Several lines of evidence indicate strong selection against hybrids.
An interesting debate in avian speciation concerns the relative importance of prezygotic versus postzygotic isolation mechanisms. Prezygotic isolation mechanisms operate before the fertilization of the egg. They can be behavioral, for example when members from different species don’t see each other as potential mates because they look or sound too different. When such behavioral isolation is imperfect and copulation does occur, fertilization might still fail if sperm and egg are incompatible (see this review paper for more on these so-called postcopulatory prezygotic barriers). Postzygotic isolation mechanisms act after fertilization and can be either intrinsic or extrinsic. Intrinsic mechanisms lead to sterility or unviability of the offspring, while extrinsic mechanisms encompass lower fitness of the hybrid offspring due to ecological or behavioral issues. For example, some hybrid hummingbirds display aberrant courtship behavior and cannot attract a mate (see this blog post). But which of these mechanisms contributes most to reproductive isolation during the formation of new bird species?
In some cases, the prezygotic isolation mechanisms are crystal-clear (although they can still be incomplete). Think of the distinct plumage patterns of the Golden-winged (Vermivora chrysoptera) and Blue-winged Warbler (V. cyanoptera, you can read more on these species in this blog post). Or just listen to the drastically different songs of the Common Chiffchaff (Phylloscopus collybita) and the Willow Warbler (Phylloscopus trochilus). However, the situation becomes more complicated when you venture into the Amazonian rainforest. Here, some bird species show minor differences in morphology or song despite millions of years of evolution. The Rondonia Warbling-antbird (Hypocnemis ochrogyna) and the Spix’s Warbling-antbird (H. striata), for example, look and sound almost similar, but have been on separate evolutionary trajectories for more than one million years. What keeps these species distinct? Could it be postzygotic isolation mechanisms? A recent study in the journal Evolution tried to find out.
Hybrid Triangles and Clines
Áurea Cronemberger and her colleagues took a genetic approach to study a hybrid zone between the two warbling-antbird species. They applied several methods to quantify the strength of selection against hybrids. First, they constructed “hybrid triangles” to determine the frequency of first-generation hybrids and backcrosses in the hybrid zone. These triangles combine information from a hybrid index (i.e. genetic ancestry of an individual) and heterozygosity to discriminate between different hybrid classes. In general, “pure” individuals are located in the lower corners, while first generation hybrids are at the top. The sides of the triangles indicate backcrosses. This analysis revealed six first-generation hybrids and a series of backcrossed individuals. One individual (in the center of the plot) looks like a second-generation hybrid (i.e. the offspring of two hybrids).
The presence of several backcrosses seems to suggest that there is no strong selection against hybrids. However, the production of backcrosses does not mean that the species are exchanging genetic material (i.e. introgression). It is possible that these backcrosses do not reproduce themselves due to genetic incompatibilities in their genomes. To check whether the production of backcrosses leads to introgression, the researchers turned to cline analyses. I have covered this approach in a previous blog post, but I will quickly recap the most important lessons here: “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.”
The researchers calculated the expected cline width under neutral conditions (no selection against hybrids), which amounted to 211 kilometers. The genetic analyses revealed that only one locus out of 5387 exceeded this threshold and 95% of the loci even showed cline widths less than 45 kilometers. In other words, a lot of steep clines. In addition, the researchers reported little variability in the geographic positions of the clines. The lack of displaced clines suggests that there is no introgression between the hybridizing species. In summary, the shape and position of the clines point to strong selection against hybrids and backcrosses.
This study provides strong evidence for postzygotic isolation between the Rondonia Warbling-antbird and the Spix’s Warbling-antbird. Despite minor differences in morphology and vocalization, these birds can thus be considered distinct species. Similar results have been reported for other Amazonian birds, such as antbirds and woodcreepers (see this blog post), suggesting that postzygotic isolation mechanisms are the main driver of avian speciation in Amazonia. This finding might have important consequences for the species richness in this area. If postzygotic isolation evolves between morphologically cryptic species, how many species are still waiting to be discovered with genetic analyses. Indeed, the researchers conclude that “Our results thus suggest the strong possibility that species richness could be dramatically underestimated in Amazonia, especially for antbirds and other understory specialists.” Time for the next expedition!
Cronemberger, Á. A., Aleixo, A., Mikkelsen, E. K., & Weir, J. T. (2020). Postzygotic isolation drives genomic speciation between highly cryptic Hypocnemis antbirds from Amazonia. Evolution, 74(11), 2512-2525.
Featured image: Spix’s Warbling-antbird (Hypocnemis striata) © Hector Bottai | Wikimedia Commons
This paper has been added to the Thamnophilidae page.