ABSTRACT: The population genetic structure of marine species lacking free-swimming larvae is expected to be strongly affected
by random genetic drift among populations, resulting in genetic isolation by geographical distance. At the same
time, ecological separation over microhabitats followed by direct selection on those parts of the genome that affect
adaptation might also be strong. Here, we address the question of how the relative importance of stochastic vs.
selective structuring forces varies at different geographical scales. We use microsatellite DNA and allozyme data
from samples of the marine rocky shore snail Littorina saxatilis over distance scales ranging from metres to
1000 km, and we show that genetic drift is the most important structuring evolutionary force at distances > 1 km.
On smaller geographical scales (< 1 km), divergent selection between contrasting habitats affects population
genetic structure by impeding gene flow over microhabitat borders (microsatellite structure), or by directly
favouring specific alleles of selected loci (allozyme structure). The results suggest that evolutionary drivers of
population genetic structure cannot a priori be assumed to be equally important at different geographical scales.
Biological Journal of the Linnean Society 01/2008; 94:31-40. · 2.19 Impact Factor