Ecology and genetics of speciation in Ficedula flycatchers

Birds have for long been popular study objects in speciation research. Being easy to observe in the field, they have traditionally been particularly important in studies of behavioural and ecological factors in speciation, whereas the genetic aspects of the process have been studied in other organisms, such as Drosophila. More recently, however, a stronger genetic focus has been placed on speciation research also in birds. Here, we review ecological, behavioural and genetic studies on speciation in the pied flycatcher (Ficedula hypoleuca) and the collared flycatcher (Ficedula albicollis). These well-studied birds provide among the few proposed examples of the process of reinforcement of premating isolation, and the evidence for reinforcement is strong. They are further characterized by having strong intrinsic postzygotic barriers (female hybrid sterility), yet the two species appear to be very similar ecologically. This is in stark contrast to another well-studied bird complex, Darwin's finches, in which the species differ vastly in ecologically important traits but have no developmental problems arising from genetic incompatibilities, and where no evidence for reinforcement is found. In the flycatchers, sex chromosome linkage of genes affecting traits associated with both pre- and postzygotic barriers to gene exchange is likely to facilitate reinforcement. We discuss whether such sex-linkage may be common in birds. The contrast between flycatchers and Darwin's finches indicate that speciation in birds cannot always be understood mainly as a result of divergent natural selection ('ecological speciation'), and generalizations from one system may lead us astray. We discuss to what extent insight from research on the flycatchers may point to fruitful avenues for future research on bird speciation and specifically call for a more systematic effort to simultaneously investigate ecology, behaviour and genetics of birds caught in the process of speciation.