“The role of geography in adaptive radiation”
John J. Schenk and Scott J. Steppan (Oct 2018)
Biogeographic transitions are important drivers of species diversity in adaptive radiations
To explain why millions of species exist on Earth, biologists seek to discover how species evolve and the factors promoting diversification. Of particular interest are groups of closely related species that rapidly evolved many new species as they adapted to new niches, a process called adaptive radiation. Even though adaptive radiation is the most iconic process in the diversification of life (most famously in the Galapagos finches), most well-studied cases occur on islands, and we know relatively little about how or if these radiations occur on continents, where most species actually live. In the absence of obvious barriers like oceanic waters, how does diversification proceed across the continuous geography of a continent? Does it start in one region, slowly spreading out as species adapt to each new habitat in turn, or does it spread everywhere quickly, and only evolve adaptations later?
In their study, Schenk and Steppan develop a new approach to examine how speciation proceeds across geographic space by studying 300 species from one of the most diverse groups of mammals, the South American sigmodontine rodents. Sigmodontines experienced an adaptive radiation following their colonization from North America approximately seven million years ago. Soon after colonizing South America, they expanded into many new regions quickly at the same time as they had a burst of diversification, and their dispersal across the heterogeneous landscape was important to their adaptive radiation. As time passed, both the rate of geographic dispersal and speciation slowed down in concert. While ecological divergence into different niches is important during adaptive radiations, movement into new and distinctive geographic regions allows speciation to repeatedly reoccur and can promote the remarkable biodiversity characteristic of adaptive radiations.
Although the importance of biogeography in the speciation process is well-recognized, the fundamental role of geographic diversification during adaptive radiations has not been studied to determine its importance during the adaptive radiation process. We examined the relationship between lineage and regional diversification patterns in the South American rodent subfamily Sigmodontinae, one of the best candidates for an adaptive radiation in mammals, to propose a conceptual framework for geographic transitions during adaptive radiations. We reconstructed a time-calibrated phylogeny from four nuclear and one mitochondrial gene for 77% of sigmodontine diversity. Historical biogeography was reconstructed among 14 regions, to which we applied a sliding-window approach to estimate regional transition rates through time. We compared these rate patterns and measured whether regions consisted of species that were more phylogenetically related than expected by chance. Following the initial South American colonization around 7 million years ago, multiple expansions from northern regions correlated with a burst of speciation. Subsequently, both diversification and regional transition rates decreased overall and within the majority of regions. Despite high regional transition rates, nearly all regional assemblages were phylogenetically clustered, indicating within-region diversification was common. We conclude that biogeographic complexity and partitioning played a profound role in the adaptive radiation of the South American Sigmodontinae (Oryzomyalia), the degree to which is determined by the relative scales of spatial variation and dispersal abilities.