Chapter 5: Adaptationism

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Chapter 5 Adaptationism

• The Power of Natural Selection

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When did the debate begin?
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When did the debate begin?
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How did Gould and Lewontin characterize
adaptationism?

• (1) An organism is atomized into "traits" and
  these traits are explained as structures
  optimally designed by natural selection for their
  functions.
• (2) After the failure of part-by-part
  optimization, interaction is acknowledged via the
  dictum that an organism cannot optimize each part
  without imposing expenses on others.

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How did Gould and Lewontin characterize
adaptationism?

• (1) If one adaptive argument fails, try another.
• (2) If one adaptive argument fails, assume that
  another must exist a weaker version of the first
  argument.
• (3) In the absence of a good adaptive argument in
  the first place, attribute failure to imperfect
  understanding of where an organism lives and what
  it does.
• (4) Emphasize immediate utility and exclude other
  attributes of form.

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What is adaptationism?

• (U) Natural selection played some role in the
  evolution of T in the lineage leading to X.
• (I) Natural selection was an important cause of
  the evolution of T in the lineage leading to X.
• (O) Natural selection was the only important
  cause of the evolution of T in the lineage
  leading to X.
• Adaptationism Most phenotypic traits in most
  populations can be explained by a model in which
  selection is described and nonselective processes
  are ignored.

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What is adaptationism?

• Some qualifications
• Adaptationists well realize that which phenotype
  is fittest depends on the biological details.
• Adaptationists might expect zebras to evolve from
  Slow to Fast but will not expect them to evolve
  machine guns with which to counter lion attacks (
  Krebs and Davies 1981). When adaptationists say
  that the fittest trait will evolve, they mean the
  fittest of the traits actually present in the
  population, not the fittest of all the traits we
  can imagine.
• No adaptationist holds that variation is
  limitlessly rich. This is why zebras have not
  evolved machine guns and why pigs don't fly.

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How genetics can get in the way

• Heterozygote has medium fitness. A allele goes to
  fixation.

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How genetics can get in the way

• Heterozygotes are superior. Stable polymorphism
  at p.
• Sickle cell anemia

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How genetics can get in the way

• Heterozygote inferiority
• Either A or a goes to fixation. Fitness is
  frequency dependent.

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Is adaptationism testable?

• Are factors that prevent the fittest trait from
  evolving (such as negative pleiotropy or
  heterozygote superiority) common or rare?
• According to critics and defenders alike,
  adaptationism seems to be an assumption rather
  than a hypothesis under test.
• Whether adaptationism is testable is a quite
  separate question from how adaptationists
  behave.
• There is no single observation that could refute
  adaptationism. Adaptationism is tested in the
  long run because it is a research program.
• The truth or falsity of adaptationism is
  determined after a lengthy chain of observations
  and experiments. How often do adaptationist
  models succeed in explaining specific phenomena?
  This is the crucial question.

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Is adaptationism an a priori truth?

• Universal Darwinism (Richard Dawkins)
• Lamarckism vs Natural Selection vs Pure Chance
  (mutation, drift, etc)
• True test natural selection (NS) vs
  NSM(utation) vs NSGD vs NSMGD