Speciation in Ficedula flycatchers.

Speciation in animals often requires that population divergence goes through three major evolutionary stages, i.e. ecological divergence, development of sexual isolation and the build-up of genetic incompatibility. There is theoretical consensus regarding favourable conditions required for speciation to reach its final and irreversible stage, but empirical tests remain rare. Here, we review recent research on processes of speciation, based on studies in hybrid zones between collared (Ficedula albicollis) and pied flycatchers (Ficedula hypoleuca). A major advantage of this study system is that questions concerning all three major sources of reproductive isolation and their interconnections can be addressed. We conclude that (i) ecological divergence is caused by divergence in life-history traits, (ii) females prefer mates of their own species based on differences in both plumage and song characteristics, (iii) male plumage characteristics have diverged but their song has converged in sympatry, (iv) there is genetic incompatibility in accordance with Haldane's rule, and (v) the Z-chromosome appears to be a hotspot for genes involved in sexual isolation and genetic incompatibility. We discuss how identification of the genes underlying the three major sources of reproductive isolation can be used to draw conclusions about links between the processes driving their evolution.