Transcript Chapter 12 Life on Earth: The Big Picture - hu

Chapter 12
Life 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)
2)
3)
4)
Progression towards homo sapiens
Progressive adaptiveness (Dawkins)
Arms races (Dawkins)
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
T
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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 antiadaptionist 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 reevaluate their importance (Sepkowski)
The Importance of Mass Extinction
Gould: 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