THE NUMBER OF ALLELES THAT CAN BE MAINTAINED IN A FINITE POPULATION

T has sometimes been suggested that the wild-type allele is not a single entity, I but rather a population of different isoalleles that are indistinguishable by any ordinary procedure. With hundreds of nucleotides, each presumably capable of base substitutions and with additional permutations possible through sequence rearrangements, gains, and losses, the number of possible gene states becomes astronomical. It is known that a single nucleotide substitution can have the most drastic consequences, but there are also mutations with very minute effects and there is the possibility that many are so small as to be undetectable. It is not the purpose of this article to discuss the plausibility of such a system of isoalleles, or the evidence for and against. Instead, we propose to examine some of the population consequences of such a system if it does exist. The probability seems great enough to warrant such an inquiry. If a large number of different states can arise by mutation, this doesn't necessarily mean that a large fraction of these would coexist in a single population. Some will be lost by random drift and others may be selectively disadvantageous. On the other hand, some may persist by being beneficial in heterozygous combinations. We shall consider three possibilities: ( 1 ) A system of selectively neutral isoalleles whose frequency in the population is determined by the mutation rate and by random drift. (2) A system of mutually heterotic alleles. ( 3 ) A mixture of heterotic and harmful mutants.