POLYMORPHISM AS AN OUTCOME OF DISRUPTIVE SELECTION

The action of selection upon a character in a population of living organisms may be analyzed in terms of three elementary components: stabilizing selection, directional selection and disruptive selection (Mather, 1953). Stabilizing selection, by acting against the more extreme variants in the distribution of the character's expression, tends to narrow the variation, to make the population more uniform. The consequent responses in development may be various, and this component includes both the stabilizing and the normalizing selection which Waddington (1953) has distinguished by developmental criteria. The second type of action, directional selection, comes into play insofar as the mean expression of the character departs from the optimal: it tends to move the mean towards this optimum. The third and final component, disruptive selection, arises when several optima are favored. The genetical consequences of stabilizing and directional selection have been discussed earlier (Mather, 1953), and some effects of disruptive selection have also been outlined (Darlington and Mather, 1949). Disruptive selection will, however, vary in its consequences according to the relations between the different optimal phenotypes and to the persistence and power of the selective forces favoring them. Not only the isolation, which Darlington and Mather discussed, but it would seem also polymorphism, can follow from disruptive selection. Selection acts primarily on the phenotype, so that disruptive selection, if it is effective, must result in that adjusted discontinuity at the phenotypic level which is a feature of both isolation and polymorphism. In the case of isolation, however, this is accompanied by disruption of the erstwhile common gene pool, whereas in polymorphism the morphic types continue to share a common gene pool (apart from any switch genes that may be involved), on whose adjustment the polymorphism indeed depends for its efficient working. Thus polymorplhism would seem to be a possible alternative to isolation as an outcome of disruptive selection, acting under conditions and in ways which we shall now examine; and being an alternative it is not necessarily a step on the way to isolation, even though both spring from the same type of selection. Where variation in the environment results in different optima being favored in different groups of the population, the resulting tendency will be towards separate adjustment of the phenotype in the different groups and, ultimately, towards genetic isolation of these groups one from another (Mather, 1943; Darlington and Mather, 1949). Certain conditions must be fulfilled, however, if this tendency is to be effective. In the first place, the action of selection on the two groups must be separate in the sense that the optimal phenotype in one group must be uninfluenced by the optimal phenotype favored in the other: or, to put it another way, the optimal phenotypes must be independent of each other. This requires in effect that neither group is an effective part of the environment of the other. Secondly, the groups must be sufficiently distinct from one another for the different forces of selection to be effective. This condition does not require that genetic exchange between the populations