Genetic study uncovers a complicated series of events.
A common mistake in discussions is the “False Dilemma”, which states that if X is wrong than Y must be true. Creationists often apply this style of reasoning when attacking evolutionary theory. They claim that if evolution is wrong, then creationism must be true (which explains why they put so much effort in discrediting evolution). This statement is obviously not true: an inconsistency in the current evolutionary theory does not automatically support creationism.
But let’s not get caught up in this useless and silly debate, because there is a much more interesting discussion about the origin of bird species in the Atlantic Forest of South America. Two main hypotheses have been put forward to explain how new species arise in this region. The refuge hypothesis states that during the Pleistocene vast stretches humid forests were replaced with dry vegetation, creating isolated forest patches in which species diversified. The river-barrier hypothesis, however, focuses on the role of rivers as isolating barriers between populations. It seems that there are only two possibilities: refuges versus rivers. But a recent study in the journal Molecular Phylogenetics and Evolution revealed that this is a false dilemma. There is another option to consider.
Refuges and Rivers
Using genetic data, Manuelita Sotelo-Muñoz and her colleagues reconstructed the evolutionary history of two Atlantic Forest species: the white-shouldered fire-eye (Pyriglena leucoptera) and the fringe-backed fire-eye (P. atra). The analyses revealed that these species diverged about 260,000 years ago, probably driven by habitat fragmentation. The species recently established secondary contact, resulting in the exchange of genetic material. Clearly, these fire-eyes were not isolated long enough for reproductive isolation to evolve. All in all, this scenario seems to support the refuge hypothesis. But wait, there is more…
Within the white-shouldered fire-eye, the researchers uncovered more fine-grained population structure. This species can be divided into three genetically distinct populations: a northern, central and southern lineage. Some of these populations come into contact around the major rivers in the Atlantic Forest. The northern and central population meet at the interfluvium of the de Contas and Pardo rivers, while the southern and the central population mix at the Doce and Grande rivers. These results support the river hypothesis, right? Well, not quite…
Extinction and Dispersal
A detailed look at the genetic patterns points to a complicated scenario. After divergence from the fringe-backed fire-eye, individuals from the white-shouldered fire-eye spread southwards and diversified into several populations. At some point, the population north of the Pardo river went extinct. This gap was later filled when birds from the central population dispersed northwards. Similarly, the southern populations originated when birds from the central population dispersed southwards. Later on, these populations re-established contact at the rivers. These rivers probably limited dispersal, accentuating the genetic differences between the populations.
Based on this information, the authors argue that “our results support neither the river-barrier nor the refuge hypothesis as originally conceived. Here, dispersal as opposed to vicariance, seems to be the main cause of intraspecific differentiation.” As I mentioned in the beginning of this blog post, there is more to evolution in the Atlantic Forest than refuges and rivers. Don’t ignore dispersal.
Sotelo-Muñoz, M., Maldonado-Coelho, M., Svensson-Coelho, M., dos Santos, S. S., & Miyaki, C. Y. (2020). Vicariance, dispersal, extinction and hybridization underlie the evolutionary history of Atlantic forest fire-eye antbirds (Aves: Thamnophilidae). Molecular Phylogenetics and Evolution, 106820.
This paper has been added to the Thamnophilidae page.