The Pleistocene Arc Hypothesis explains the evolution of the Rufous-fronted Thornbird

Genetic patterns in this species follow the distribution of dry forests around Amazonia.

The evolution of South American birds is a complicated story, to put it mildly. Different species have been affected by different environmental barriers, such as the open vegetation corridor between the Andes and Amazonia or the myriad of rivers that crisscross the Amazon rainforest. And don’t forget about the climatic fluctuations during the Pleistocene (between 2.5 million and 11,000 years ago) which impacted vegetation patterns across South America, and consequently shaped the distribution patterns of the avifauna inhabiting certain vegetation types. For example, the evolutionary history of the Variable Antshrike (Thamnophilus caerulescens) was mainly driven by the expansion and contraction of forest habitat (see this blog post for the details). The evolution of this species follows the so-called “Rainforest Refugia Hypothesis”, which proposes that the fragmentation of the rainforests during cold and dry periods resulted in allopatric speciation by separating birds into distinct rainforest “refugia” surrounded by open habitat.

The “Rainforest Refugia Hypothesis” mainly focuses on birds in the wet rainforest, but what about species that inhabit dry forests? Their evolution might adhere to a related scenario: the “Pleistocene Arc Hypothesis” which states that dry periods might have promoted the expansion of dry forests, culminating in a continuous arc around the southern half of Amazonia from Peru to Brazil. A recent study in the journal Molecular Ecology tested this hypothesis for the Rufous-fronted Thornbird (Phacellodomus rufifrons), a common dry forest bird species.

Past Connections

Eamon Corbett and his colleagues collected samples across the range of the Rufous-fronted Thornbird, which has been divided into several subspecies that follow the distribution of dry forests around Amazonia (see figure below). Genetic analyses of the different populations uncovered patterns that are in line with the “Pleistocene Arc Hypothesis”. Specifically, the researchers found evidence that certain populations in currently distinct patches of dry forest were connected in the recent past.

For example, even though the subspecies peruvianus and sincipitalis are separated by more than 1000 kilometers of lush rainforest, they share genetic variants and show signatures of recent divergence. The authors write that “the most likely scenario is that peruvianus and sincipitalis were connected through formerly suitable habitat in central and southern Peru in the recent past, and that the large modern-day disjunction between them is a recent phenomenon.” Similarly, the subspecies sincipitalis and rufifrons are currently isolated by 500 kilometers of unsuitable Cerrado habitat, but they nonetheless show low genetic differentiation. Interestingly, the genetic splits between the different subspecies did not occur in the same time period, suggesting that particular patches of dry forest along the Pleistocene Arc became isolated at different times.

Range and sampling localities of the Rufous-fronted Thornbird, showing a highly disjunct distribution that corresponds to the extent of the dry forest biome. From: Corbett et al. (2020) Molecular Ecology.

Putative Hybrid Zone

Apart from the recent divergence between geographically isolated subspecies, the authors uncovered the opposite pattern between two neighboring subspecies (rufifrons and specularis). Although these subspecies have overlapping distributions in eastern Brazil, the genetic analyses pointed to a relatively deep divergence compared to the other subspecies. The exact geographic barriers responsible for this genetic divergence remain to be determined. The São Francisco River seems like an unlikely candidate because several rufifrons individuals were found on the other side of the river (where specularis resides) when following it inland. Perhaps the ecological transition between the dry Caatinga and wet Cerrado habitats can explain this genetic pattern?

Moreover, despite the deep genetic divergence between rufifrons and specularis, the researchers reported evidence for recent gene flow. There might thus be a hybrid zone between these subspecies in Brazil. The authors nicely set the stage for future research: “Detailed vocal, morphological, and genetic data at a fine geographic scale will be needed to illuminate the evolutionary dynamics at work in this putative contact zone.” As I wrote at the beginning of this blog post: the evolution of South American birds is a complicated story, to put it mildly.

References

Corbett, E. C., Bravo, G. A., Schunck, F., Naka, L. N., Silveira, L. F., & Edwards, S. V. (2020). Evidence for the Pleistocene Arc Hypothesis from genome‐wide SNPs in a Neotropical dry forest specialist, the Rufous‐fronted Thornbird (Furnariidae: Phacellodomus rufifrons). Molecular Ecology29(22), 4457-4472.

Featured image: Rufous-fronted Thornbird (Phacellodomus rufifrons) © Hector Bottai | Wikimedia Commons

This paper has been added to the Furnariidae page.

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