Discrepancies in population differentiation at microsatellites, mitochondrial DNA and plumage colour in the pied flycatcher - inferring evolutionary processes

Genetic differentiation between three populations of the pied flycatcher Fi cedula hypoleuca (Norway, Czech Republic and Spain, respectively) was inves tigated at microsatellite loci and mitochondrial DNA (mtDNA) sequences and compared with the pattern of differentiation of male plumage colour. The Cz ech population lives sympatrically with the closely related collared flycat cher (F. albicollis) whereas the other two are allopatric. Allopatric popul ations are on average more conspicuously coloured than sympatric ones, a pa ttern that has been explained by sexual selection for conspicuous colour in allopatry and a character displacement on breeding plumage colour in sympa try that reduces the rate of hybridization with the collared flycatcher. Th e Czech population was genetically indistinguishable from the Norwegian pop ulation at microsatellite loci and mtDNA sequences. Recent isolation and/or gene flow may explain the lack of genetic differentiation. Accordingly, di fferent selection on plumage colour in the two populations is either suffic iently strong so that gene now has little impact on the pattern of colour v ariation, or differentiation of plumage colour occurred so recently that th e (presumably) neutral, fast evolving markers employed here are unable to r eflect the differentiation. Genetically, the Spanish population was signifi cantly differentiated from the other populations, but the divergence was mu ch more pronounced at mtDNA compared to microsatellites. This may reflect i ncreased rate of differentiation by genetic drift at the mitochondrial, com pared with the nuclear genome, caused by the smaller effective population s ize of the former genome. In accordance with this interpretation, a genetic pattern consistent with effects of small population size in the Spanish po pulation (genetic drift and inbreeding) were also apparent at the microsate llites, namely reduced allelic diversity and heterozygous deficiency.