Local populations of the spiny pocket mice, Liomys pictus, were sampled from the tropical deciduous and semideciduous forests from Chamela, Jalisco, Mexico, where L. pictus experiences profound population fluctuations and high rates of population turnover, associated with the strong environmental seasonality and heterogeneity characteristic of these forests. In order to review the hypothesis stating that the level of genetic variation in a population is correlated with the degree of environmental heterogeneity, the genetic differences between subpopulations from these two contrasting habitats were evaluated. Thirty presumptive gene loci were analyzed using starch-gel electrophoresis. Allele frequencies, number of heterozygous loci and observed and expected heterozygosity values were not statistically different between subpopulations. Population differences and sex-biased dispersal were also evaluated with an assignment test based on individual genotypes. Seventy-two percent of individuals were correctly assigned to their population of origin, which we considered indicative of low interpopulation differentiation. The short-term changes in genetic variability, as a response to the fluctuations in population density and high rates of population turnover, together with movement of individuals (a marked male-biased dispersal was observed), could conceal or buffer divergences between these subpopulations. L. pictus subpopulations were also characterized by inbreeding, which might be interpreted as a Wahlund effect, resulting from mixing of subpopulations of differing allelic frequencies. On the other hand, we suggested that female philopatry and male-biased dispersal, main components of the social structure of this species, may result in either dispersion of individuals (population structuring) or mixing subpopulations (Wahlund effect). Short-term population and genetic changes are important aspects of many small mammalian populations, which should be considered when assessing demography and genetic structure.
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