Résumé
Loci with higher levels of population differentiation than the neutral
expectation are traditionally interpreted as evidence of ongoing selection
that varies in space. This article emphasizes an alternative explanation
which has been largely overlooked to date: in species subdivided into
large subpopulations, enhanced differentiation can also be the signature
left by the fixation of an unconditionally favourable mutation on its
chromosomal neighbourhood. To discriminate among the two alternative
scenarios one needs to investigate how genetic structure varies along the
chromosomal region of the locus. Local hitchhiking is shown to generate a
single sharp peak of differentiation centred on the adaptive polymorphism
and the standard signature of a selective sweep only in those
subpopulations where the allele is favoured. Global hitchhiking produces
two domes of differentiation on either side of the fixed advantageous
mutation and signatures of a selective sweep in every subpopulation,
albeit of different magnitude. Investigating population differentiation
around a locus that strongly differentiates two otherwise genetically
similar populations of the marine mussel Mytilus edulis, plausible
evidence for the global hitchhiking hypothesis has been obtained. Global
hitchhiking is a neglected phenomenon that might prove to be important in
species with large population sizes such as many marine invertebrates.