Sexual Signals and Speciation : A Study of the Pied and Collared Flycatcher

Abstract: Speciation is the process in which reproductive barriers evolve between populations. In this thesis I examine how sexual signals contribute to the maintenance, reinforcement or breakdown of reproductive barriers.Male pied flycatchers (Ficedula hypoleuca) and collared flycatchers (F. albicollis) differ in song and plumage traits. However, where the two species coexist, several pied flycatchers sing a song resembling the collared flycatcher (mixed song). Mixed song is not caused by introgression from the collared flycatcher but is due to heterospecific copying. Mixed song provokes aggressive behaviour in collared flycatcher males and leads to heterospecific pairing and maladaptive hybridization. The species differences in song were found to be larger in an old than a young hybrid zone. This was due to a reduction in the frequency of mixed song in the pied flycatcher and a divergence in the song of the collared flycatcher. Apparently, mixed song causes maladaptive hybridization, which over time leads to reinforcement of reproductive barriers by a song divergence.Previous studies have shown that a character displacement in male plumage traits reinforces species barriers. Hence both plumage and song divergence reduce the incidence of hybridization. The evolution of male plumage traits has been so rapid, or selection has been so strong that rapidly evolving molecular markers are unable to trace it.Hybrid females mate with a male of the same species as their father. Previous studies have shown that females use male plumage traits controlled by genes linked to the sex chromosomes (the Z) in species recognition. An association between preference and a sex-linked trait through the paternal line may render reinforcement of reproductive barriers more likely.In conclusion, sexual signals are affected by species interactions that cause breakdown or reinforcement of reproductive barriers.