Speciation, introgressive hybridization and nonlinear rate of molecular evolution in flycatchers

Evolutionary history of Muscicapidae flycatchers is inferred from nuclear a nd mitochondrial DNA (mtDNA) sequence comparisons and population genetic an alysis of nuclear and mtDNA markers. Phylogenetic reconstruction based on s equences from the two genomes yielded similar trees with respect to the ord er at which the species split off. However, the genetic distances fitted a nonlinear, polynomial model reflecting diminishing divergence rate of the m tDNA sequences compared to the nuclear DNA sequences. This could be explain ed by Haldane's rule because genetic isolation might evolve more rapidly on the mitochondrial rather than the nuclear genome in birds. This is because hybrid sterility of the heterogametic sex (females) would predate that of the homogametic sex (males), leading to sex biased introgression of nuclear genes. Analyses of present hybrid zones of pied (Ficedula hypoleuca) and c ollared flycatchers (E albicollis) may indicate a slight sexual bias in rat e of introgression, but the introgression rates were too low to allow prope r statistical analyses. It is suggested, however, that the observed deviati on from linearity can be explained by a more rapid mutational saturation of the mtDNA sequences than of the nuclear DNA sequences, as supported by ana lyses of third codon position transversions at two protein coding mtDNA gen es. A phylogeographic scenario for the black and white flycatcher species i s suggested based on interpretation of the genetic data obtained. Four spec ies appear to have diverged from a common ancestor relatively simultaneousl y during the Pleistocene. After the last glaciation period, pied and collar ed flycatchers expanded their breeding ranges and eventually came into seco ndary contact in Central and Eastern Europe and on the Baltic Isles.