Recent study provides several lines of evidence that natural selection is driving divergence in this tropical species.
Ecological speciation is still a controversial idea. This process describes the evolution of reproductive isolation between populations due to divergent natural selection. Environmental gradients provide an ideal setting for ecological speciation to take place: populations adapt to the changing conditions along this gradient and diverge in certain characters, possibly culminating in the origin of new species. A recent study in the Journal of Ornithology argues that this is happening in the White-tipped Plantcutter (Phytotoma rutila).
This bird species can be found across the Chaco-Andes dry forest belt in South America. It is divided into two subspecies that occupy different altitudes: rutila occurs in the lowlands, while angustirostris lives in forests between 2000 and 3900 meters. Could these subspecies be the products of divergent natural selection? Let’s have a look at the evidence.
First, there is a significant correlation between body size and elevation: larger birds tend to live at higher altitude. This is probably an adaptation to cope with particular conditions in the mountains, such as low temperatures and low levels of oxygen. However, larger body size can also be the outcome of neutral processes: birds at higher altitude just happen to be bigger by chance.
To rule out the possibility of neutral evolution, the researchers used the Pst-Fst comparison. Pst measures the differentiation in certain phenotypes, whereas Fst captures genetic differentiation. If morphology is evolving by neutral processes, Pst and Fst should be similar. If the two measures are different, adaptive processes might be involved. The analyses of several genetic markers revealed that Pst was consistently higher than Fst, supporting the idea that natural selection is driving body size evolution.
More detailed genetic analyses provided further evidence for incipient speciation in the White-tipped Plantcutter. The highland birds formed a clade that was nested within the lowland birds, indicating that a lowland ancestor probably colonized higher altitudes at some point. Moreover, the researchers found gene flow from highland into lowland birds. This pattern also fits with the scenario of ecological speciation where ecological divergence occurs in the face of gene flow.
The White-tipped Plantcutter provides a nice example of ecological speciation. However, other processes than natural selection have probably contributed to the observed divergence. For example, the Yungas forests seem to present a partial barrier between lowland and highland birds. More research will be needed to sort out the details of this interesting case of incipient speciation.
Rodríguez-Cajarville, M. J., Calderón, L., Tubaro, P. L., & Cabanne, G. S. (2019). Body size and genetic variation in the White-tipped Plantcutter (Phytotoma rutila: Cotingidae) suggest ecological divergence across the Chaco–Andes dry forest belt. Journal of Ornithology, 1-15.
This paper has been added to the Cotingidae page.