ZOOL 304

Analysis of Adaptation

The following notes (and quoted section headings) are adapted from Chapter 11 of Mark Ridley's textbook, EVOLUTION, 2nd ed. (1996), Blackwell Science, Inc., Cambridge MA. ISBN 0-86542-495-0.

"The way organisms are adapted may not be obvious."

• This cuts two ways.  
  • Sometimes apparently insignificant traits might actually be adaptations
  • Sometimes traits that look like adaptations really are not.
• Far too often in biology, "adaptation" is simply presumed.

  In the words of George C. Williams (in Adaptation and Natural Selection, 1966, p. 4), "The ground rule ... is that adaptation is a special and onerous concept that should be used only where it really necessary."

• To establish a trait as an adaptation, it is necessary to demonstrate several things:
  • Demonstrate that the trait is (or has been) variable.
  • Demonstrate that the putatively adaptive variant could be more fit than the previously-existing (ancestral) variant.
  • Demonstrate that the potential fitness contribution of the putative adaptation outweighs its cost.
  • Demonstrate that selection is actually operating (or has operated in the past) to favor the putative adaptation over other variants.
  • There are additional criteria, including identification of the appropriate unit of selection and understanding how selection for the adaptation could be undone by mutation or drift, but that's enough for now.
     

  "Three main methods are used to study adaptation."

• One may consider three "conceptual stages" in the study of adaptation.  This is the basic scientific method, applied to adaptation.
  • Postulate alternatives (genetic variants).
  • Develop a hypothesis for adaptive function of one particular variant.
  • Test the hypothesis (see criteria above).
     
• Without such careful formulation and testing, hypotheses of adaptation are little more than wishful thinking and story-telling.
• One may consider also at least three distinct methods for testing hypotheses of adaptation.
  • Description (confirmation of prediction based on design criteria).
  • Experimentation (manipulation, with appropriate controls).
  • Comparative analysis (correlation of the putative adaptation with phylogeny and ecological circumstance).
     
• Any test of adaptation should address several distinct concerns (see section above).  A thorough analysis may require more than one method to address these concerns.
• Note that in general, it is the evolutionary acquisition of a novel trait that warrants special explanation.  For traits inherited by many species from a common ancestor, only the acquisition by the ancestor may require special explanation.  For the several descendents which share the trait, inheritance alone may be sufficient explanation.  However, selection may also be invoked to explain maintenance of a trait against loss in descendent lineages.

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