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The House mouse (Mus musculus L.) population on the small Welsh island of Skokholm was founded in the 1890s when a few animals were inadvertently introduced. It has thrived ever since. The animals breed from late March to early October, during which time the population increases 8–10 fold. Winter survival is related to temperature: the spring population can be as low as 100 individuals after a cold winter; if the winter is warm, so many animals survive that high numbers may occur in the following autumn. There are no resident predators of the mice, and no other small mammals on the island.

The population was sampled on 20 occasions between 1957 and 1972, and genetical changes measured by the incidences of non-metrical skeletal variants (which describe a substantial proportion of the genome) and (from 1968–72) by allele and genotype frequencies of electrophoretically detected genes. Two sorts of changes occurred: an irregular but increasing deviation from the genetical composition of the population at the beginning of the study period, and an annual cycle with frequencies changing in one direction during the breeding season and the opposite direction in the winter. Samples collected in the autumn were more similar to each other than spring caught samples, which conforms with independent evidence about the differing environmental contributors to death in different winters, as compared with general homogeneity of mortality factors during the summers. It is concluded that the genetical composition of the population is largely dependent on natural selection, although the agents and intensity of selection change with both seasons and years.