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We report rapid change of morphology and mitochondrial genes in white-footed mice (Peromyscus leucopus) in the Chicago (Illinois, USA) region. We sequenced mitochondrial DNA COX2 from 55 museum skins of white-footed mice caught in the Chicago area since 1855 and from 44 mice recently trapped in the same locations. We found consistent directional genotype replacement at five separate collection locations. We later focused on a single one of these locations (Volo Bog State Natural Area) and sequenced mitochondrial D-loop control region from 58 museum skins of mice collected in 1903-1976 and 32 mice recently trapped there. We found complete and more recent replacement of D-loop haplotypes, apparently occurring between 1976 and 2001. We tested whether these genetic changes were mirrored by changes in morphology by comparing 15 external and cranial traits. We found no significant morphological differences between mice collected in 1903-1976; however, mice collected in 2001-2003 showed 9 of 15 measurements to be significantly changed relative to the earlier samples. Recent mice were longer in total length, with broader, longer noses, and longer but shallower skulls(1). Discriminant function analysis allowed for 100% correct classification using these traits. Principal components analysis shows variance over time is well distributed across both external and cranial measures. The sequential replacements of haplotypes and the rapid change of morphology can best be explained by replacement of the regional population with immigrants from genetically distinct neighbouring populations, likely facilitated by the large environmental changes occurring over the time period. Replacement with genotypes from external populations may be a common mechanism of evolution of newly adaptive local forms in an increasingly human-impacted world.

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In general, rapid morphological change in mammals has been infrequently documented. Examples that do exist are almost exclusively of rodents on islands. Such changes are usually attributed to selective release or founder events related to restricted gene flow in island settings. Here we document rapid morphological changes in rodents in 20 of 28 museum series collected on four continents, including 15 of 23 mainland sites. Approximately 17,000 measurements were taken of 1302 rodents. Trends included both increases and decreases in the 15 morphological traits measured, but slightly more trends were towards larger size. Generalized linear models indicated that changes in several of the individual morphological traits were associated with changes in human population density, current temperature gradients, and/or trends in temperature and precipitation. When we restricted these analyses to samples taken in the US (where data on human population trends were presumed to be more accurate), we found changes in two additional traits to be positively correlated with changes in human population density. Principle component analysis revealed general trends in cranial and external size, but these general trends were uncorrelated with climate or human population density. Our results indicate that over the last 100+ years, rapid morphological change in rodents has occurred quite frequently, and that these changes have taken place on the mainland as well as on islands. Our results also suggest that these changes may be driven, at least in part, by human population growth and climate change.

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