Genetic analyses help to explain the peculiar circum-Amazonian distribution of this species.
In February 2020, I started a new position as lecturer at Wageningen University (the Netherlands). I was looking forward to interacting with students during a variety of ecology courses. Then the Corona-virus spread across Europe and I was forced to start teaching online. Although the switch to an online format was challenging, I tend to focus the bright side and reflect on all the new skills I learned during the past months. For example, I recently recorded two knowledge clips about the evolution of birds that you can watch on YouTube (on the origins of feathered flight and the diversity in beak morphology).
Today, I came across a TEDxtalk by Aaron Barth about the importance of story-telling in online education. He criticized the one-way style of teaching where an instructor gives a bullet-point-rich lecture and provides some reading material. Teaching (and by extension, science communication in general) should be interactive and can benefit from storytelling. That is also something I try to do on this blog – for instance, I announce new blog posts as “Avian Hybrids stories”. In this post, I will share the evolutionary story of the Variable Antshrike (Thamnophilus caerulescens), based on a recent study in the journal Molecular Phylogenetics and Evolution.
Around the Amazon
As the name suggests, the Variable Antshrike shows extensive plumage variation. Another interesting feature about this species is its peculiar distribution. Populations are namely wrapped around the Amazon – spanning the Atlantic Forest, Cerrado, Chaco, and the foothills of the Andes – culminating in a so-called circum-Amazonian distribution (see map below). This geographic arrangement does not fit neatly into the classical South American domains, such as Amazonia, the Atlantic Forest or the Llanos.
What processes led to this circum-Amazonian distribution? The evolutionary history of South American birds is thought to be shaped by two main processes: geological and climatic events that affected the spread of tropical forests and the birds living in them (the Forest Refugia Hypothesis) or the isolating effects of rivers (the Riverine Barrier Hypothesis). Do these hypotheses also apply to the Variable Antshrike?
Sergio Bolívar-Leguizamón and his colleagues extracted DNA from 53 museum specimens, representing all known subspecies of the Variable Antshrike. Genetic analyses of one mitochondrial gene and the flanking regions of several ultraconserved elements (UCEs) uncovered three main geographical groups. If you are not familiar with the different South American regions, you can compare the colors on the phylogenetic tree below with the distribution map of the Variable Antshrike. But I will walk you through this circum-Amazonian trip.
The caerensis group contains individuals from the Atlantic Forest north of the river Rio São Francisco (in darkblue). The caerulescens group is composed of samples from southeastern Cerrado and central Atlantic Forest (in orange, green, and purple). Finally, the aspersiventer group spans the transition from the drier environments in the Chaco and southern Yungas in Argentina, Bolivia, and Paraguay (in red and grey) to the more humid forests in the northern Yungas and Central Andes in northern Bolivia and Peru (in pink and lightblue).
To reconstruct the evolutionary history of these three main genetic clusters, the researchers ran several demographic models. The results from these analyses provided them with divergence times, patterns of gene flow and past population dynamics. It seems that the evolution of this passerine was mainly driven by the expansion and contraction of forest habitat, supporting the Forest Refugia Hypothesis. Putting all this information together, we can tell the story of the Variable Antshrike.
The results of our analyses suggest that the history of T. caerulescens began in the Late Miocene-Pliocene, with an initial widespread population distributed across the Cerrado, Atlantic Forests, the Chaco and the central Andes. During the Early – Middle Pleistocene (0.81–0.59 Ma), climatic fluctuations promoted expansions and contractions of forested habitats separating populations in the northern Atlantic Forest from those farther south. During the Middle Pleistocene (0.50–0.36 Ma), the continued effects of wet-dry cycles caused the contraction of populations in the southern Atlantic Forest and in the Chaco-Andes, but at the same time allowed sufficient opportunities to maintain gene flow via the onset of dry-forest corridors that connected these two regions.
Sounds like a wonderful, ornithological bedtime story.
Bolívar-Leguizamón, S. D., Silveira, L. F., Derryberry, E. P., Brumfield, R. T., & Bravo, G. A. (2020). Phylogeography of the Variable Antshrike (Thamnophilus caerulescens), a South American passerine distributed along multiple environmental gradients. Molecular Phylogenetics and Evolution, 148, 106810.
Feature image © Francesco Veronesi | Wikimedia Commons
This paper has been added to the Thamnophilidae page.