Blackwell Publishing

Adaptive explanation - Is perfect adaptation possible?


Genetic constraints

Adaptations will often be imperfect because of genetic constraints. An example of such a constraint occurs when the heterozygote at a locus has a higher fitness than either homozygote, and the population evolves to an equilibrium at which all three geneotypes are present. A proportion of the population must therefore have the deleterious homozygous genotypes.

This situation arises because the heterozygotes cannot, under Mendelian inheritance, produce purely heterozygous offspring: they cannot breed true. Insofar as heterozygous advantage exists, some members of the natural populations will be imperfectly adapted. The importance of heterozygous advantage is controversial; but there are undoubted examples, such as sickle cell anemia (blood cells suffering from this condition are shown opposite), which is a practical example of imperfect adaptation due to a genetic constraint.

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