Davis proud.

As noted above, ‘convergence’ has often been touted as quintessential evidence for hard

adaptationism. However, many instances of homoplasy are better explained by a hybrid analysis

involving both functionalist (non-contingent) and historicist-structuralist (contingent)

interpretations of the constraints on available design space (Wake, 1991). Such an analysis is

more characteristic of parallelism than it is of convergence. SCM fails to recognize this

difference and therefore does not consider the possibility that frozen, canalized contingencies

may underlie many of the paradigmatic examples of ‘convergent’ evolution that he invokes.25

As a result, he repeatedly peddles putative examples of ‘convergence’ that are in fact parallel

adaptations underwritten by a homologous body plan, physiology, and genetic-developmental

architecture.

For instance, SCM appeals to numerous examples of ‘striking convergence’ within

taxonomic classes in response to common selective pressures. Such convergences better

interpreted as parallelisms (in which underlying homologous generators cannot be ruled out)

include: the ‘convergent’ saber-toothed morphology between placental and marsupial felids

(1998, p. 202-204; 2003, p. 130-132), ‘convergences’ within specific orders of mammals,

25 This crucial point is forcefully made by Sterelny (2005) in his critical review of SCM’s 2003 book.

22

including fossorial lifestyles (p. 132, 140), ‘convergence’ with respect to ‘pike morphology’ in

several genus of freshwater fish (p. 133), ‘convergence’ in raptorial forelimbs of the mantids and

neuropterans (p. 129) (two orders within the class hexapoda), ‘convergences’ within orders of

birds in plume coloring, wing shape, and hummingbird morphology (p. 138), and ‘convergence’

in stem morphology of the New World cactus and the African spurge, to name just a few (p.

134).26

As to this last example, SCM states (Ibid) that “cacti and spurges are only distantly

related, and their common convergence is because of the rigors of living in an arid environment.”

But this is only a partial truth, as a significant (if not dominant) causal determinant of the above

recognized similarities in form is a common Bauplan which restricts the set of formal solutions

to shared design problems. In such cases, both selection and development constrain adaptational

design space, and the latter may reflect contingent rather than robust macroevolutionary patterns.

Pictorial renditions of these parallelisms are on their face quite remarkable, and appear at first

blush to reflect the inexorable power of natural selection to steer form toward certain pre-

ordained evolutionary ‘endpoints’ or ‘functional islands.’ Nevertheless, they are not all that

surprising or impressive, given a more thorough appreciation of developmental pathway

conservation. As noted by Wake (1991, p. 555) in the context of the evolution of the attenuate

body form in salamanders, the fact that “related lineages independently have adapted to similar

microhabitats by assuming essentially identical ecomorphologies” is suggestive not of formal-

functional invariance but significant design limitations due to entrenched developmental

26 SCM also invokes the results of controlled experiments by Travisano et al. (1995) on populations of E. coli that he believes cut against a radically contingent view of life. After being left to diverge for 2000 generations and then exposed to a challenging biochemical substrate (maltose), adaptive responses were initially dominated by history (contingency), but (according to SCM) adaptationism ultimately prevailed via the swamping effects of convergence. This represents yet another example of parallel evolution involving underlying homologous generators whose extrapolatability to the history of life is seriously questionable. Furthermore, as Sterelny states (2005, p. 587), “[i]t is one thing to multiply evolve the ability to digest maltose; it is another to evolve meiosis.”

23

pathways. For these reasons, SCM cannot invoke many or even most of his ‘striking’ examples

of homoplasy without accounting for positive developmental constraints. SCM may be correct

that natural selection will (ceteris paribus) tend to find the optimal solution to a given design

problem, but it will be forced to do so within the strictures of a contingently pre-defined

Bauplan.

For example, selection seems to have arrived at an essentially optimal ratio of filament

length to strike speed in squid tentacles (Kier & Curtin, 2002), but such a compromise is made

within the larger, constrained and perhaps ultimately contingent framework of the molluskan, or

more specifically cephalopod, body plan. Similarly, biologists have made arguments for long-

term evolutionary progress by pointing to iterative examples of competitive exclusion or

incumbent replacement, such as the replacement of straight-necked turtles with those capable of

folding their necks underneath their shells (a parallel rather than convergent event which,

according to Rosenzweig & McCord, 1991, occurred on four to five separate occasions). While

these examples might indeed be indicative of phyletic competition driving patterns of

macroevolution, there is no reason to believe that such intra-class or intra-order homoplasy is

generalizable to higher taxonomic levels. Therefore, such touted examples of parallelism are not

going to give SCM the kind of extrapolatability that he needs to support his universal biology,

which requires a set of laws regarding the evolution of biological form that is stable across

higher taxonomic counterfactuals.27

27 For sure, the number of variants within a given Bauplan may be infinite, but it is the disparity or range of morphological variation that is of critical import for purposes of understanding the scope and severity of constraints on the evolution of form. Amundson (1994) offers an elegant analogy in this regard between Chomskian theories of language and Bauplanian constraints—he states (p. 570) that “just as all languages generated by a universal grammar are governed by certain constraints, so are all of the possible outcomes of the embryological processes of a given phylum.”

24

After discussing the remarkable case of ‘convergence’ between the marsupial fauna of

the super-islands of Australia and South America, and the placental mammals, SCM goes on to

render the following most astonishing conclusion (1998, p. 205): “Does it follow then that

contingent processes are an irrelevance in the way we see the world? I have argued that, so far

as the history of life is concerned, they are.” Simply by looking at putative homoplasies between

two mammalian orders, he concludes that contingency is not important in the history of life.

What about the evolution of body plans themselves in the late Ediacaran? What about the

seemingly arbitrary extinction of many successful phyla in the Cambrian? Why did it take

between 1 and 2 billion years for the first single-celled organisms to appear, and another billion

years or so for prokaryotic symbiosis to produce eukaryotic cells, a major transition which only

happened once?28 The answers to these questions do not necessarily invoke contingency, but

these are critical questions for which we do not currently have answers—questions that clearly

cannot be resolved definitively or even confidently by appeals to marsupial parallelisms.

That said, for the comparative physiologist, instances of marsupial-placental convergence

represent a “treasure of information distinguishing between crucial and incidental features of

mammals that have taken up different habits and habitats” (Vogel, 1996, p. 301). Any such

extrapolations, however, are taxonomically limited to the mammalian class. On the other hand,

if the most recent common ancestor of toothed and baleen cetaceans was terrestrial, their

similarities would tells us much more about the external constraints on aquatic mammalian

lifestyle then they currently do. Nevertheless, even such broader generalizations would be

taxonomically confined—for example, “[t]he fact that some members of every major arthropod

class [] have extremely long and thin legs quite likely implies some basic but as yet

28 In each of his major works discussing the philosophical implications of convergence, SCM’s fails to address or otherwise mention the term ‘major transitions’, a concept which, according to a review by Szathmary (2005, p. 851), represents a “great challenge” to SCM’s proffered view of life.

25

unappreciated functional property of the basic skeletomuscular scheme of arthropods” (Ibid), but

probably not of bilaterians generally.

While SCM does point as well to genuine instances of convergence, many of these

examples, such as the analogy between arthropod eusociality and the reproductive caste systems

of naked mole rats (2003, p. 141-142), are probably not counterfactually robust and only weakly

extrapolatable. Eusociality, for instance, is only a key innovation (a trait connected with

macroevolutionary diversification as compared to a paleontological sister taxa without the trait in

question) if at all in its arthropod instantiation(s), which in some sense militates against

recognizing it as a universal functional attractor. Moreover, as Vogel (1996) points out, the

dozen or so independent origins of sexually non-egalitarian eusociality in Hymenoptera reflect

parallel rather than convergent adaptations underwritten by a peculiar haplo-diploid genetic

system (as compared to a one single case of eusociality arising in egalitarian insects). In any

event, if eusociality is an inevitable evolutionary endpoint, why has it not arisen in other

vertebrate clades?

Problems of counterfactual instability apply to many of SCM’s proffered exemplars of

convergence, including those between beetles and spiders on ant morphology (in subverting the

‘immune system’ of the hive), which themselves may depend on layers of ecological and formal

contingency. On the other hand, SCM does draw upon some examples of genuine macroscopic

convergence that could be the subject of robust macroevolutionary generalizations, such as those

pertaining to particular sensory modalities (Ibid, Ch. 7);29 lamentably, he offers no principled

method for comparing the extrapolatability of different types of homoplasy.

29 Sterelny (2005) suggests that there may be significant limitations on sensory adaptations given the constrained set of reliable signals an organism can get about its environment, but such constraints are probably much weaker than SCM contends.

26

SCM must find some way, then, to rescue the philosophical import of many of his

favorite examples of convergence properly reinterpreted as developmentally-entrenched (and

positively constrained) instances of parallel evolution. One possibility is to offer the following

argument: In order to develop a homology-free perspective on macroevolution, particularly in

relation to the evolution of form, we must (ironically) accede to the developmental-structuralist

contention that conserved internal channels underlie many homoplastic events; however, this

need not entail capitulation to a historicist, contingency-infested view of life, so long as each

entrenched developmental component of any given homoplasy arose not by contingent but

predictable, functional processes. Lather rinse repeat, so the argument goes, we arrive at the

current inhomogenous distribution of morphospace occupation through the optimizing hand of

selection working synergistically with the ‘ratcheting’ effect of internal Bauplanian constraints.

There are good reasons for rejecting this argument, however. While there are instances in

which natural selection has been shown to act as an optimizing agent (see examples involving

squid strike speed and folding turtle necks above), there are innumerable occasions in the

evolution of form in which a simple engineering analysis demonstrates that selection acts more

often as a tinkerer; that is by tweaking existing sub-optimal developmental plans to meet

transient local adaptive challenges. In fact, it is the ubiquitous sub-optimality of organismic

design that is often showcased (by Darwin, no less) as among the best evidence for evolution by

natural selection, a blind mechanistic process that works with what is at hand and cannot plan

ahead. Indeed, as Vermeij states (1994, p. 233), “the fact that degrees of specialization [i.e.

optimality] among ecologically similar species vary widely in the world today strongly implies []

that design standards in nature are relative, not absolute.” Thus, “given that the correlation

between structure and function is often far from precise, we should expect organisms to embody

27

ad hoc and often rather clumsy solutions to functional demands, solutions that bear a deep stamp

of history and ancestry” (Ibid, citations omitted).30 In sum, there are few good reasons for

believing that selection has produced primarily optimal, counter-factually stable developmental

constraints, particularly given the stochastic and ephemeral nature of the selective environment.

Furthermore, SCM has failed to demonstrate that the specific ineluctable evolutionary

endpoints suggested by widespread homoplasy are associated with diversification (as measured

either by species number or functional disparity), nor has he established that lineages bearing

these ‘inevitable’ adaptive complexes enjoy longer evolutionary durations than those without the

traits. To the contrary, Van Valen (1973) famously showed that the probability of extinction for

any group is steady through time, irrespective of its phylogenetic age. These findings have been

interpreted as the result of a perpetually decaying selective environment, due in part to strategic

interactions between co-evolving species, in which global evolutionary progress is virtually

impossible. At the same time, the ability of selection to subvert contingent Bauplanian

constraints has not been shown, and in fact the opposite conclusion flows from the massive

conservation of kernels and the prevalence of sub-optimal design. Having failed to establish

these two propositions, SCM’s central argument—and by implication his view of life—cannot

stand.

Thus, we may properly conclude that SCM cannot appeal to parallelisms to bolster his

core contention (2003 p. 11) that ‘[while] the landscape of biological form...may in principle be

almost infinitely rich...in reality the number of roads through it may be much, much more

30 Other factors interfering with selection, such as gene flow between neighboring populations, have been shown to undermine the evolution of local optimality. For instance, genes selected for in contiguous but ecologically different environments can hybridize during inter-populational mating, impeding optimal adaptation to local ecological challenges (Brandon, 1990). And there is evidence to suggest that even low levels of gene flow can have significant homogenizing effects on gene frequencies within a population (e.g. Maruyama, 1972).

28

restricted.’ A better explanation of irregular morphospace occupation is reflected in Gould’s

(2002, p. 1178) remarks:

I do not doubt that many discontinuities in morphospace represent the colonization by optimal phenotypes of widely dispersed peaks in maximal biomechanical efficiency. But I am equally confident that more of nature’s evidently nonrandom, and oddly dispersed, clusters in morphospace, bearing such enormously different weights ranging from single ‘outliers’ to millions of species, primarily record the historical constraints imposed by workable solutions with adaptive origins—developmental designs that then congealed, enforcing reiteration and change within their internally directed channels forever after.

Importantly, some of SCM’s claims tiptoe around the evolution of organismic form per se

into the more Platonic realm of disembodied functional properties. Nonetheless, convergence

between phyla or other distant taxonomic groups on certain broad functional solutions, such as

‘mastication,’ will not admit of interesting predictions regarding the evolution of form absent

some specification of a particularized Bauplan that natural selection can tweak in furtherance of

the common ecological task. Beyond forecasting a greater hardness of the proximate organs of

mastication relative to the objects being masticated (a ratio with otherwise unspecified

parameters), in addition to the ‘muscles’ and ‘neural’ architecture necessary to correlate their

grinding movements, not much can be said about the evolution of formal solutions to mastication

disembodied from an explicit body plan. This is because the limitations and positive (non-

isotropic) channels imposed by internal homologous generators are a much more significant

determinant of the evolution form then the invariant physical laws. Although not totally

insignificant, the latter constraint alone will not restrict ecologically viable morphospace to a

manageably finite set of formal solutions. While exobiologists would likely be able to identify

organs of mastication regardless of the particularized form they assumed, there is no reason to

conclude that vertebrate teeth, arthropod mandibles, or mollusk beaks are inevitable solutions to

29

such a broad ecological design problem as ‘chewing.’31 Thus, we may be able to conclude a

priori that big fierce animals will be relatively rare in any ecosystem (Colinvaux, 1980), but this

generalization tells us virtually nothing about what they will look like.32

What in any event are these Platonic functional attractors that SCM refers to? He

believes that they are “biological properties” that represent the predictable evolutionary

endpoints or functional attractors toward which biological form gravitates. There are deep

problems with this view, however, and precisely how deep will depend on how we cash in the

term biological property. If we define it broadly to include such mechanisms as ‘predator

evasion,’ ‘metabolism,’ ‘thermo-regulation,’ or ‘propulsion,’ such properties may indeed be

universal but they would be of no use whatsoever in predicting the evolution of form if there is

an unmanageably large set of configurations that can realize the same function. The biological

property of ‘a behavior that increases one’s chances of reproduction’ would be about as

uninformative.33 The concepts of Batesian and Mullerian mimicry, for instance, in and of

31 Another mistake that SCM makes in citing a litany of putative examples of ‘convergence’ is that he simply assumes without argument that the morphological similarities he cites represent similar formal solutions to similar functional design problems. In fact, the strict morphological definition of homoplasy at work in phylogenetic reconstruction includes no such assumption. That is, similar non-homologous features are convergent or parallel regardless of their particular functions—“the convergent forms may function similarly, but similarity in physiological performance or ethological or ecological biological role is neither assumed not expected” (Simpson, 1961). Thus, if SCM endeavors to show that the examples of convergent (often actually parallel) evolution that he cites imply particular invariant constraints on adaptational design space, he must show that these instances of homoplasy involve common selective effects. 32 Although a good case can be made for significant non-contingent (chemico-physical) constraints on biological mechanisms (Sterelny & Griffiths, 1999), this paper is concerned with the evolution of form, to which mechanism does not necessarily speak. 33 SCM suggests that the optimizing agency of natural selection biases macroevolutionary processes toward certain predictable evolutionary ‘endpoints.’ It is true that even if throughout the course of evolution we do not see a repetitious return to the same old formal functional solutions, we do find an undeniable trend in certain meta-variables, such as the increase associated with hierarchical structure and complexity of design. However, pioneering work by Dan McShea (1994 / 2001), operationalizing complexity to determine whether an evolutionary tendency (bias) toward complexity is present across all hierarchical levels of all major taxa, has shown the evidence for a driven (rather than passive) trend to be inconclusive. As such, SCM must countenance the adequacy of stochastic, ‘random walk’ null models of macroevolutionary change to explain the emergence of (homoplastic) complexity without upward biases toward particular ‘functional attractors.’ In other words, while many instances of homoplasy

30

themselves offer nothing by way of the evolution of form because they are contingent on the

sensory capabilities of predators, rather than any factors intrinsic to mimicry (Vogel, 1996).

Viewed as such, SCM’s conception of biological properties is at best overly broad and at worst

tautologous and devoid of interesting conclusions. Moreover, it is particularly odd that SCM

would argue for the existence of disembodied functional attractors largely by appealing to the

literature on parallel and convergent evolution, as functional convergence without formal

convergence does not confound phylogenetic reconstruction and is generally not considered

homoplasy.34

In any event, SCM offers no reason to believe that the set of functional attractors, even if

it is significantly less than the infinite morphospace of the logically possible, is small enough that

it will allow for robust generalizations. Given the complexities of local, Earth-based ecology, it

is easy to envision a near-infinite set of functional attractors in the hyper-heterogeneous cosmic

ecology—one so large that science can never make a dent in it, let alone invoke local ecological

conditions to formulate laws of form on its basis. Even many of the adaptive complexes which

can be defined solely by their selective effects because they represent functional solutions to

widespread design problems in the animal kingdom, such as ‘gut’ or ‘heart,’ are probably not

restricted to a manageable set of forms that can realize the relevant function. For this reason, as

far as the evolution of organismic form is concerned, purely functional biological categories of

may be the result of similar selective pressures, overall macroevolutionary trend dynamics may be random with respect to those particular instances. Moreover, convergence in one-dimensional variables such as hierarchy would be no more impressive than that with respect to say body size or color, since none of these features enable biologists to make robust predictions regarding the evolution of particularized form. Thus, even if a selection-biased trend toward hierarchical complexity were established, this would be but a very small brick in foundation of a universal biology. 34 SCM also points to convergences in behaviors, such as ‘agriculture’ (e.g. between certain hymenoptera and modern humans). While behavioral homoplasy may be more amenable to operationalized comparison than disembodied functions, it nonetheless presents definitional difficulties that make the comparison more recalcitrant than traditional investigations in structural homoplasy.

31

the sort relied upon by Neander (1991) and referenced by SCM do not have much to contribute

to the quest for non-accidental macroevolutionary generalizations.

To be clear, I am not arguing that the holy grail of biology—a universal research

program—is but a pipe dream. In fact, I have put forward ideas elsewhere (Powell, 2005,

unpublished manuscript) designed to develop a conceptual-empirical research program toward

precisely that effect. In order to overcome the obstacles imposed by functional supervenience on

a heterogeneous formal base, and strategic interaction operating in a stochastically fluctuating

selective environment, we must identify more than simply nominal instances of homoplasy that

are merely suggestive of design constraints, as SCM has done. Rather, we must identify

iteratively evolved traits that are robust and Bauplan-independent, in that they represent the

recurrent independent realization of adaptive solutions that, due to unusually severe physical

constraints and near-universal ecological applicability, are limited to a manageable set of forms

and largely impervious to strategic interaction. If we could confidently identify such an adaptive

complex,35 it would offer a powerful refutation of radical contingency, or the view that not only

is the emergence of specific taxa highly contingent on initial conditions, but so too are the

complex adaptive solutions hit upon by those taxa (as espoused by Gould, 1989, p. 290).

Unfortunately, SCM’s unsystematic work on convergence takes us no closer to realizing

this goal. He maintains that unlike the specific adaptations of a particular species, general

35 Candidates that are in my view probably robust across higher taxonomic counterfactuals and constrained to a limited set of potential forms include (inter alia) image-forming eyes (see e.g. Land, 1992), circulatory systems which depend on bulk flow rather than simple diffusion and hence require nonlinear elasticity (Vogel, 1996), and load-bearing structures which depend on a crucial twist-to-bend ratio (Vogel, 1998). The latter two examples involve mechanical variables that can be (and indeed have been) achieved in forms as diverse as the hind-tibia of Orthoptera to tree trunks and flight-associated feathers.

32

biological properties are robust, since they will inevitably manifest at some spatio-temporal

point in the unfolding of the evolutionary process.36 He states (2003, p. 308):

[A]lthough any history is necessarily unique, the resultant complex end-form is not simply the contingent upshot of local and effectively random processes. On any other suitable planet there will I suggest be animals very much like mammals, and mammals much like apes. Not identical, but surprisingly similar.

In essence, then, SCM’s challenge is not directed at uncontroversial theories of contingency

associated with “the destiny of a given lineage” (1998, p. 201), but toward the more radical

Gouldian notion that most or all interesting biological properties are themselves highly

sensitively dependent on initial conditions. He states (2003 p. 307) that “whenever the known

edge of the evolutionary envelope is reached...then it will be explored independently several

times.” However, he does not offer any concrete guidelines as to how in any particular case we

are to know when the putative “edge” is reached. How many cases of convergence are to

suffice, and how are we to determine the degree of similarity necessary to recognize them as

meaningful homoplastic events? In this vein, how similar must the structural solutions be

(formally and functionally) for our projections to be meaningful? Does parallel evolution count

for or against the inevitability of evolutionary endpoints? Even more importantly, why are

certain endpoints at the end? These questions confront any strong appeal to the frequency of

homoplasy as evidence for a highly non-contingently constrained adaptational design space.

Finally, SCM appeals to widespread exaptation as evidence of the supremacy of natural

selection to escape developmental constraints in driving the compass of life toward particular

36 SCM would argue that although we might be unable to predict which particular lineages (e.g. small theropod dinosaurs with feathers aiding in thermoregulation) will co-opt existing structures for particular predictable functions (e.g. powered flight), we need only show that it is virtually inevitable that some lineage will evolve the relevant biological property.

33

formal-functional solutions.37 His view seems to be that ubiquitous pre-adaptation implies that

life will find a way to ‘leap frog’ as it were from one developmental constraint to another,

shifting functions and co-opting spandrels, inexorably moving toward more complex functional

solutions embodied by homoplastic events. As such, he views exaptation as a positive causal

contributor to the frequency of homoplasy. Gould (2002) also emphasizes the

macroevolutionary role of exaptation, but he argues that it points instead toward a severely

contingent view of life. He maintains (p. 1160) that the ubiquity of “quirky functional

shift...inevitably suggests (but admittedly does not prove) a high degree of fortuity, as implied by

the required capacity of features built for one function to act in another way that could not have

influenced or regulated their original construction by any functional evolutionary mechanisms

like natural selection.” Thus, one can just as easily read the ubiquity of exaptation as frustrating

rather than facilitating homoplastic events by forcing natural selection to work within severely

sub-optimal developmental constraints. As a result, we are unable to analytically or empirically

infer either predictability or contingency from pervasive exaptation.

3. DEEP HOMOLOGY AND ITS IMPLICATIONS FOR MACROEVOLUTIONARY EXO-BIOLOGICAL EXTRAPOLATION

Deep homology refers to the recent surprising discovery in the emerging field of

evolutionary developmental biology (‘evo-devo’) that all major phyla share certain key

developmental pathways whose origin dates back to the Pre-Cambrian, well over 500 million

37 Throughout his 2003 book (e.g, p. 4-5), SCM vaguely refers to a concept he calls “inherency,” which interpreted mechanistically appears to refer to the notion of an ‘exaptive pool’ (sensu Gould, 2002, p. 1270) and is often invoked in the context of discussions of phenomena that are best described as exaptation (a term conspicuously absent from an otherwise thorough index). This reading is supported by a definition SCM offers in another publication that same year (2003b, p. 505), in which he characterizes inherency as “the potentiality of structures central to evolutionary advancement…[which] are already ‘embedded’ in more primitive organisms.” Szathmary (2005, p. 856) has recently also interpreted SCM’s use of ‘inherency’ in this way.

34

years ago, a notion unexpected by the Modern Synthesis. The presence of deep homology

suggests a much more prominent role for parallel evolution in shaping the history of life than

previously thought. This raises the possibility that bilaterian diversity, as Gould (2002, p. 1159)

suggests,

represents an extensive set of modifications and tinkerings upon a basic pattern set by history at the outset, and then adumbrated in one geologically brief episode to establish all fundamental building plans. Forever after, for more than half a billion years, the subsequent evolution of complex animals [] has been restricted to much more limited permutation within the confines of these early congealed designs.

The core question, then, is whether the metazoan Bauplane represent a “subset of workable, but

basically fortuitous, survivals among a much larger set that could have functioned just as well,

but either never arose, or lost their opportunities, by historical happenstance” (Gould, 2002, p.

1160-1161). This question is currently unanswerable, although the foregoing analysis

concerning the decaying nature of the selective environment combined with the radical

entrenchment of kernels suggests that the various morphospatial clusterings are unlikely to

represent globally optimal evolutionary endpoints.

Research probing the phylogenetic depth of homologous development, such as work done

on the inter-phylum substitutability of master control genes such as Pax-6/eyeless (Zuker, 1994)

and Hox genes (Carroll, 2005), and their ability to generate ectopic or supernumerary organs in

different phyla, demonstrates the substantial macroevolutionary importance of phylogenetic

conservation. Take, for instance, that locus classicus of convergent evolution in the camera-type

eyes of vertebrates and cephalopods. While the macroscopic arrangements of such eyes may be

convergent, their underlying neural-developmental architecture is homologous, dating back to

35

the Pre-Cambrian. SCM notes as much (2003, p. 239-241), but does not follow the implications

of deep homology to its natural, larger conclusion. As Gould (2002, p. 1069) states,

Eyes of such strikingly similar design owe their independent origin as much to genetic and developmental parallelism, based on internal constraints of homologous genes and developmental pathways, as to selection’s capacity for iterating nearly identical adaptations from scratch by convergence.

SCM notes that there may be a common organization for highly developed sensory systems

which he believes, if true, suggests that the evolution of the various sensory modalities may be

surprisingly likely once the neural architecture is in place for one system. However, he offers no

guess as to the evolutionary probability of that initial suite of neural architecture which he

believes was placed into the exaptive pool for later cooption into various sensory modalities as

selection sees fit. Presumably, he would argue that the massive conservation of entrenched

developmental pathways speaks not to the existence of frozen accidents but rather to the power

of natural selection to stabilize good adaptive solutions.

Nevertheless, deep homology raises the following striking possibility: at the heart of many or

even most of the quintessential examples of convergence is a massively conserved, monophyletic

developmental architecture. If this is the case, and the evolution of such underlying homologous

constraint (what Saether (1989, p. 619) referred to as the “capacity to develop synapomorphy”) is

itself sensitively dependent on initial conditions, then adaptationists such as SCM cannot freely

cite such examples of parallel evolution as evidence of non-contingent constraints on

adaptational design space. Deep homology may indeed imply deep contingency—and it only

takes one major contingency to collapse the regime of adaptive predictability!

Nevertheless, non-accidental generalizations regarding the evolution of form can obtain even

if the entire history of life is derived from a single massive contingency (say at the earliest stages

36

of replicating molecules, 3.5 bya), or even from several phylum-level contingencies during the

Cambrian ‘experimentation phase’ which resulted in the entrenchment of only a small subset of

equally likely metazoan phyla. This is because our extrapolations could specify particular

Bauplanian constraints (which function as a limited set of ceteris paribus qualifications), thereby

presupposing certain major transitions in the evolution of life or the origins of major body plans.

For example, given the developmental underpinnings of the mammalian dental arcade, and an

epistemologically manageable set of ecological factors, we could perhaps make certain robust

predictions regarding the evolution of mammalian molars. This may be done even if one

monumental contingency (e.g. a massive bolide impact 65 mya) lies at the casual heart of

mammalian adaptive radiation in the Paleocene.

The precision of our generalizations will, of course, be inversely correlated with the

taxonomic abstraction at which they are made—that is, intra-order extrapolations will fare better

than those made within phylum, class, etc., due to the increased significance of shared

homology. Indeed, it is for this reason that we should expect to find higher frequencies of

homoplasy at lower taxonomic categories, as we in fact do. If this analysis is correct, I would

regard it not as a failure of a priori biology per se, but rather as an important (if ineliminatable)

limitation on the scope of exobiological generalizations.

4. THE NEED TO SPECIFY HIERARCHICAL LEVELS OF ANALYSIS IN ASSESSING HOMOLOGY AND HOMOPLASY

Homology has always been a non-functional category, identified by anatomical,

morphological, or developmental criteria alone, irrespective of selective-functional

considerations (Amundson & Lauder, 1994). Importantly, organisms possess character traits at

37

multiple levels of organization and can be examined at different structural tiers (Roth, 2001). As

developmental biology has matured, it has become increasingly clear that “homology at one level

does not necessitate homology at another” (Abouheif et al., 1997). According to Van Valen

(1982), one fundamental property of homology is “continuity of information.” Because different

aspects of structures are controlled by different developmental programs (Roth, 1984), changes

at one level, such as genotype or gross phenotype, may be ‘screened off’ (sensu Brandon, 1990)

from changes at other levels of the hierarchy (Roth, 2001 / 1991). Phenotypic traits may be

decoupled, as it were, from their underlying patterns of gene expression and development.38 For

these reasons, we are compelled to identify our level of inference in assessing homology, be it

genes, their patterns of expression, their developmental functions, or the structures to which they

ultimately give rise.

A similar analysis should apply to our identification of homoplasy. Not surprisingly, a

number of authors (Gould, 2002; Bolker & Raff, 1996; Roth, 1991; Wagner, 1989) have

emphasized that the assessment of homoplasy can only be done effectively by specifying a

hierarchical level of analysis. For example, many aspects of the wings of birds and bats are

homologous—for they share a common suite of vertebrate plesiomorphic traits—however, the

macroscopic modification of those traits (for powered flight) evolved homoplastically between

these lineages. Similarly, a given morphology may be parallel, while the underlying molecular-

developmental generators are homologous. Thus, in order to claim that two traits are

homoplastic, we must specify particular hierarchical levels of complexity, such as the molecular-

genetic, cellular, tissue, organ, morphological suite, or ecological archetype. This is a crucial

38 Roth (2001) offers the example of a gut, which is a synapomorphy shared by all vertebrates (and hence homologous), but with patterns of gastrulation which differ between vertebrate clades.

38

element missing from many discussions of convergence, including that of SCM. To be fair,

however, the champions of contingency are frequently guilty of the same confusion by also

failing to consider traits as multi-dimensional, heterogeneous entities. In sum, any similarity, be

it homologous or homoplasious, can be said to disappear at different levels of detail. Thus, as

Gould (2002, p. 1128) notes, “convergence may prevail on one level, and constraint at another.”

Furthermore, in order to identify true homology, we need only show “overall historical

continuity in developmental mechanics,” not uniformity of underlying development; that is to

say, we must only demonstrate that in “stepwise substitution of some elements of the process [],

correspondence remains for other elements” (Roth, 2001, p. 94).39 Presumably, since ‘true

homology’ requires contiguity in developmental mechanics, ‘true homoplasy’ requires a

discontinuity in developmental mechanisms, which is supposedly reflected in phyletic disparity.

However, as we have seen, parallel evolution involves both a continuity and discontinuity in

underlying developmental generators, which compels us to recognize this gray area between

homology and homoplasy—a category that (as findings in deep homology suggest) is becoming

ever more important for understanding the inhomogenous occupation of morphospace.

Even if we find substantial convergence at higher vertical levels of analysis, such as the

macroscopic arrangement of the camera-type eye, the generalizability of such homoplasy may be

undercut by the presence of an earlier horizontal contingency, such as the monophyletic origin of

the underlying neural-developmental architecture or, even more broadly, the advent of

prokaryotic symbiosis to form eukaryotic cells (the single pre-cursor to multi-cellularity, as

noted above).

39 Thus, according to Roth (2001), X is homologous in all instances of the following four-step evolutionary transition sequence: (1) a,b,c produce trait X; (2) a,b,k produce X; (3) p,b,k produce X; (4) p,g,k produce X.

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5. DIFFICULTIES IN OPERATIONALIZING THE DEGREE OF HOMOPLASY

As intimated above, homology and homoplasy are not all-or-nothing phenomena (Roth,

1983), and the degree of homology may depend on the extent to which two traits share a

developmental pathway. In the weakest forms of homology, for example, two traits may merely

derive from the same germ layer. Just as it is important to specify the hierarchical levels under

consideration, it is necessary to assess the degree of homoplasy when appealing to examples of

convergence as evidence for limitations on the evolution of organismic form. In listing

numerous examples of homoplasy, SCM makes no attempt to assess their respective strengths,

and offers no method for quantifying the significance of the examples he cites.

Given the inherent difficulties of assessing homoplasy in multi-dimensional structures, it

is not surprising that no method to my knowledge has yet been conceived by which to quantify

the degree of homoplasy between two lineages or forms. In theory it is possible to operationalize

the degree to which a given set of organs have become more or less similar over time—this

could be accomplished by methods analogous to the determination of morphospace occupation

(but in reverse, since disparity measures morphological divergence, rather than convergence).

Even under such an analysis, however, instances of meaningful homoplasy will often be limited

to examples of parallel rather than truly convergent evolution, since (as noted above) it is only

within lower-taxonomic contexts that form will significantly converge on multiple hierarchical

levels.

For example, the parallel evolution of saber-toothed morphology between orders of

mammals will garner a higher homoplasy rating than the convergence between mollusk and

vertebrate camera-type eyes with respect to the macroscopic arrangement of structural parts, due

in large part to the higher propinquity of descent in the former. In order to compensate for this

40

problem, we might attempt to calibrate for phyletic distance (say, for instance, between the

incisors of mammals and the mandibles of hexapods) by adjusting for the importance of

between-phyla convergence. This might allow us to better control for homology and assess the

significance of external, functional constraints on the evolution of form. While it is certainly

worth a try, it is presently unclear how such calibration can be carried out in a principled way.

Another potential method for operationalizing the degree of homoplasy is to construct a

null model of morphological change over time, assessing the probability that by chance alone

two lineages ‘randomly walked’ (according to purely stochastic processes) toward one another in

morphological variation.40 Although the null model of morphological change does not posit an

underlying cause of the homoplasy, where two lineages resemble each other more than either

does their common ancestor, the most parsimonious presumption is that such commonalities

result from similar selective pressures (e.g. Vogel, 1996). This presumption is rebuttable,

however, and must be supported by an ecological engineer’s analysis.

Apart from issues regarding its causal agnosticism, however, another limitation of the

null model of morphological change is that it does not take into account the various hierarchical

components of parallelism—that is, the contribution of underlying monophyletic developmental

pathways to homoplastic trends. As discussed above, developmental canalization may act as a

positive constraint on the evolution of form and a facilitator of homoplastic events by

functioning as a bias in an otherwise stochastic trend. Therefore, assuming that selection is

equally strong in each case, trends involving parallel and convergent evolution respectively may

be differentially biased by their corresponding degrees of developmental entrenchment.

40 For this point I am indebted to Dan McShea, personal communication. While random walks will frequently admit of statistically significant regressions and obvious trends, they often generate fallacious intuitions about bias, since such trends can emerge even where ‘ups’ and ‘downs’ are equally probable.

41

CONCLUDING REMARKS

A contingent view of life does not entail denying a significant role to natural selection in

governing evolutionary change. However, it does claim that selection is forced to respond to

stochastically changing ecological environments and to work within entrenched, sub-optimal

developmental constraints. Rather than snubbing the only known mechanism for producing

function, the historicist may grant some importance to the “striking phenomenon” of

convergence, but maintain that SCM “overestimates its sway” in guiding evolution to particular

endpoints (Gould, 1998, p. 57). In searching for the holy grail of biology, we must be cautious

to avoid what we might call the ‘anti-naturalistic fallacy,’ or the penchant to believe that nature

(in this case macroevolution) is how we would like it to be—orderly, intelligible, and governed

by a manageable set laws. I leave it to the reader to decide whether Charles Darwin himself was

a functionalist or historicist at heart—that is, whether the oft-quoted final passage of the Origin

(reproduced below) makes an analogy or disanalogy between the invariance of the physical laws

and the evolution of biological form:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved (1859).

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