COS 235-5 - Do the species with large geographic range diversify faster?

Abstract: Range size is a universal characteristic of every biological species, and is often assumed to influence diversification rates. However, the relationship between range size and past and future diversification of species remains elusive. On one hand, there are strong theoretical arguments that large-ranged species should have higher rates of diversification. On the other hand, the small-ranged species often have shorter phylogenetic branches in the empirical datasets, leading to a negative relationship between range size and phenomenological diversification metrics, and suggesting fast diversification of small-ranged species. This discrepancy between theory and the data may be caused by the fact that typical methods of data analysis do not account for range size changes during speciation, and that the phylogenetic aggregations of small-ranged species may reflect past rather than incipient radiations. Understanding the relationship between range size and diversification is important for predicting species future evolutionary potential, especially in the context of nature conservation concepts such as endemism or IUCN threat status.

Here we use a state-dependent diversification model accounting for changes of range size both during (cladogenetic) and outside (anagenetic) speciation, and apply them to range size data and phylogenies of birds and mammals. We show that range size changes during speciation are ubiquitous and small-ranged species indeed diversify generally slower, as theoretically expected. Interestingly, the diversification processes with range size shrinkage during speciation systematically produce phylogenies with negative relationship between range size and phenomenological diversification metrics, similar to the empirical phylogenies. Although the diversification is driven by large-ranged species in most of our datasets, we also identified multiple clades where this general pattern is reverted. According to the ancestral state reconstructions, these radiations of small-ranged species often take place in volant taxa inhabiting isolated insular landscapes such as oceanic archipelagos.

Our results suggest that, while range size is an important factor influencing species diversification, its effect in the real-world systems is context-dependent, making the range size itself an unreliable predictor of species evolutionary future. At the same time, the phenomenological measures of diversification such as diversification rate metric, or various methods for identifying neoendemism, may often identify species or areas that underwent fast diversification in the past, but their future potential is low. We argue that future efforts for estimating species evolutionary potential should integrate range size and phylogenetic data using more realistic assumptions about range-size evolution, account for local spatial and phylogenetic context, and possibly also integrate population-level genetic data.