Ecology explains the genetic differences in two Atlantic Forest species.
One of my favorite science stories is the discovery of the neutrino by the Austrian physicist Wolfgang Pauli. During an experiment, he found that energy appeared not to have been conserved. Reluctant to give up the universal idea of conservation of energy, Pauli developed an explanation. He speculated that the missing energy was carried off by a new particle. Next, he developed a mathematical model to predict certain properties of this new particle, so that its existence could be verified. Twenty-five years later this new particle was found and is now a well-established member of particle physics, even if still hard to detect. This story illustrates the power of formulating explanations and hypotheses which can consequently be tested with new experiments and observations.
A recent study in the journal Molecular Phylogenetics and Evolution took a similar approach when examining the genetic population structure of of two species in the Montane Atlantic Forest: the blue manakin (Chiroxiphia caudata) and the drab-breasted bamboo tyrant (Hemitriccus diops). What explanations could account for the similarities and differences in population structure between these species?
Last Glacial Maximum
Reconstructing the past distributions of these species revealed that they responded similarly to the climatic conditions during the Last Glacial Maximum (about 20,000 years ago). At this time, the Montane Atlantic Forest covered a larger area of South America, allowing both species to expand their range. This finding was also supported by the genetic analyses where several statistics (Fu’s Fs and R2) indicated population expansion in both species.
Despite these similarities, additional genetic analyses of the mitochondrial ND2 gene revealed some striking differences. The blue manakin did not display a clear phylogeographic structure, whereas the drab-breasted bamboo tyrant showed a phylogeographic break near the Doce River. What could explain these differences?
The researchers discuss several explanations for this phylogeographic incongruence. A first possibility is that there has been more gene flow between several blue manakin populations, preventing the build-up of genetic differences. This higher exchange of individuals (and genes) between populations could relate to the diet and mating system of this species. The blue manakin is a frugivore. Given that fruit is a more ephemeral resource in time and space, frugivorous species are expected to travel large distances to find food. In addition, the blue manakin exhibits lekking behavior in which females visit several locations where multiple males display (see video below). Again, lekking species are expected to disperse over larger distances during their visits. The drab-breasted bamboo tyrant, on the other hand, is a insectivore (plenty of those around) and has a territorial mating system (no need to travel far). These characteristics might lead to less gene flow between populations and the accumulation of genetic differentiation.
A second explanation concerns the different generation times of both species. Female manakins start breeding when they are 2–3 years old and have a generation time of 4.9 years. This is roughly three times larger than the average generation time of non-lekking passerine birds (ca. 1.7 years). Hence, the authors suggest that “the shorter generation time of H. diops could have favored the accumulation of differences between geographically isolated populations.”
Similar to Pauli postulating the existence of a new particle, the formulation of all these explanations is just the first step in this scientific endeavor. Now, the researchers will need to dig deeper and test these explanations with other analytical tools and new datasets. Indeed, they mention that “the incongruent population structure pattern shown in our comparative study indicates that life history and ecological traits can be important in diversification processes. Further investigations of these traits are needed to clarify their micro-evolutionary role.” Whether you are a physicist or a biologist, further investigations are the way forward.
da Silva Ribeiro, T., Batalha-Filho, H., Silveira, L. F., Miyaki, C. Y., & Maldonado-Coelho, M. (2020). Life history and ecology might explain incongruent population structure in two co-distributed montane bird species of the Atlantic Forest. Molecular Phylogenetics and Evolution, 153, 106925.
Featured image: Blue Manakin (Chiroxiphia caudata) © Dave Curtis | Flickr