Studying Willow and Alder Flycatcher at different stages in the speciation process

Researchers follow the evolution of reproductive isolation in two contact zones.

When two species start hybridizing after a period of geographic isolation several scenarios are possible. Barriers of reproductive isolation might break down and the species collapse into one panmictic population. Or new interactions between the hybridizing species might push reproductive isolation towards completion. In most cases, we can only guess what happened in the past by studying present-day patterns of genetic variation. In my own work, for example, I reconstructed the evolutionary history of two Bean Goose species that established secondary contact about 60,000 years ago. It seems that these geese are in the merging into one species, but it is tricky to draw conclusions on a process that could take thousands to millions of years. Sometimes, however, we come across a situation where we can directly study different stages of speciation process. In a recent study in the journal Molecular Ecology, researchers could compare different contact zones between two Empidonax flycatchers to understand how reproductive isolation evolves between these species.

Contact Zones

Willow Flycatcher (E. traillii) and Alder Flycatcher (E. alnorum) interbreed across two contact zones in North America: a broad overlapping area in the east (more than 1000 kilometers) and a narrow one (less than 200 kilometers) in the west. Jordan Bemmels, Ashley Bramwell and their colleagues used whole genome data to investigate patterns of introgression at these contact zones. In the western zone, there was clear evidence for introgression (2.4-8.2%), while no admixture was detected in the eastern zone. These patterns indicate that reproductive isolation is strong in the east and still incomplete in the west. Additional analyses revealed that the western contact zone is of recent origin, whereas the formation of the eastern contact zone can be traced back to the last glacial maximum. Birds in the east have thus had more time to build-up reproductive isolation.

Genomic analyses uncovered different levels of introgression in the western and eastern contact zones between Willow Flycatcher and Alder Flycatcher. From: Bemmels et al. (2021) Molecular Ecology.

Testing Traits

The traits underlying reproductive isolation between these flycatchers remain to be determined. These traits can often be identified by looking for signatures of divergent character displacement. For instance, selection for reduced competition could result in differences in habitat use or beak morphology when species adapt to distinct food sources. Selection can also directly contribute to reproductive isolation. If hybridization is maladaptive, traits involved in species recognition will evolve along different trajectories to prevent birds from hybridizing. The researchers tested several traits, such as beak morphology and the colors of crown feathers, but found no clear evidence for character displacement. Moreover, the species did not appear to differ in habitat use or timing of breeding.

However, there are still plenty of other traits that can be tested, such as differences in song or sperm morphology (see this blog post). The researchers also mentioned that “the genomes of the two species are well differentiated, with numerous Fst peaks that occur on almost every chromosome.” These peaks in genetic differentiation could contain some interesting candidate genes that might point to the traits underlying reproductive isolation between Willow Flycatcher and Alder Flycatcher.

Clear genetic differentiation across the genome of the flycatcher species. Some of these peaks might contain candidate genes involved in reproductive isolation. From: Bemmels et al. (2021) Molecular Ecology.

References

Bemmels, J. B., Bramwell, A. C., Anderson, S. A., Luzuriaga‐Aveiga, V. E., Mikkelsen, E. K., & Weir, J. T. (2021). Geographic contact drives increased reproductive isolation in two cryptic Empidonax flycatchers. Molecular Ecology30(19), 4833-4844.

Featured image: Willow Flycatcher (Empidonax traillii) © VJ Anderson | Wikimedia Commons

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