chapter 12 life on earth: the big picture introduction to philosophy of biology: sex and death

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