Defining Adaptation
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© 1997
David H.A. Fitch
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Adaptation:  What is and what is not an adaptation?

I.  "Adaptation" is used differently in evolutionary biology than it is in physiology
i.e., it is not an adjustment to environment, but is a heritable feature shaped by natural selection acting on variation because of the variation's effect on fitness

II.  Some aspects of a feature may suggest it is an adaptation

A.  Complexity of structure may suggest function (and thus adaptation)
B.  Correspondence between structure and engineering design may suggest function (and thus adaptation)
C.  The "comparative method" (or more precisely, the phylogenetic method) provides a powerful means of correlating species differences with ecological factors:  Similarities of species in an environment could be due to shared ancestry or convergence; inference of the latter indicates (but does not prove) adaptation
D.  Direct evidence for adaptation can only come from experimental studies

III.  Problems recognizing adaptation (an "onerous" concept)

A.  An adaptation may not appear to result in better performance with respect to the environment
1.  Adaptation increases relative fitness, not necessarily absolute fitness (relative to what?)
2.  Better competition with other genotypes may not result in increased population size (why?)

B.  A variation of a structure could simply be neutral
C.  Adaptations may not have evolved for purposes for which they now appear to be useful
D.  A trait might not be determined genetically, but be a direct consequence of environment or learning
E.  A trait might be a simple consequence of chemical or physical "laws"
F.  A trait may have evolved by genetic drift
G.  A trait may have evolved as a pleiotropic consequence of allelic differences that evolved for other selective reasons or by genetic drift
H.  Different species may have (neutral) variation for the same adaptive feature merely because of different ancestry; e.g., although a pattern that provides good camouflage is likely to be an adaptive trait, alternative patterns that also provide good camouflage may work just as well (i.e., although the camouflage is adaptive, particularities of the pattern might have resulted from arbitrary historical "choices")
I.  Many variations that appear may be "constrained" by the developmental system or the genome (e.g., that tetrapods have 4 limbs may not be an adaptation--why couldn't 6 work just as well?--but this number depends on what is allowed by the developmental mechanisms that pattern the organism and on the ancestry of the genome)
J.  Any trait is likely to be anachronistic, since the conditions under which a feature evolved existed in the past

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Exercises

  1. Pick some features that are expressed by humans that you know (bald spots, the shape of lips, ear lobes, laughing, etc.).  For each feature that you pick, what are the indications that it is an adaptation?  What are the indications that it is not?
     
  2. George Williams (1992, Natural Selection; placed on reserve at Bobst Library) suggests that adaptation is "demonstrated by observed conformity to a priori design specifications".  Can you think of some clearly adaptive features that do not easily conform to this definition?  How would you modify this definition to include such features?

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 Examples of
Adaptations  Defining
Adaptation  Levels of
Selection  Optimal
Models  Tradeoffs  Sexual
Selection
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[Examples of Adaptations] [Defining Adaptation] [Levels of Selection] [Optimal Models] [Tradeoffs] [Sexual Selection]

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