Adaptations are not perfect

As John Maynard Smith (pictured opposite) remarked,

“if there were no constraints on what is possible, the best phenotype would live forever, would be impregnable to predators, would lay eggs at an infinite rate, and so on”.

However, adaptations in nature are not perfect, for various reasons:

• Natural selection may be a slow process, and time lags sometimes cause a species to be less than perfectly adapted.

• The dynamics of the developmental mechanisms serve to constrain which phenotypes are biologically realizable.

• Genetic constraints on adaptation occur when the heterozygote at a locus has a higher fitness than either homozygote.

• Some natural populations may now be imperfectly adapted because accidents of history pointed their ancestors in what would later become the wrong direction.