Microgeographic Morphological Differentiation in Muskrats

Cranial morphometry was studied in local populations of muskrats inhabiting ponds along various tributaries in a single river drainage (150 km(2)) in southern Belgium. Despite the absence of noticeable environmental heterogeneity across this region, significant morphological differences were found among local populations. This differentiation was not related to gender, and it remained after controlling for the age and size components of variation. Spatial analysis confirmed the existence of a microgeographic pattern of differentiation, which is related to variation in shape of skull. Isolation by distance along corridors is proposed as a mechanism to account for the observed pattern of differentiation. This mechanism involves population isolation, genetic drift, and a small amount of socially-induced gene flow among local populations that mainly are connected to one another through the river network. According to this model, the distance between local populations is suitably represented by the number of ''decisions'' a muskrat must make when travelling along the course of the river from one tributary to another. Data are consistent with 10 quantitative predictions derived from the proposed model involving morphological, straight-line, and ''decision'' distances among populations