Transcript Lecture 7: Adaptive Explanation

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
Gould & Lewontin (1979)
The Spandrels of San Marco
• Flaws:
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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
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
Ho explaining traits
• But, shouldn’t throw away adaptive
approach entirely
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
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)
Intermediate Stages
“What good is half a wing?”
Intermediates:
1. Adaptive: half is better than none
2. Exaptations: Δ in function w/o Δ in
structure
Lobe-finned Fishes
• Original Function:
Stabilizers
Movement on Land
• Later Function:
Tetrapod Limbs
More exaptations
Archaeopteryx:
Feathers
– Insulation  flight
Bone
– Phosphate storage
 support
Adaptation
• character that confers a fitness advantage &
is maintained in a population by natural
selection
ALL adaptations can be explained by
Natural Selection
Are all characters adaptive?
Adaptationists might argue yes, but…
1) Drift
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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
Water Ouzel
• Feeds under water but looks like a thrush
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
Selection vs. Constraint
Test via:
• Adaptive prediction
• Direct measure of selection
• Heritability of characters
• Cross-species evidence
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