Lecture 7: Adaptive Explanation

1. Lecture 7: Adaptive Explanation Are all phenotypic traits adaptations? • The Adaptationist program • Gould & Lewontin • Mayr

2. 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

3. Gould & Lewontin (1979)The Spandrels of San Marco • Flaws: • Assume all traits adaptive • NS only explanation for all traits • All organisms optimally designed • Split organisms into optimally engineered parts • Look for an adaptive explanation until one is found

4. 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

5. Consensus • Not all characters are adaptations • there are alternative, non-adaptational Ho explaining traits • But, shouldn’t throw away adaptive approach entirely

6. Natural Selection • NS is NOT the only cause of evolution mutation, gene flow, genetic drift • NS can slow down evolution : removes variation from populations • NS IS the only cause of adaptation b/c other processes don’t act via fitness diff’ns

7. Barriers to Adaptationist Program • Complex Characters • Intermediate Stages

8. 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)

9. Intermediate Stages “What good is half a wing?” Intermediates: • Adaptive: half is better than none • Exaptations: Δ in function w/o Δ in structure

10. Original Function: Stabilizers Movement on Land Later Function: Tetrapod Limbs Lobe-finned Fishes

11. Archaeopteryx: Feathers Insulation  flight Bone Phosphate storage  support More exaptations

12. Adaptation • character that confers a fitness advantage & is maintained in a population by natural selection ALLadaptations can be explained by Natural Selection

13. Are all characters adaptive? Adaptationists might argue yes, but… • Drift • Random Δs in gene frequency in a pop’n • Only when alleles have same fitness • Characters can be lost by chance events

14. 2) Time Lags / Anachronisms • Major change in env’t – need time to react • e.g. Neotropical fruit • Overprotected , large • Recent extinction of large herbivores

15. Water Ouzel • Feeds under water but looks like a thrush

16. 3) Why does Trait Exist? e.g. Tyrannosaurus rex arms: • Adaptive explanations don’t make sense • Non-adaptive – Byproduct of allometry • Therefore spandrels

17. 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

18. Selection vs. Constraint Test via: • Adaptive prediction • Direct measure of selection • Heritability of characters • Cross-species evidence

19. 5) Linkage with trait under selection • Pleiotropy – 1 gene with many effects • 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