HOW STABLE IS BALANCED POLYMORPHISM?

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Abstract
As a method of adaptation, balanced polymorphism is a costly one, since it entails production of some relatively unfit homozygotes. Balanced polymorphism is nevertheless widespread in natural populations of Drosophila and probably in other sexually reproducing organisms as well. There is every reason to think that the chromosomal balanced polymorphisms in Drosophila are stable. This is shown by the wide geographical distribution and the apparent antiquity of many natural polymorphisms, and also by their retention in experimental populations in which opportunities are afforded for adaptive reconstruction of the genetic system. On the other hand, establishment of balanced polymorphism in a population is not an evolutionary blind alley, as shown by the emergence of secondary monomorphisms in some experimental populations. From a long-range view, the evolutionary advantages of genetic systems based on balanced polymorphisms may lie, as suggested by Lerner and others, in their great adaptational plasticity and in the possession of the property of genetic homeostasis. It may also be that the fitness conferred upon their carriers by heterozygosis for balance supergenes is not easily equalled in homozygotes. How important are balanced polymorphisms in the genetic systems of man and of other higher organisms with limited fecundity is an open question. Chromosomal inversions of the kind known in Drosophila are unlikely to be common in mammals, but other genetic mechanisms which form and maintain balnaced supergenes may exist. The losses of the homozygotes of lower fitness are more difficult to put up with, but they can be reduced by development of numerous heterotic supergenes acting like multiple alleles of ginle loci.