Pleistocene expansion and connectivity of mesic forests inside the South American Dry Diagonal supported by the phylogeography of a small lizard*

Abstract

Traditionally focused on Amazonian and Atlantic rainforests, studies on the origins of high Neotropical biodiversity have recently shifted to also investigate biodiversity processes in the South American dry diagonal, encompassing Chaco, Cerrado savannas, and Caatinga seasonally dry tropical forests. The plateau/depression hypothesis states that riparian forests in the Brazilian Shield in central Brazil are inhabited by Pleistocene lineages, with shallow divergences and signatures of population expansion. Moreover, riparian forests may have acted as a vegetation network in the Pleistocene, allowing gene/species flow across the South American dry diagonal. We tested these hypotheses using Colobosaura modesta, a small gymnophthalmid lizard from forested habitats in the Cerrado savannas and montane/submontane forests in the Caatinga. We conducted phylogeographic analyses using a multi‐locus dataset, tested alternative demographic scenarios with Approximate Bayesian Computation, and also employed species delimitation tests. We recovered a history of recent colonization and expansion along riparian forests, associated with Pleistocene climate shifts, and the existence of a new species of Colobosaura restricted to the Serra do Cachimbo region. We also present evidence that riparian forests have provided an interconnected network for forest organisms within the South American dry diagonal and that Pleistocene events played an important role in their evolutionary history.

Digest: A lizard evolutionary history illustrates a past Neotropical dispersal route*

“…In this issue, Ledo et al. (2020) explored the tempo and mode of population divergence of a small lizard, Colobosaura modesta , which inhabits forested habitats, including dry and riparian forests, from eastern Amazonia to Caatinga, a seasonal dry forest of Northeastern Brazil, as well as the Atlantic Forest. Using this biological system, the authors tested a hypothesis that considers the geomorphological subdivision of the landscape between older (Late Tertiary to Early Quaternary) plateaus dominated by the savanna‐like Cerrado ecosystem and Caatinga, and younger (Quaternary) valleys where riparian and dry forests prevail (Werneck 2011).…”

“…The “plateau/depression” hypothesis (Werneck 2011) tested by Ledo et al. (2020). This hypothesis (A) considers a geomorphological landscape subdivision between older (Late Tertiary to Early Quaternary) plateaus dominated by Cerrado and Caatinga, and younger (Quaternary) valleys.…”

“…Finally, coalescence‐based methods and species distribution modelling independently showed signs of demographic instability, remarkably for Late Pleistocene (Ledo et al. 2020). These findings not only fail to reject the geomorphological‐based plateau/depression hypothesis, but they also provide novel within‐species insights on the past connections of key Neotropical biomes (Fig.…”

“…Ledo et al. (2020) were able to show that the majorly dry Brazilian plateau was not a barrier to gene flow for this species, suggesting a network of riparian forests as a cross‐biome dispersal route during the climatically unstable Pleistocene (Cheng et al. 2013).…”

The two South American dry diagonals

“…The first notion of SADD refers to the concatenation of sub-humid, dry and semiarid domains stretching from northeastern Brazil into northwestern Argentina formed by the Caatinga, Cerrado and Chaco domains (Fig. 1A; Prado and Gibbs 1993, Werneck et al 2012, Santos-Silva et al 2013, Fouquet et al 2014, Azevedo et al 2020a, Collevatti et al 2020, Ledo et al 2020, Neves et al 2020, Rocha et al 2020, Masa-Iranzo et al 2021. The second refers to the arid and hyperarid domains distributed from southern Argentina into southwestern Ecuador formed by Patagonia, Monte, Prepuna, dry Puna and the Atacama and Peruvian deserts (Fig.…”