Population Size, Polymorphism, and the Rate of Non-Darwinian Evolution

The possibility that a high rate of selectively neutral mutation can account for both genetic polymorphism and gene substitution in evolution is investigated theoretically. A species divided into partially isolated local populations is considered, and relations found between the local population size, the number of populations, the migration and mutation rates, and the degree of polymorphism within and between populations. It is shown that if selectively neutral mutation rates are high enough (perhaps 10-6 per generation per cistron) to account for observed rates of evolution, then either the total population size of a species, now or in the geologically recent past, must be small (⩽106), or hybrids between local populations will be heterozygous at almost all loci.