Process Responsible for Temporal Changes in the Percentages of Pelage Color Types Among the Young Recruited Into a House Mouse-Population in an Outdoor Pen

Abstract

In populations of the house mouse, Mus musculus, temporal changes in allelic frequencies have been attributed to both selective and non-selective processes. During a 12-year study on a mouse population carrying recessive alleles at loci affecting the colour of the pelage, the percentages of homozygous recessive mice among the young recruited were found to change with time. These changes occurred in both the total population living in the experimental pen and in the subpopulations living in the five shelters housing the mice. In the total population, bottlenecks preceded changes in the percentages of recessive homozygotes among the mice recruited each year. In the subpopulations, changes in the percentages of recessive homozygotes were preceded by bottlenecks and by movements of mice between shelters. When the data for the males and females of each pelage colour were pooled, the males of all colour types appeared to be equally fit, and females of only one colour type appeared to be slightly less fit than the other three. We concluded that the temporal changes in the percentages of mice homozygous for the recessive alleles were due largely, if not wholly, to drift and migration rather than to natural selection, and that the relative importance of these non-selective processes was determined by the degree to which the populations were isolated from one another.