Habitat Mosaic Limits Gene Flow and Promotes Morphological Adaptation in a Generalist Mammal

Many habitat generalist species exploit habitat patches of differing types and quality, yet the influence of such habitat mosaics on genetic structure remains poorly understood. Here, we tested whether fine-scale habitat heterogeneity affects the population structure of the European wood mouse (<i>Apodemus sylvaticus</i>) by sampling three matched forest parks in Northern Ireland across hedgerow, forest edge, and inner forest habitats. Microsatellite analysis revealed strong genetic differentiation among sites and consistent divergence between habitat types within sites. Stable isotope data showed that hedgerow mice fed at a higher trophic level than inner forest individuals, with forest edge mice intermediate. Mandible shape also differed by habitat and was correlated with δ¹⁵N, though differences were subtle and may reflect both drift and plasticity. Together, these results indicate that habitat mosaics can promote repeated, fine-scale population divergence even in the absence of physical barriers. This highlights the role of ecological heterogeneity in structuring genetic variation in widespread generalists and cautions against assuming panmixia in continuous landscapes.