Two hybrid zones between Barn Swallow subspecies reveal the strength of reproductive isolation.
When you read papers on hybrid zones, you will definitely come across “cline analyses”, a powerful technique (largely developed Nick Barton) to disentangle the change of morphological and genetic traits across a hybrid zone transect. I will not bore you with the mathematical details. Instead, I will quickly explain the rational behind clines.
Cline Theory for Dummies
Constructing clines is rather straightforward. You collect data on morphological characters across the hybrid zone and you plot it on a graph. Let’s say you have a white species and a black species that produce gray offspring in a contact zone. You observe birds along a transect and note down their plumage color. When you put the data in the graph, you will see a transition from white birds (when you were in the habitat of the white birds) through grayish birds (in the hybrid zone) to black birds (in the black bird habitat). Easy, no?
The interesting part is that the shape of a cline can tell you something about the biology of the birds. For example, if the gray hybrids interbreed with their parental species, there will be a variety of backcrosses of different colors. Some more white and some more black, depending on the species they crossed with. This will result in a smooth transition from white through different (perhaps 50) shades of gray to black. In other words, a wide cline. However, if gray birds cannot find a mate, there will be mostly gray hybrids in the contact zone. This will result in a rapid transition from white to black plumage, a steep cline.
Finally, the position of a cline can also provide useful information. If there is no hybridization outside the hybrid zone, all clines should be centered on the midpoint of the hybrid zone. But if there is gene flow from one species into the other, the cline center can be displaced into the distribution of one of the species. The displaced cline often corresponds to a certain phenotypic character (e.g., plumage color), which indicates that this character is spreading from one species into the other because it confers some kind of advantage, such as in finding a suitable partner.
To recap, a steep cline suggests strong reproductive isolation between hybridizing species, while a wide cline points to weak isolation. And a displaced cline suggests gene flow from one species into the other. Now that you have some understanding of cline theory, let’s put the theory into practice with a recent paper by Elizabeth Scordato and her colleagues in Molecular Ecology. They studied two Russian hybrid zones between three subspecies of Barn Swallow (Hirundo rustica).
There are six Barn Swallow subspecies that breed across the Northern Hemisphere. Previous analyses indicated an African origin (with subspecies savignii and transitiva) with a consequent colonization of Eurasia (rustica and gutturalis). About 25,000 years ago the Asian gutturalis crossed the Bering Strait to North America, giving rise to the subspecies erythrogaster. Later (roughly 10,000 years ago) the North American birds recolonized Siberia, which resulted in the subspecies tytleri. The present study focused on the three Eurasian subspecies (rustica, gutturalis and tytleri) that interbreed in two hybrid zones.
For both hybrid zones (rustica-tytleri and tytleri-gutturalis), the researchers constructed clines for several morphological characters, such as wing length and breast color, and for genetic data. The clines in the rustica-tytleri hybrid zone were steep and narrow, whereas the clines in the gutturalis-tytleri hybrid zone were much wider. This suggests that there is stronger reproductive isolation between rustica and tytleri than between gutturalis and tytleri. This suggestion is supported by other analyses as they found fewer hybrid and backcrosses between the former two species.
In addition, the cline for breast color was displaced in the gutturalis-tytleri hybrid zone. This might indicate the spread of darker plumage from tytleri into gutturalis. Indeed, previous work in the North American subspecies has shown that females prefer darker males.
Scordato, ES, MR Wilkins, G Semenov, AS Rubtsov, NC Kane, RJ Safran. (2017) Genomic variation across two barn swallow hybrid zones reveals traits associated with divergence in sympatry and allopatry. Molecular Ecology, 26:5676-5691. http://onlinelibrary.wiley.com/doi/10.1111/mec.14276/full
The paper has been added to the Hirundinidae page.