Studies of hybrid zones offer important insights into the process of speciation. Much of the knowledge to be gained is dependent on an accurate estimation of the strength of pre- and post-zygotic isolation between hybridizing taxa. My results demonstrate that hybridization can variously affect different components of fitness. In Ficedula flycatchers, late-breeding females may directly benefit from pairing with a heterospecific male by gaining access to superior territories. The hybrid offspring possess an immune system that is as equally well functioning as in the parental species (the collared, F. albicollis, and pied flycatcher, F. hypoleuca). However, I found that a severe reduction in fertility persists for at least three generations after the actual hybridization event. Combining all information about the reproductive success of hybridizing individuals and their descendents revealed that postzygotic isolation between flycatchers is very strong; hybridizing individuals leave almost no descendents. This thesis presents one of few comprehensive summaries of the selection for/against assortative mating in a natural hybrid zone. These findings suggest a central role for intrinsic postzygotic isolation as a reproductive barrier separating newly evolved bird species, and contrast previous suggestions that postmating isolation is the slowest of the reproductive barriers to evolve in birds.
Despite this strong selection against hybridization, pre-mating isolation is incomplete. Hybridization often results from females lacking conspecific partners, but appears to be also caused by errors in species recognition. Much of this error probably reflects the short period of time that pied flycatchers on Gotland and Öland have been in sympatry. Compared to collared flycatchers, pied flycatchers are poorer able to discriminate between conspecific and heterospecific song, and male pied flycatchers more often falsely signal their own identity through heterospecific song copying. However, despite colonising the study site from other sympatric populations and having very little gene flow from allopatry, collared flycatchers also possess traits (e.g. delayed plumage maturation) that increase their hybridization risk. Once pre-mating isolation is strong, the rarity of hybridization probably inhibits further selection against traits promoting interspecific mating, especially when such traits may be beneficial in other contexts. This thesis highlights complex interactions between factors affecting hybridization rate that would not be detected if such a study were not field-based. Furthermore, it showcases likely examples in nature of a number of theoretical objections to the evolution of pre-mating barriers between populations living in sympatry.