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Darwin's Principle of DivergenceAuthor(s): Ernst MayrSource: Journal of the History of Biology, Vol. 25, No. 3 (Autumn, 1992), pp. 343-359Published by: SpringerStable URL: http://www.jstor.org/stable/4331227 .

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Darwin's Principle of Divergence

ERNST MAYR

Museum of Comparative Zoology Harvard University Cambridge, Massachusetts 02138

One must avoid making the history of science a hagiography of its greats. Even the greatest of the scientists had their blind spots and fell victim to various contradictions. In spite of my almost unbounded admiration for Charles Darwin, I must confess that even he was human. This has recently been pointed out by another great admirer of Darwin's, David Kohn, who showed how often Darwin vacillated and, in order not to hurt the feelings of friends and members of his family, sometimes even concealed his real views.'

Darwin's principle of divergence and its application to the process of speciation is an area where Darwin was quite unable to come to a clearcut solution, and where his writings show that he had not yet emancipated himself completely from "pre-Darwinian" modes of thinking.2 In his autobiography, he says:

But at that time [the 1840s- 1850s] I overlooked one problem of great importance. ... This problem is the tendency in organic beings descended from the same stock to diverge in character as they become modified . .. and I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me; and this was long after I had come to Down. The solution, as I believe [and this can be considered Darwin's definition of the principle of divergence] is that the modified offspring of all dominant and increasing forms tend to

1. David Kohn, "Darwin's Ambiguity: The Secularization of Biological Meaning," Brit. J. Hist. Sci., 22 (1989), 215-239.

2. I have dealt with this subject once before (Ernst Mayr, "Isolation as an Evolutionary Factor," Proc. Amer. Phil. Soc., 103 [19591, 221-230), arriving at some tentative conclusions. A new situation, however, was created by the publication of Darwin's "Big Species Book" (Charles Darwin, Natural Selection, ed. R. C. Stauffer [Cambridge: Cambridge University Press, 19751), in which Darwin presented his reasoning and his evidence in far greater detail than in the Origin of Species. This permits a new and indeed more definitive analysis.

Joumal of the History of Biology, vol. 25, no. 3 (Fall 1992), pp. 343-359. ? 1992 KluwerAcademic Publishers. Printed in the Netherlands.



344 ERNST MAYR

become adapted to many and highly diversified places in the economy of nature.3

In his writings from the 1850s on, Darwin never failed to emphasize the evolutionary importance of this principle. In fact, he stated in a letter to Joseph Hooker that together with the principle of natural selection it was the most important concept he had ever developed: "the 'principle of Divergence,' which with 'Natural Selection' is the keystone of my Book & I have very great confidence it is sound."4

Among the students of Darwin's writings there has been considerable disagreement as to when Darwin actually discovered this principle, and even more disagreement as to why he considered it so important. Janet Browne thinks that it was discovered in 1857,5 but Kohn has advanced strong arguments that Darwin had already developed the new concept in 1854, although it seems that he did not use the definite terminology "principle of divergence" until 1857, in correspondence with Hooker and Asa Gray.6 On September 5, 1857, Darwin wrote to Asa Gray: "One other principle, which may be called the principle of divergence plays, I believe, an important part in the origin of species. The same spot will support more life if occupied by very diverse forms: we see this in the many generic forms in a square yard of turf."n7

The basic point of the principle of divergence is simplicity itself: the more the coinhabitants of an area differ from each other in their ecological requirements, the less they will compete with each other; therefore natural selection will tend to favor any variation toward greater divergence. The reason for the principle's importance to Darwin is that it seemed to shed some light on the greatest of his puzzles - the nature and origin of variation and of speciation.

To solve these puzzles Darwin asked himself two important questions: (1) Where in nature do we encounter the greatest

3. Charles Darwin, The Autobiography of Charles Darwin, ed. Nora Barlow (London: Collins, 1958), pp. 120-12 1.

4. Frederick Burkhardt and Sydney Smith, eds., The Correspondence of Charles Darwin (Cambridge: Cambridge University Press, 1991), VII, 102.

5. Janet Browne, "Darwin's Botanical Arithmetic and the 'Principle of Divergence,' 1854-1858," J. Hist. Biol., 13 (1980), 53-89.

6. David Kohn, "Darwin's Principle of Divergence as Internal Dialogue," in The Darwinian Heritage, ed. D. Kohn (Princeton: Princeton University Press, 1985), pp. 245-257.

7. Burkhardt and Smith, Correspondence, VI, 448.



Darwin's Principle of Divergence 345

amount of variation? (2) How is this variation connected with the origin of new species? To answer the first question, Darwin employed the "botanical arithmetic" so carefully analyzed by Browne.8 He finally came to the conclusion that there were three kinds of genera: large genera (that is, genera with very numerous species), and two kinds of small genera. The first category of small genera contained similar species occurring in a rather restricted contiguous area. The second category encompassed genera with rather distinct or aberrant species, and a rather scattered distribution. When calculating the number of varieties per species, Darwin found that "on an average the species in the larger genera in any country oftenest present varieties in some degree permanent, and likewise a greater average number of such varieties, than do the species of the small genera."9

From this Darwin concluded that genera went through a cycle. At first they were small, with the species still rather similar to each other and occurring in a relatively compact area. By further evolution they would become rich in species, and these species would develop a tendency to have numerous new varieties. And finally there were certain somewhat senescent genera that had lost their ability to produce variation, had a scattered distribution, and were likely to become extinct sooner or later. The question whether Darwin's calculations were truly relevant, and his results statistically valid, I shall leave unanswered because the answer is not pertinent to my further arguments. Let us assume instead that Darwin had truly discovered the source of new varieties.

VARIETIES AND SPECIES

But why did Darwin consider varieties so important, and what is their relation to the principle of divergence? To answer these questions, it is necessary to review Darwin's concepts of varieties and species. What did he mean when he used the term "species"?

There was a drastic change in Darwin's species concept between the time of the Transmutation Notebooks (1837-1838) and the 1850s. In the Transmutation Notebooks Darwin had an amazingly modem biological species concept, evidently based on the observation of sympatric species of animals. It includes such statements as "my definition of species ... is simply, an instinctive impulse to keep separate," "The dislike of two species to each other is evidently an instinct - & this prevents linterjbreeding,"

8. Browne, "Darwin's Botanical Arithmetic" (above, n. 5). 9. Darwin, Natural Selection, p. 235.



346 ERNST MAYR

and "Hence species may be good ones & differ scarcely in any external character."10

Almost no trace of this kind of thinking about species can be found in Darwin's later writings. There is no doubt that by the mid-1850s he had shifted to a rather nominalist-typological species concept. "Species," he said now,"... [likel genera ... are merely artificial combinations made for convenience."'II Naturally, the members of a species were derived, as Alphonse de Candolle said (following John Ray), "from a common source after a considerable number of generations."'I2 But none of Darwin's later nominalist-typological definitions permitted any demarcation between species and variety.

Darwin is quite inconsistent in what he calls a variety. At one place he states that he calls a variety that which is connected at the present day by intermediate gradations,'3 but he also recognized discontinuous variants. Varieties, so Darwin thought, were on the whole less distinct than species - or, as Browne has said, they were "little species." Nor is ordinary individual variation separable from the presence of distinct varieties: "we should look at all individual differences (independently of those produced by crossing) as having the same nature & origin with those marked by naturalists as varieties." 14

What was far more injurious for Darwin's analysis was that he quite consistently used the same term, "variety," for two entirely different natural phenomena: on the one hand, for geographically delimited populations, currently called geographic races or subspecies; and on the other, for intrapopulation variants - that is, for individuals differing from the majority of the population by a diagnostic character, which, as we now know, may be due to a single gene difference. With respect to these two very different categories of varieties, a definite shift had occurred over the years in Darwin's thinking. Can we reconstruct the stages in his conver- sion from a biological to a more typological position?

The first stage occurred in March 1837 when the three island populations of mockingbirds which he had considered to be varieties were declared by John Gould to be three species. For the

10. P. H. Barrett, P. J. Gautrey, S. Herbert, D. Kohn, and S. Smith, Charles Darwin's Notebooks, 1836-1844 (Ithaca, N. Y.: Cornell University Press, 1987), C, 161;B, 197; B, 213.

11. Charles Darwin, On the Origin of Species (Cambridge: Harvard Uni- versity Press, 1964), p. 485.

12. Darwin, Natural Selection, p. 96 (translation mine). 13. Darwin, Origin of Species, p. 485. 14. Ibid., pp. 105-106.




first time Darwin realized that accepting degree of difference as the species criterion might lead to differences of opinion: what one author considered a species, another might consider a variety, and vice versa. More importantly, he learned that this was particularly true for what we now call "allopatric" (= geographically representative) populations.

It is quite evident from the discussions in his species book that Darwin was rather uncertain when to use the terms "subspecies" and "variety." Of subspecies he says that they are "the geographi- cal races of some Zoologists. But the term subspecies is used by some authors, to define ... very close species."'" In agreement with prevailing zoological custom, Darwin in the 1830s and 1840s called geographic races "varieties," and referred to the situation "when two varieties inhabit two distinct countries as is often the case and as is very generally the case with the higher animals."'6 The problem of what to call geographically isolated populations played a considerable role in the working out of Darwin's zoological Beagle collections, not only for the Galapagos, but also for Chiloe and the Falkland Islands: "I was much struck how entirely arbitrary the distinction is between varieties & species, when I witnessed different naturalists comparing the organic productions which I brought home from the islands, off the coast of S. America."'7

His work on the barnacles was the next stage in the develop- ment of Darwin's species concept. It did not produce any major shift in his thinking, but it forced him scores of times to make difficult decisions as to which forms to consider species, and which others subspecies, in a highly variable group of species. In letters to his friends he detailed again and again his difficulties and frustrations: "After describing a set of forms, as distinct species, tearing up my M.S., & making them one species tearing that up & making them separate, & then making them one again (which has happened to me) I have gnashed my teeth, cursed species, & asked what sin I had committed to be so punished."'18 In the end Darwin used varieties copiously, particularly for allopatric populations. As a leading barnacle specialist (William A. Newman) writes me: "most of his varieties are now recognized as good species."'9 For instance, of the ten varieties of Balanus tintinnabulum

15. Ibid., p. 99. 16. Ibid.,p.138. 17. Ibid., p. 115. 18. Burkhardt and Smith, Correspondence, V, 156. 19. William A. Newman, personal correspondence, December 17, 1991.



348 ERNST MAYR

recognized by Darwin, one is a synonym, while all the other nine are now considered good species. Congeneric species of barnacles are morphologically much more similar to each other than are most species of birds.

Two points are evident from Darwin's barnacle work. The enormous effort he made to arrive at the right conclusion in the question "species or variety?" indicates (1) that at that time he considered the species a real phenomenon of nature, and not an arbitrary artifact of human sorting; and (2) that he considered varieties to be a stage in the process of speciation. He described most of his barnacle varieties from wide-ranging species, which, as Newman tells me, "showed variation especially when samples from distant localities were compared."20 This, indeed, was hinted at by Darwin himself.

In contrast to the prevailing custom in zoology, the term "variety" in the botanical literature was applied more often than not to individual variants within a population. Furthermore, there was rather general agreement among botanists that there is no real difference between varieties and species, species simply being more distinct than mere varieties. For instance, Darwin was informed by one of his botanist friends that among the varieties listed in the London Catalogue (of British Plants) there are 182 varieties that "have been ranked by some one botanist as species."21 Quite logically, Darwin stated: "by our theory two closely allied species do not differ essentially from a species and its strongly defined variety."22 Hence, "According to the views dis- cussed in this work, species do not differ essentially from varieties; - two closely allied species usually differing more from each other than two varieties, & being much more constant in all their characters."23 Just how uncertain naturalists were at that time about the nature of species is indicated by the comment made by Darwin that "more than one-third of the varieties marked by Asa Gray are considered by him as possibly deserving to be called species."24

THE ARBITRARINESS OF THE DISTINCTION BETWEEN VARIETY AND SPECIES

By the mid-1 850s, and afterwards, Darwin frequently expressed

20. Ibid. 21. Darwin, Natural Selection, p. 137. 22. Ibid., p. 139. 23. Ibid., p. 165. 24. Ibid., p. 137.




his conviction that the distinction between variety and species was purely arbitrary: "it is no wonder that there should be difficulty in defining the difference between a species & a variety; - there being no essential, only an arbitrary difference ... there is often a wide neutral territory in which the terms species & varieties are bandied about according to the state of our knowledge & our ideas of the term species."25 If one adopted Darwin's claim, then the problem of the origin of new species was virtually solved: every variety was at the threshold of becoming a new species, and it required only a little push of natural selection to complete the process. Therefore, varieties and species are smoothly connected by ";a graduated series from the finest shades of individual differences, to well defined races, distinguished with great difficulty, if really distinguishable at all, from sub-species & closely allied species."26 This observation was formulated by Darwin in the assertion that "species do not essentially differ from varieties, & that varieties by further modification may be converted into species."27 Again and again Darwin states that he looks at varieties as incipient species.

One of the ambiguities in Darwin's thinking about species stemmed from his uncertainty about the nature of variation. Sometimes, when talking about the gradual change of a variety owing to natural selection, he uses the language of population thinking. On other occasions he sounds rather typological, as if a species consisted of a mosaic of varieties: "As each new variety is formed through natural selection, solely from having some advantage over its parent, each new variety will tend to supplant and exterminate its predecessor."28

WHAT IS SPECIATION?

A species, as we now see it, is characterized not only by being different, but also by being distinct from other species (separated by a gap, reinforced by isolating mechanisms). In his more nominalistic treatment of species in the later 1850s Darwin was concerned only with difference, the first criterion of this dual characterization. As a result he apparently never fully realized that there was a fundamental difference between the phyletic evolution of a lineage (the typological change of a species in the

25. Ibid., p. 98. 26. Ibid., pp. 164-165. 27. Ibid., p. 164. 28. Ibid., p. 272.



350 ERNST MAYR

time dimension) and genuine speciation (that is, the multiplication of species).

Darwin's description of the change of a variety into a species is a description of phyletic evolution. In a discussion of various forms of the origin of varieties, he says that a new variety "will tend in the long run to supplant & exterminate its parent-stock; for its formation is due to some new advantage gained under the conditions to which it is exposed, & it will generally largely inherit the advantages of its parent."29 He describes this process of "supplanting" as follows: "for of those living at one time & within one area we should see only [in the fossil record] the parent-stock and one or two varieties, which if destined to become triumphant will increase in numbers & range & so ultimately supplant the parent; the parent, I may add, being ranked as the variety, as soon as its range became less than that of the conquering variety."30 There is no question as to Darwin's focus on phyletic evolution ("vertical speciation"), even though in a few passages (see below) he also refers to the speciation of geographically isolated "varieties."

HOW DOES A VARIETY BECOME A SPECIES?

A species arises simply, says Darwin, by a variety gradually becoming more and more different through natural selection:

I believe ... that by far the most effective origin of well marked varieties and of species, is the natural selection or preservation of those successive, slight, & accidental (as we in our ignorance must call them) variations, which are in any way advantageous to the individuals thus characterized: hence there would be a better chance of varieties & species being thus formed amongst common than amongst rare [species]. I may add, to illustrate what I mean, that a nurseryman who raises seedling of a plant by the hundreds of thousand far oftener succeeds in his life- time in producing a new & valuable variety, than does a small amateur florist.3'

When trying to answer the question why new varieties might be successful in the struggle for existence, Darwin introduces his principle of divergence. He points out that Man in the production

29. Ibid., p. 263. 30. Ibid. 31. Ibid., p. 136 (italics added).




of his domestic races has tended to select the most extreme forms; and he cites examples from pigeons, fowl, horses, and so forth. "Now in nature, I cannot doubt, that an analogous principle, not liable to caprice, is steadily at work, through a widely different agency; & that varieties of the same species, & species of the same genus, family or order are all, more or less, subjected to this influence."32 As illustration, Darwin mentions variable species.

The importance of the principle of divergence lies in the fact that "as in the long run, more descendants from a common parent will survive, the more widely they become diversified in habits, constitution & structure so as to fill as many places as possible in the polity of nature, the extreme varieties & the extreme species will have a better chance of surviving or escaping extinction, than the intermediate & less modified varieties or species."33 And it is the principle of divergence that is responsible for the observation that "the average difference between two species of the same genus, the parents of which by our theory once existed as mere varieties, is greater than the average difference between two such varieties."34 Darwin does not realize how close he is to a circular argument, considering that he determines species status by the degree of difference.

Much of Darwin's argument suffers from his failure to make a distinction between ecological segregation (occupation of a different niche) and geographic segregation (occupation of an isolated area). With reference to vacant ecological niches, he says: "The expression of variation in a right direction implies that there is a place in the polity of nature, which could be better filled by one of the inhabitants, after it has undergone some modification: the existence, therefore, of an unoccupied or not perfectly occupied place is an all-important element in the action of natural selection."35 Unfortunately, under "unoccupied place" Darwin con- founds ecological niche with a newly colonized geographic area, for he continues: "both Mr. [Thomas] Wollaston & Alph. de Candolle have strongly insisted that isolated areas are the chief scenes of what they consider, like most naturalists, as the actual creation of new species & likewise of varieties."36 Darwin and Wollaston - de Candolle talked about two entirely different things: ecological niches and geographic isolates. These two

32. Ibid., p. 228. 33. Ibid., p. 238. 34. Ibid., p. 249. 35. Ibid., p. 252. 36. Ibid.



352 ERNST MAYR

phenomena play utterly different roles in the process of speciation, and it is impossible to understand the process of speciation if one does not make a distinction between them.

Equally misleading is Darwin's failure to distinguish between the ecological niche of a species and the slight ecological sin- gularities of individuals in a population. His principle of divergence applies almost exclusively to different coexisting species, and he produces no evidence whatsoever that it also applies to members of a single population. If it did, then every population would consist of ecotypically highly different individuals - but this is not what one finds. Highly polymorphic species are rare; the individuals of most species, and certainly of most populations, resemble one another quite extraordinarily - like peas in a pod, as the saying goes. Therefore intrapopulation variation does not supply the material for a multiplication of species. Darwin's nominalist approach was unable to solve the problem of speciation. Perhaps not surprisingly, one finds exactly the same confu- sion of ecological and geographic isolation in the writings of Darwin's disciple George Romanes.

THE ROLE OF GEOGRAPHIC ISOLATION

The Darwin of the Galapagos mockingbirds was aware that geographic isolation can play a role in converting a variety (geographic race) into a species. He realized that there is a difference between sympatric varieties and situations where "two varieties inhabit two distinct countries, as is often the case & as is very generally the case with the higher animals."37 Not only isolation as such but the degree of isolation is important for the completion of speciation. Hence, on Madeira, strong flying birds have not developed any notable endemism, whereas poorly flying beetles and even more so the sedentary terrestrial mollusks have produced great numbers of endemics. Incidentally, all the particular forms mentioned by Darwin in this discussion as being the product of geographic isolation are animals.38 This is not surpris- ing since, beginning with Pyotr Simon Pallas, there had been in zoology a long tradition of the study of geographic variation of the description of geographic varieties and subspecies, while there was much less study of geographic variation in plants, particularly in Britain and America before the mid-nineteenth century. The numerous difficulties in the delimitation of sympatric species of

37. Ibid.,p. 138. 38. Ibid., pp. 253-257, 269.




plants may have been the reason for the neglect of the study of geographic variation by botanists.

Darwin's main contact after the completion of the barnacle monographs was with botanists, particularly with Hooker, Hewett C. Watson, and Asa Gray. This is well reflected in his correspondence. The new information that he received from these botanists made him change his thinking in a number of ways. They con- vinced him, for instance, that sterility was not the secure species criterion he had once thought. Also, they had a much more morphological species concept than he had had earlier. If Darwin wanted to convince his botanical friends of the validity of his ideas on the origin of species, it was a good tactic to adopt a theory that might appeal to them - and this, he thought, was one of the virtues of the principle of divergence.

It is obvious that Darwin had trouble appreciating the importance of geographic isolation. The main reason for this is that he could see only one kind of geographic barriers, oceanic ones. Rises and falls of sea level seem to have been the only continental barriers conceived by him. Thus he concluded: "in the case of the southern extremity of Africa, which is so extraordinarily rich in species, [one must assume] that it formed at no very remote epoch a large archipelago of islands."39 (He later abandoned this con- jecture.) If only water barriers lead to isolation, and "considering the whole world, from the fewness of the completely isolated spots, & from the difficulty of the subsequent diffusion of new forms therein produced, such isolated spots, will probably not have played a very important part in the manufacturing of species."40 In other words, most new species, according to Darwin, originated in continuous areas: "I do not doubt that many species have been formed at different points of an absolutely continuous area, of which the physical conditions graduate from one point to another in the most insensible manner."41 The only discontinuous factor is the occurrence of competitors whose species border does not coincide with that of the species with which it competes. Hence "I do not doubt that over the world far more species have been produced in continuous than in isolated areas."42

It was this downgrading of the importance of geographic isolation that was later attacked by Moritz Wagner.43 Wagner, of

39. Ibid., p. 265. 40. Ibid.,p.261. 41. Ibid., p. 266. 42. Ibid.,p. 254. 43. Moritz Wagner, Die Darwin'sche Theorie und das Migrationsgesetz der

Organismen (Leipzig: Duncker und Humblot, 1868).



354 ERNST MAYR

course, was quite right, even though Darwin answered him with the words "It would have been a strange fact if I had overlooked the importance of isolation, seeing that it was such cases as that of the Galapagos Archipelago, which chiefly led me to study the origin of species."44 Evidently by then (1876) Darwin had forgot- ten how adverse he had been in the 1850s to attributing an important role to geographic speciation. In spite of the opposing claims by some historians, it would seem that Wagner clearly had the better of the argument. He demonstrated that in the scores, if not hundreds, of cases of ongoing speciation that he had studied, geographic barriers had without exception isolated the incipient species. That Wagner had a rather erroneous understanding of natural selection is irrelevant for the speciation argument. Darwin, by contrast, was unable to demonstrate even a single credible case of sympatric speciation. August Weismann's later endeavor to document sympatric speciation in the case of the Steinheim snails is at best inconclusive, since these snails have enormous phenoty- pic plasticity and it is most likely that different phenotypes lived at the same period in different springs in the by-then largely dried up Steinheim Basin.45

THE ALTERNATE: SYMPATRIC SPECIATION

If geographic isolation is insufficient to account for the origin of the majority of species, what then did Darwin imagine to occur? He thought that all that was necessary was that natural selection would enhance the differences among the coexisting varieties of a species until they had reached species level. And this is the point where the "principle of divergence" entered the picture. Darwin suddenly conceived the idea that the more different the varieties became from each other, and the more they used different resources and occupied different niches in order to avoid competition with each other, the better such varieties could coexist in the same area and the more quickly they would become different good species. Interestingly, in his big manuscript Darwin

44. Francis Darwin, Life and Letters of Charles Darwin (London: Murray, 1888), III, 159, letter of October 13, 1 876.

45. Ernst Mayr, "Weismann and Evolution," J. Hist. Biol., 18 (1985), 295- 329; Frederick Churchill, "Weismann's Continuity of the Germ Plasm in Histori- cal Perspective," Freiburger Universitdts-blitter, 87/88 (1985), 107-124; W.-E. Reif, "Endemic Evolution of Gyraulus Kleini in the Steinheim Basin," in Sedimentary and Evolutionary Cycles, ed. Ulf Bayer and Adolf Seilacher (Berlin: Springer, 1985), pp. 256-294.




compared this process with "our domestic productions": here too the breeder selects "the most extreme forms. He has made the race-horse as fleet & slim as possible & goes on trying to make it fleeter: the cart-horse he makes as powerful as he can," and so forth.46 As a consequence, "in any country, a far greater number of individuals descended from the same parents can be supported, when greatly modified in different ways, in habits constitution & structure, so as to fill as many places, as possible, in the polity of nature, then when not at all or only slightly modified."47

Darwin found evidence for his principle also in other areas, as when he pointed out that "the view that the greatest number of organic beings ... can be supported on any area, by the greatest amount of their diversification ... is in fact that of 'the division of labour', so admirably propounded by Milne-Edwards."48 Indeed, Darwin specified of what this diversification consists: the diverging varieties are being kept apart because they have become "isolated from haunting different stations, disliking each other, breeding at different times &c, so as not to cross."49 I have shown previously that the numerous putative instances of sympatric speciation in the literature are highly vulnerable.50 The populational characteristics menticned by Darwin are clearly secondary by-products of the genetic divergence of isolated populations; hence, they are the result of ongoing speciation rather than being the cause of it. The "disliking," the "haunting of different stations," and the "breeding at different times" relate to different populations, not to different individuals within a population. Indeed, such ecological and behavioral properties in individuals would sexually isolate them and doom them to have no offspring.

Darwin's putative evidence plays such an important role in his thinking that it might be worthwhile to look at it in more detail. Thirteen cases of incipient sympatric speciation are listed in Natural Selection.5' The first six cases involve domestic races of mammals and birds, where members of one race are reported to have preferred to associate and mate with members of their own race rather than with those of the other; the artificial isolation that resulted in the production of these races evidently preceded the

46. Darwin, Natural Selection, p. 227. 47. Ibid., p. 228. 48. Ibid., p. 233. 49. Ibid., p. 269. 50. Ernst Mayr, Animal Species and Evolution (Cambridge, Mass.: Harvard

University Press, 1963), pp. 449-450. See also Ernst Mayr, "The Why and How of Species," Biol. Phil., 3 (1989), 431-441.

51. Darwin, Natural Selection, pp. 258-259.



356 ERNST MAYR

testing of the preferences. A sixth case, the story of the two kinds of wolves in the Catskill Mountains, was never confirmed. The next case is the two kinds of caribou, and here clearly geographic isolation was the primary factor. Not enough information is given about Gould's case of two varieties of a Tasmanian bird species to analyze this case, but the occurrence of both migratory and sedentary individuals in the same population of birds is well established in both hemispheres and has never, so far as I know, resulted in speciation in spite of Darwin's claim (tenth case). Finally, Darwin lists three cases, one in crows, one in rollers, and one in ground beetles, in which two varieties or species hybridize in a zone of contact. All three cases have since been well studied, and it is now quite clear that this zone of contact is the line at which the two previously isolated incipient species met after the breakdown of the extrinsic isolation. Thus, in none of the cases cited by Darwin were the behavioral differences primary, leading to the isolation of the incipient species. It is obvious from this analysis to what an extent he underestimated the importance of geographic isolation.

In the application of his principle of divergence to the descendants of a set of parents, Darwin argued as if every variety during the process of divergent speciation were an asexual clone, not interacting at all genetically with the other varieties of the species but subject only to divergent selection. But we are actually dealing with sexually reproducing species, and no mechanism is known that would keep these diverging individuals from interbreeding with each other.

If Darwin's reasoning were valid, all species ought to be highly variable, consisting of ecological specialists. However, nothing of the sort is found in nature. Highly variable polymorphic species are very rare. The uniformity of nearly all species, as well as the frequency of sibling species, documents on the contrary that such centrifugal variation within a population hardly ever occurs. What variation is found is mostly geographic variation, but this does not at all illustrate the principle of divergence since each geographic variant (subspecies) is the only representative of the species where it occurs.

The discriminating reader will notice that throughout Natural Selection Darwin talks only about ecological specialization. Now- here does he mention the aspects of species he had so emphasized in his Transmutation Notebooks; for example, nothing is mentioned about the acquisition of the instinct of repugnance to intermarriage, or the instinct to keep separate, which at the time of writing the Notebooks had been for him among the chief




characteristics species.52 Nothing is mentioned about the origin of that which we now call the isolating mechanisms. In fact, in his later controversy with A. R. Wallace about the origin of cross- sterility of species, Darwin very specifically stated that it could not have been acquired by natural selection under sympatric conditions; as he said in a letter to T. H. Huxley, "Nature never made species mutually sterile by selection, nor will men."53 It is quite curious that Darwin seems never to have realized that his process of sympatric speciation by character divergence would not work unless he supplied simultaneously an explanation of the acquisition of isolating mechanisms. As I showed above, all the suggested incipient isolating mechanisms described by Darwin are clearly properties of populations that had been previously isolated.54

Perhaps the greatest weakness of his argument is that Darwin applied the principle of divergence not only to species, where it is indeed largely supported by modem ecological research, but also to the offspring of a set of parents: "I consider it as of the utmost importance fully to recognize that the amount of life in any country, & still more that the number of modified descendants from a common parent, will in chief part depend on the amount of diversification which they have undergone, so as best to fill as many & as widely different places as possible in the great scheme of nature."55 Actually, Darwin had no evidence for putative ecological differences among the individuals of a single population; the use of the words "common parent" is equivocal and misleading. And if the intrapopulation varieties do not differ from each other ecologically, then the principle of divergence cannot operate.

THE VALIDITY OF THE PRINCIPLE OF DIVERGENCE

It is now evident that Darwin failed to prove that the principle of divergence plays a primary role in speciation. This failure must be attributed to

52. Barrett et al., Notebooks, B, 197 and 213; C, 161. 53. Francis Darwin, More Letters of Charles Darwin (New York: D.

Appleton, 1903), vol. 1, p. 27 7. 54. See Ernst Mayr, Evolution and the Diversity of Life, (Cambridge, Mass.:

Harvard University Press, 1976), pp. 120-128; M. Kottler, "Charles Darwin's Biological Species Concept and Theory of Geographic Speciation," Amer. Sci., 35 (1978), 275-297; and Frank J. SuHoway, "Geographic Isolation in Darwin's Thinking: The Vicissitudes of a Crucial Idea," Stud. in Hist. Biol., 3 (1979), 23- 65.

55. Darwin, Natural Selection, p. 234.



358 ERNST MAYR

(1) his typological-nominalist conceptualization of species and of the process of speciation;

(2) his mistake of not discriminating between intrapopulation variants and geographic subspecies, calling both of them varieties; and

(3) his failure to distinguish between isolation in an ecological niche and in a geographically isolated area.

It would be whiggish to criticize Darwin for his nominalist way of looking at species and varieties, for this was the universal attitude in his time. Indeed, he frequently did break away from it, particularly in his treatment of natural selection and the acquisition of adaptedness, where he introduced population thinking.56

Typological thinking also plagued one of Darwin's contem- poraries in a like manner: it contributed to a failure of Mendel's later researches. Not making a distinction between two very different kinds of "hybrids" - those of intraspecific variants, like the pea varieties Mendel was crossing, and hybrids between real species (such as in Hieracium, Aquilegia, Verbascum, Nicotiana, etc.) - Mendel got conflicting results, which greatly frustrated him. It may have given him the feeling that he had not really solved the problem of the nature of hybrids.

Too many problems were as yet unsolved in Darwin's day to allow a resolution of the problem of speciation. Although several naturalists (Henry Seebohm, Edward Poulton, Karl Jordan, Erwin Stresemann) eventually applied consistent population thinking to the species problem and greatly clarified the issues, it was not until the evolutionary synthesis that the problem of speciation was clarified to such an extent in the writings of Bernhard Rensch, Theodosius Dobzhansky, and myself that the main questions could be considered as solved. It is now no longer necessary to invoke a principle of divergence for this particular process of evolution.

It is ironic that of the two principles, which Darwin considered equally important, that of divergence was far less often criticized than that of natural selection. Any yet eventually it was natural selection that was victorious, while it is now evident that the principle of divergence is invalid.

56. See Ernst Mayr, "Darwin and the Evolutionary Theory in Biology," in Evolution and Anthropology: A Centennial Appraisal, ed. Betty J. Meggers, (Washington, D.C.: Anthropological Society of Washington, 1959), pp. 3-12.




Acknowledgments

I acknowledge with deep gratitude the valuable suggestions received from F. J. Sulloway, John Beatty, and an anonymous reviewer, and particularly from William A. Newman on Darwin's treatment of varieties and species of barnacles.



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