Lecture%207:%20Adaptive%20Explanation

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Lecture 7 Adaptive Explanation

• Are all phenotypic traits adaptations?
• The Adaptationist program
• Gould Lewontin
• Mayr

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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 popn is
  optimal in terms of design function
• 4) if not
• construct new functl argument
• invoke other factors to explain discrepancy
  b/w optimal observed

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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

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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
• Dont ignore whole for parts
• Dilemma of the adaptationist must look at all
  adaptive explanations before rejecting adaptation
  entirely

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Consensus

• Not all characters are adaptations
• there are alternative, non-adaptational Ho
  explaining traits
• But, shouldnt throw away adaptive approach
  entirely

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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 dont act via fitness diffns

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Barriers to Adaptationist Program

• Complex Characters
• Intermediate Stages

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Complex Characters

• Co-adaptation simultaneous ?s in many parts
• But, gradual ?s do not need to be simultaneous
• e.g. Eyes (Nilsson Pelger)
• - independent evoln 40-60 x
• - vertebrate eye in 2000 steps (400,000 genns)

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Intermediate Stages

• What good is half a wing?
• Intermediates
• Adaptive half is better than none
• Exaptations ? in function w/o ? in structure

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Lobe-finned Fishes

• Original Function
• Stabilizers
• Movement on Land
• Later Function
• Tetrapod Limbs

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More exaptations

• Archaeopteryx
• Feathers
• Insulation ? flight
• Bone
• Phosphate storage ? support

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Adaptation

• character that confers a fitness advantage is
  maintained in a population by natural selection
• ALL adaptations can be explained by Natural
  Selection

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Are all characters adaptive?

• Adaptationists might argue yes, but
• Drift
• Random ?s in gene frequency in a popn
• Only when alleles have same fitness
• Characters can be lost by chance events

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2) Time Lags / Anachronisms

• Major change in envt need time to react
• e.g. Neotropical fruit
• Overprotected , large
• Recent extinction of large herbivores

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Water Ouzel

• Feeds under water but looks like a thrush

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3) Why does Trait Exist?

• e.g. Tyrannosaurus rex arms
• Adaptive explanations dont make sense
• Non-adaptive Byproduct of allometry
• Therefore spandrels

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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

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Selection vs. Constraint

• Test via
• Adaptive prediction
• Direct measure of selection
• Heritability of characters
• Cross-species evidence

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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
• Devt is integrated genes are often pleiotropic