The importance of different processes seems to vary per location.
Hybridization often leads to introgression, the exchange of genetic material between the interacting species. The resulting patterns of introgression are determined by the complex interplay between numerous factors. First, local population sizes can affect hybridization rates. When one species is rare, its members will have more difficulty finding a mate and they might settle with a partner from another species. This phenomenon – known as Hubb’s Principle – has been nicely illustrated on the Falkland Islands where a numerical imbalance between Speckled Teal (Anas flavirostris) and Yellow-billed Pintails (Anas georgica) led to hybridization. Next, selection on hybrids might play a role. The selection pressure might be endogenous, affecting the fertility or viability of the hybrids. Or hybrids might have to deal with exogenous selection when they are badly adapted to local environmental conditions. Disentangling all these different factors is a challenging endeavor. But that did not stop Logan Maxwell and colleagues from studying how neutral and selective processes determine introgression patterns between Saltmarsh Sparrow (Ammospiza caudacuta) and Nelson’s Sparrow (Ammospiza nelsoni). Their findings recently appeared in the journal BMC Ecology and Evolution.
The researchers genotyped more than 500 birds at two locations (coastal and inland) along the coast of New England. The genetic data allowed them to determine the number of “pure” individuals, first-generation hybrids and backcrosses. If introgression is mainly determined by neutral processes, such as local population sizes, you would expect the distribution of genotypes to follow a random pattern. This was not the case: the coastal site had less backcrossed Saltmarsh Sparrows than expected while at inland site there were more hybrids, backcrosses and “pure” Saltmarsh Sparrows than predicted. These findings suggest that neutral demographic processes are not sufficient to explain introgression rates in this hybrid zone. Selection plays a role as well.
But is the selection on hybrids endogenous or exogenous? The results suggest that both types of selection are at work. The researchers detected a reduction in hybrid females among the adults. Because there was no difference in viability of females at the egg stage, it seems that hybrid females have a lower chance of survival. This pattern is in accordance with Haldane’s Rule which states that in a hybrid cross the sex with two different sex chromosomes (i.e. the female in birds) will suffer the greatest fitness reduction. In addition, there were more individuals with Saltmarsh Sparrow DNA at the coastal site, suggesting that genetic variants from this species provide an adaptive advantage in that area (see also this blog post). There is thus exogenous selection against Nelson’s Sparrows at the coast.
Finally, the researchers tested whether the birds mated assortatively in the hybrid zone (i.e. finding a partner of the same species). In general, this was certainly the case: the majority of mating events (79%) occurred between the same species. However, the patterns differed by site. Assortative mating was strong at the coastal site, but not at the inland site. The inland population is smaller which could increase the frequency of hybridization there. The exact mechanisms underlying mate choice remain to be determined, but could be related to differences in song and mating behavior.
All in all, the hybridization and introgression dynamics between Saltmarsh and Nelson’s Sparrow are determined by the complex interplay of numerous factors which differ between locations. The authors nicely summarized the situation at the beginning of the discussion-section:
We found that neutral demographic factors—relative abundances of the two species—alone could not explain the observed patterns of introgression between Saltmarsh and Nelson’s Sparrows and that spatial variation in the distribution of parental and offspring genotypes was a result of both exogenous and endogenous selective forces. In addition, sexual selection played a role in maintaining species boundaries through assortative mating. However, these patterns differed on the coastal and inland site, suggesting local differences in the strength of selection.
Maxwell, L. M., Walsh, J., Olsen, B. J., & Kovach, A. I. (2021). Patterns of introgression vary within an avian hybrid zone. BMC Ecology and Evolution, 21(1), 1-18.
Featured image: Nelson’s Sparrow (Ammospiza nelsoni) © Andy Reago & Chrissy McClarren | Wikimedia Commons