A Genetical System admitting of Two Stable Equilibria

Abstract

IT has long been known that, in a population where different genotypes possess different selective advantages, it is possible for balanced polymorphism to exist. That is to say, there is an equilibrium between the proportions of different genotypes which is stable under random mating with differential viability and fertility. I do not know, however, whether any case has been recognized in Nature, or discussed theoretically, in which it is possible for two distinct non-trivial stable equilibria to exist. (A trivial equilibrium is an extreme case when, for example, some genes or genotypes are unrepresented in the population, and is of little interest.) It may therefore be of some theoretical value to construct a genetical system admitting of two distinct stable states of balanced polymorphism.