It turns out that this moving hybrid zone didn’t move very far.
Hybrid zones can move. In 2007, Richard Buggs presented evidence for 25 hybrid zones that have shifted over time. Since then, at least 11 more hybrid zones have been added to that list. Perhaps hybrid movement is more common than stability?
An often-cited example of such a moving zone is the contact zone between Hermit Warbler (Setophaga occidentalis) and Townsend’s Warbler (S. townsendi). These songbirds interbreed in three hybrid zones: two in Washington and one in Oregon. A genetic study found Hermit mtDNA in phenotypically pure Townsend’s populations up to 2000 km north along the Pacific coast. This was interpreted as a genetic wake left behind by the moving hybrid zone. A recent study in the Journal of Evolutionary Biology focused on one of the Washington hybrid zones (in the Cascade Mountains) and compared samples from 1987-1994 with samples from 2015-2016 to reconstruct the movement of this hybrid zone.
Silu Wang and her colleagues measured eight species-specific plumage characters to create a hybrid index score. A low score points to Hermit Warbler, while a high score indicates Townsend’s Warbler. Hybrids can be detected by intermediate scores. When you plot these hybrid indices on a map, you can see a gradual transition from Hermit Warbler-like birds to Townsend’s Warbler-like birds. The resulting figure is a so-called cline (for a quick lesson in cline theory, you can read this blog post). By comparing the cline from 1987-1994 with the one from 2015-2016, you can deduce how much the hybrid zone has moved. The result from this analysis was quite surprising: the Setophaga hybrid zone did not move…
A New Scenario
Genetic analyses (based on 21,852 SNPs) confirmed the plumage patterns: the hybrid zone did not move. So, what happened then? The authors propose a new biogeographic scenario to explain these results. During the last glacial maximum, Hermit Warblers resided in various refugia along the coast. After this cold period, inland Townsend’s Warblers (from Idaho or Montana) moved to coastal area and made contact with the Hermit Warblers. This ancient hybridization event could have occurred “along a long front paralleling the coastal mountains, just inland from the coastline.” Later, the hybrid zones moved southwest to their current location. This scenario only requires slow movement of the hybrid zones and is more consistent with the stability observed in this study. We can thus remove this hybrid zone from Richard Buggs’ list…
Buggs, R. J. A. (2007). Empirical study of hybrid zone movement. Heredity, 99(3), 301.
Krosby, M., & Rohwer, S. (2008). A 2000 km genetic wake yields evidence for northern glacial refugia and hybrid zone movement in a pair of songbirds. Proceedings of the Royal Society B: Biological Sciences, 276(1657), 615-621.
Wang, S., Rohwer, S., Delmore, K., & Irwin, D. E. (2019). Cross‐decades stability of an avian hybrid zone. Journal of Evolutionary Biology.
This paper has been added to the Parulidae page.