Evolution__3rd_Edition

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10Adaptive Explanation
T his chapter considers a series of points about adaptation.
Many of them are matters of controversy in the
professional literature and this chapter aims to explain the
main positions in those controversies, and to provide the
background to understanding them. We look first at the
argument to show that natural selection is the only known
explanation of adaptation; and second at whether natural
selection can explain all adaptations, including such complex
organs as the eye. Then we move on to the genetics of
adaptation, particularly Fisher’s model, which suggests that
adaptations evolve in many small genetic steps. We briefly
look at the main methods of studying adaptation: the
relation between the predicted and observed forms of a
character, experiments, and interspecies comparisons.
Most of the rest of the chapter is about various reasons why
adaptations may not be perfect. Adaptations may be out
of date or may be constrained by genetics, developmental
mechanisms, historic origins, or trade-offs between multiple
functions. We finish by looking at the definitions of
adaptation.