Eight bird lineages with a trans-Beringian distribution show clear signatures of past gene flow.
Most people know Beringia as the land bridge that allowed humans to travel from Siberia to North America during the Ice Ages. Indeed, throughout the Pleistocene (between 2.5 million to 11,000 years ago) cold snaps resulted in low sea levels, which led to the formation of land bridges between the two continents. When the climate warmed again, these land bridges disappeared under the rising sea levels. This cycle of exposure and inundation of central Beringia occurred at least nine times (and perhaps up to twenty times or more). Not only did this facilitate the spread of humans across the globe, it also affected the numerous bird species that can be found on both sides of the Bering Strait.
These climatic cycles have probably shaped the genetic make-up of several Beringian bird species. During periods of high sea levels, bird populations were geographically isolated and diverged genetically. When sea levels dropped and land bridges formed, some of these populations might have come into secondary contact and exchanged some genetic material. This raises the question how much gene flow (if any) occurred during these periods of contact. A recent study in the journal Molecular Ecology took a closer look at eight population pairs to answer this question.
Jessica McLaughlin and her colleagues focused on eight bird lineages that cover the taxonomic range from populations over subspecies to distinct species. They used ultraconserved elements (UCEs) to determine the level of genetic divergence and the amount of gene flow between the following pairs:
- Long-tailed duck (Clangula hyemalis – populations)
- Bluethroat (Luscinia svecica – populations)
- Green-winged teal (Anas crecca crecca and A. c. carolinensis – subspecies)
- Whimbrel (Numenius phaeopus variegatus and N. p. hudsonicus – subspecies)
- Pine grosbeak (Pinicola enucleator kamschatkensis and P. e. flammula – subspecies)
- Eurasian and American wigeon (Mareca penelope and M. americana – species)
- Grey-tailed and wandering tattler (Tringa brevipes and T. incana – species)
- Eurasian and black-billed magpie (Pica pica and P. hudsonia – species)
The researchers tested several demographic models to reconstruct the evolutionary history of these species. In each case, gene flow was an integral part of the divergence process. Three pairs (whimbrel, pine grosbeak and magpies) followed a scenario of divergence-with-gene flow, while the evolution of two other pairs (long-tailed duck and wigeons) was best captured by gene flow at secondary contact. For the remaining three pairs (bluethroat, green-winged teal and tattlers), the analyses could not discriminate between divergence-with-gene-flow or secondary contact.
Given that these eight pairs span the taxonomic range from populations to species, you might expect to see this reflected as a continuum of genetic divergence and gene flow. This was, however, not the case. First, the genetic divergence (measured as Fst) did not follow the taxonomic classification. Some distinct species – such as Eurasian and American wigeon – were genetically quite similar (Fst = 0.04), while some subspecies – such as whimbrel (Fst = 0.27) or pine grosbeak (Fst = 0.44) – were more genetically distinct. Taxonomy is thus not a good predictor of genetic divergence.
Second, there is no clear continuum when plotting the relationship between genetic divergence and gene flow. Instead, two distinct groups are visible (see figure below). This result follows recent theoretical work that considers speciation as a two-state system with most populations pairs clustering near the two ends of the continuum (either showing genetic small differences or full reproductive isolation). Diverging populations are moving towards the right end of this continuum, but can be pulled back to the left end when gene flow occurs. Once a certain threshold of reproductive isolation has been achieved, populations will remain on the right end of the spectrum. The Beringian birds nicely represent both sides of this continuum.
Out of curiosity, I returned to my recent paper on the evolution of taiga and tundra bean goose (which I covered in this blog post). Using whole genome re-sequencing data, we found low genetic divergence (Fst = 0.033) and high levels of gene flow (M = 13). These numbers clearly put the bean geese on the left side of the spectrum. How general these patterns are remains to be determined, but Beringia seems like the perfect place to start.
McLaughlin, J. F., Faircloth, B. C., Glenn, T. C., & Winker, K. (2020). Divergence, gene flow, and speciation in eight lineages of trans‐Beringian birds. Molecular Ecology, 29(18), 3526-3542.
Featured image: Whimbrel (Numenius phaeopus) © Andreas Trepte | Avi-Fauna