Maintenance of Polymorphism in an Island Population of the California Vole, Microtus californicus
Evolution, 31 (3):
512-525 (1977)Abstract
A detailed investigation of a polymorphic island population of the California vole, Microtus californicus, led to the conclusion that heterozygote superiority in reproduction countered by seasonal reduction of one of the homozygotes may be important in maintaining the polymorphism at its present level. This is a single locus polymorphism in which the homozygous recessive has a lighter coat color (buffy) than the wildtype (agouti) and the heterozygote is morphologically indistinguishable from the wildtype homozygote. Animals of the three genotypes were bred in the laboratory to compare their reproductive performance. Genotypically dependent differences were found in the fertility of the crosses and in the number of offspring born and weaned. Both physiological and behavioral factors, especially with regard to the male's behavior, seemed to be important in establishing reproductive differences. Another factor that could affect the numbers of the different genotypes is a sex ratio favoring heterozygous males over females. The buffy allele is maintained only on Brooks Island in San Francisco Bay, while it is nearly absent on the mainland. The proportion of buffy voles on the island varies seasonally, being highest after the breeding season. Selective predation may be an important factor in the seasonal decrease in buffies on the island. Predation is much heavier on the mainland and may be responsible for the absence of the buffy phenotype there, despite the presence of the buffy allele in low frequency.