Jeison M Barraza, Jorge Avaria-Llautureo, Marcelo M Rivadeneira

We inferred the geographic dispersal routes and the environmental conditions that shaped the ~30-million-years historical biogeography of Brachistosternus scorpions in South America. We evaluated the role that altitude and aridity had on the geographic distance that each species dispersed from the location of the genus common ancestor. Based on previous studies, we evaluated the hypothesis postulating that the species’ geographic expansion was promoted by arid conditions in higher altitudes.
To test the hypothesis, we integrated two methodological approaches, the phylogenetic Geographical model and the Conductance model. The Geo model infers the locations of ancestral species in a phylogenetic tree, assuming a spherical space and using samples of georeferenced locations for every species as input data. It allows us to estimate the historical species dispersal routes and distances from the location of the genus common ancestor. The Conductance model is based on circuit theory and infers the geographic routes and distance of least resistance between two points, i.e., an origin and destination. We defined the origin as the location of the genus common ancestor obtained from the Geo model and the destination point as the current geographic location of each species. This model infers the geographic routes with the least cost of resistant for dispersal in a landscape of varying altitude and aridity. Finally, we evaluated the correlation between the two dispersal distances each species has moved from the location of the common ancestor, i.e., the distance inferred from the Geo model and from the Conductance model.
The Geo model shows that Brachistosternus’s geographical origin was most likely along the coast of south Peru, and central Chile. From this location, extant species dispersed through routes ranging from 873 to 2,800 km on average. The Conductance model that considers the routes with less resistance to elevation and aridity simulated dispersal distances that are highly correlated with the species dispersal distances obtained from the Geo model. We revealed the geographic dispersal routes, with the least resistance to the pressures imposed by changes in altitude and aridity, that 55 species of scorpions have probably followed in the last 30 million years in South America. These geographic routes that went along the Andean Mountains and the arid zones of South America shaped the current spatial distribution radiation of the genus Brachistosternus.