Would they recommend the dry Chaco and the open-vegetation Cerrado?
“A wealth of phylogeographic data is available for many terrestrial and aquatic organisms of the Northern Hemisphere. In fact, a disproportionately 77% of all empirical surveys of the field (or 1874 papers) have focused exclusively on Northern Hemisphere study systems.” This statement comes from a 2008 review by Luciano Beheregaray on the state of phylogeography. His analysis clearly showed a bias towards studies in the Northern Hemisphere, indicating that more studies in the Southern Hemisphere are needed. I do not know whether this bias is still so pronounced, but there seems to be a clear increase in the number of phylogeographic papers on South American birds.
I have covered some of these studies on this blog, telling the evolutionary story of several species, such as the Buff-browed Foliage Gleaner (Syndactyla rufosuperciliata) and the Variable Antshrike (Thamnophilus caerulescens). Both species occur in the Atlantic Forest on the east coast of South America and the Andean region in the west. Interestingly, these regions are separated by the dry Chaco and the open vegetation of the Cerrado. Could it be that bird populations in the Atlantic Forest and the Andean region were connected in the past? And which route did the birds take from one region to the other? These questions provided the starting point for a recent study in the journal Molecular Phylogenetics and Evolution.
Cerrado or Chaco?
Natalia Trujillo-Arias and her colleagues decided to focus on three species that occur in the Atlantic Forest and the Andes: the Ochre-faced Tody-flycatcher (Poecilotriccus plumbeiceps), the Mottle-cheeked Tyrannulet (Phylloscartes ventralis) and the Golden-winged Cacique (Cacicus chrysopterus). Genetic analyses clearly differentiated between populations in the Atlantic Forest and the Andes, supporting the idea that both regions have been isolated for some time and probably acted as refugia during the Pleistocene.
To test whether these regions have been connected by gene flow, the researchers ran several demographic models (using Approximate Bayesian Computation, for the interested readers). The models with the highest statistical support pointed to a connection through the Cerrado. There was no evidence for gene flow through the Chaco, suggesting that this area forms a formidable barrier for small passerines due to its dry forests, savannahs and wide rivers (e.g., the Paraná-Paraguay river).
Morphology Lagging Behind
The clear genetic differences between the populations on both sides of the South American continents were not reflected in their morphology. The researchers noted that “A morphologic-genetic disagreement suggests that the phenotype of these species has been impacted by factors other than the demographic history of populations.” What other factors could have prevented the morphological traits from catching up with the genetic differentiation. In other words, why do birds from Andean and Atlantic Forest populations still look alike?
One possibility is neutral evolution: the phenotypes might have been changing by chance, which results in slower evolutionary change compared to strong natural or sexual selection. Alternatively, there might have been balancing selection that maintained certain morphological traits in both environments (also known as phenotypic conservatism). At the moment, the researchers could not discriminate between these possible explanations, opening the door for future research (check out this PNAS paper for more information on phenotypes in phylogeography). This is a common theme in science: answer one question (finding a Cerrado connection) and you are faced with a collection of new challenges (explaining morphological evolution).
Trujillo-Arias, N., Rodríguez-Cajarville, M. J., Sari, E., Miyaki, C. Y., Santos, F. R., Witt, C. C., … & Cabanne, G. S. (2020). Evolution between forest macrorefugia is linked to discordance between genetic and morphological variation in Neotropical passerines. Molecular phylogenetics and evolution, 149, 106849.
Featured image: An Ochre-faced Tody-flycatcher in Brazil. © Dario Sanches | Wikimedia Commons