Adaptive Explanation - Evolution - Lecture Slides, Slides of Theory of Evolution

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Lecture 7: Adaptive Explanation

Are all phenotypic traits adaptations?

• The Adaptationist program
• Gould & Lewontin
• Mayr

The Adaptationist Program

1. identify trait likely to be under selection
2. construct functional argument for how trait has adaptive value (relate to fitness)
3. determine if mean value of trait in pop’n is optimal in terms of design & function
4. if not: construct new funct’l argument invoke other factors to explain discrepancy b/w optimal & observed

Mayr to the rescue!

• Argues:
  • Flawed understanding of researchers
  • Do not assume organisms are perfect
  • Assuming traits are adaptive useful as starting point for research
  • Don’t ignore whole for parts
  • “Dilemma of the adaptationist” – must look at all adaptive explanations before rejecting adaptation entirely

Consensus

• Not all characters are adaptations

• there are alternative, non-adaptational

H o explaining traits

• But, shouldn’t throw away adaptive

approach entirely

Barriers to Adaptationist Program

• Complex Characters
• Intermediate Stages

Complex Characters

Co-adaptation – simultaneous Δs in many parts

But, gradual Δs do not need to be simultaneous

e.g. Eyes (Nilsson & Pelger)

• independent evol’n 40-60 x
• vertebrate eye in 2000 steps (400,000 gen’ns)

Lobe-finned Fishes

• Original Function:

Stabilizers

Movement on Land

• Later Function:

Tetrapod Limbs

More exaptations

Archaeopteryx:

Feathers

• Insulation → flight Bone
• Phosphate storage → support

Are all characters adaptive?

Adaptationists might argue yes, but…

1) Drift

• Random Δs in gene frequency in a pop’n
• Only when alleles have same fitness
• Characters can be lost by chance events

2) Time Lags / Anachronisms

• Major change in env’t – need time to react
• e.g. Neotropical fruit
  • Overprotected , large
  • Recent extinction of large herbivores

3) Why does Trait Exist?

e.g. Tyrannosaurus rex arms:

• Adaptive explanations don’t make sense
• Non-adaptive – Byproduct of allometry
• Therefore spandrels

4) Developmental Constraints

• bias on type of variants b/c of developmental system
• Gaps in morphospace:

selection or constraint?

• e.g. Raup - shells
• Best of all possible forms vs. only form possible

5) Linkage with trait under selection

a) Pleiotropy – 1 gene with many effects

b) Genetic linkage between traits

-low recombination -trait ↑ in freq as passive consequence

Organisms cannot be decomposed into separate traits – Dev’t is integrated & genes are often pleiotropic