Chapter 12 Life on Earth: The Big Picture Introduction to Philosophy of Biology: Sex and Death
Dec 28, 2015
Chapter 12Life on Earth: The Big Picture
Introduction to Philosophy of Biology: Sex and Death
History of life & directionality
Weak
Particular taxa evolve at particular points in time
Strong
Particular kinds of taxa evolve at particular points in time
‚Murky‘ Directionality is…
• Progress (12.1 )
• Changes in Disparity (12.2/12.3)
• disputed and contingency of life‘s history emphasized (12.4)
• possibly cut down by mass extinctions (12.5)
Directionality is „progress“
Possible conceptualizations of progress:
1) Progression towards homo sapiens
2) Progressive adaptiveness (Dawkins)
3) Arms races (Dawkins)
4) Progressive increase in complexity (Gould)
Progress is adaptiveness (Dawkins)
Weak:
Organisms of today are better adapted to the environment than earlier ones (i.e. comparing two organisms of one population)
Strong:
Weak thesis + „…over million of years“ (requiring a ‚general‘ property of adaptiveness)
Progress is visible in arms races (Dawkins)
Thesis: Arms races between competing lineages define a direction of progress
Problems: - Prolonged arms races reconstruct the
environment- May involve a „rock/paper/scissors“
evolutionary shuffle
Progress is Increase in Complexity
- Intuitive
- Complexity measures
- Property status (relative/ objective)1) Dawkins: ascribed though objective:
complexity = length of description of an organism at a fixed level of description
2) Gould: complexity = spread of ‚variation‘
Gould‘s variations
• Life starts off simply and usually stays there• Complexity increases by passive diffusion from a
point of origin (undirected, stochastic process)• Real change is increase in total variance
(bias in the direction of complexity)– Facts, presumably:
• No mechanism of adaptation/ speciation/ extinction favours complexity
• Bacteria dominate
=> Complexity drifts upwards undirected
Smith and Szathmary vs. Gould
Thesis:
Series of major transitions and hence inherent directionality (RNA, DNA, eucaryotes, (plants, animals, fungi), human language)
Crux (according to Sterelny& Griffith)– Different pictures of variation
Gould vs. S.&S.: Structures of variance
Gould: - lower limit to complexity
- no upper limit
- Gradual spread to higher complexity
S.&S.:-„evolution of evolvability“, i.e. dynamic re-limitation
- Major transitions= movements of points of max. complexity (=> min. complexity)
Gould‘s challenge (12.2)
Claims
- expectation that complexity/diversity of life increases gradually over time due to natural selection is mistaken
- Therefore the received view is also mistaken
Gould‘s case: The Burgess Shale fauna
- Cambrian explosion
- 7-8 phyla found that are not existent today
Therefore:
- Orthodox conception of the shape of tree of life is wrong
- Diversity increased, but disparity DEcreased
Trees
o f
lif e
The received view
Gould‘s view
Gould‘s conclusions & interests
- „overestimation of the role of selection in evolution“- Selection plays no role in generating/
reducing disparity- History of life is contingent
- Small change (t0) => big change (t1)
- Outcomes sensitively dependent on initial conditions
=> „Survival of the luckiest“
The Concept of Disparity
Question:
What is it and how (if at all) has it changed?
Model:
morphosphere= space that represents the physical forms of all actual and possible organisms– Similar forms close together b/c similar sets of
physical propertied describe them– Disparity = Size of morphospace for life existing
Disparity and Morphospace
Challenges
• Distances in morphospace (if any) are not measureable)
- trait choice, weighting
- The Cladist‘s anti-subjectivist argument
- Properties important for genealogy ≠ Properties important for disparity- Property lability and retrospective fallacy
Contingency (12.4)
Contingency hypothesis= Important features of life are not
counterfactually resilient
– Importance of particular events in shaping history of life and unpredictability of consequences
– Some features of life not predictable by physics
• No robust process explanations possible
3 Types of Contingency
1) Contingency of specific taxi- Implication of the received view
2) Contingency of Adaptive Complexes- undercuts idea that traits are robustly explained by a selective
environment- Inconsistent w/ any kind of empirical adaptionism
3) Contingent Explorations of Morphospace
It seems that: 1 uncontroversial, 2/3 are relatives of Gould‘s anti-adaptionist criticism and besides, hard to test
Mass extinction (12.5)
Claim (Whose)- Major transitions of life are defined by mass
extinction, not routine or background extinction- Disparity of life depends on extirpation of
dominant groups
Challenge: - Difference b/w mass and background extinction- Accept importance of mass distinctions but re-
evaluate their importance (Sepkowski)
The Importance of Mass ExtinctionGould: YES, b/c mass extinctions
- Change the ‚rules‘ of evolution- Have a profound effect on biota Make explanations extrapolating from changes
in local populations into ‚ecological time frames‘ impossible
Sterelny: NO, b/c mass extinctions- Just change the outcome: normal operations in an abnormal
world- Is consistent w/ mass extinction fundamentally reshaping the
tree of life
Mass extinction is no threat to the received view
Review: The concepts of
• Directionality
• Progress
• Complexity
• Disparity
• Contingency
• Mass extinctions