EPIGENETIC POLYMORPHISM OF THE RODENT SKELETON

Abstract

One type of discontinuous variation, genetic polymorphism, has been studied frequently, in animal populations. This paper deals with another type, here called epigenetic polymorphism, which is developmental in origin and due to the action of a threshold on underlying continuous variation. Both genetic and environmental factors are important in causation. This type of polymorphism is known to be common in wild mouse populations with respect to many skeletal characters. The present survey is concerned with the situation in the rodents as a whole, samples of the following species being classified for forty‐nine variants: Sciurus carolinensis, Cavia porcellus, Peromyscus maniculatus, Lemmus lemmus, Microtus agrestis, Micromys minutus, Mus musculus, Rattus norvegicus, Rattus rattus and Rattus exulans. All show extensive polymorphism, some variants being ubiquitous while others are confined to one or a few species. A rare variant in one species is sometimes present in all representatives of another species. There is some association between the frequency of a few variants and overall size, but none was found with respect to domestication. The genetic and evolutionary aspects of this kind of polymorphism are discussed, as well as its possible use in systematic studies. Various types of epigenetic polymorphism (transient etc.) seem to occur, some probably being important in micro‐evolution. Changes in the frequencies of variants may often be due entirely to chance, but selective forces may also operate and are probably best envisaged on the lines of Waddington's (1953 a, 1961) concept of genetic assimilation. However, their relative importance cannot be assessed in the present investigation.