Résumé
Identifying the presence and magnitude of population genetic structure
remains a major consideration in evolutionary biology as doing so allows
one to understand the demographic history of a species as well as make
predictions of how the evolutionary process will proceed. Next-generation
sequencing methods allow us to reconsider previous ideas and conclusions
concerning the distribution of genetic variation, and what this
distribution implies about a given species evolutionary history. A
previous phylogeographic study of the crustacean Daphnia magna suggested
that, despite strong genetic differentiation among populations at a local
scale, the species shows only moderate genetic structure across its
European range, with a spatially patchy occurrence of individual lineages.
We apply RAD sequencing to a sample of D. magna collected across a wide
swath of the species' Eurasian range and analyse the data using
principle component analysis (PCA) of genetic variation and Procrustes
analytical approaches, to quantify spatial genetic structure. We find
remarkable consistency between the first two PCA axes and the geographic
coordinates of individual sampling points, suggesting that, on a
continent-wide scale, genetic differentiation is driven to a large extent
by geographic distance. The observed pattern is consistent with unimpeded
(i.e. no barriers, landscape or otherwise) migration at large spatial
scales, despite the fragmented and patchy nature of favourable habitats at
local scales. With high-resolution genetic data similar patterns may be
uncovered for other species with wide geographic distributions, allowing
an increased understanding of how genetic drift and selection have shaped
their evolutionary history.