What drives the big changes in evolution, biological interactions or geography?

Why are there so many flowering plants? Where does all the diversity come from? Is it scrambled genes? Or colonization of new lands? Or even from interactions with other species? A paper by Robin Aguilée and colleagues argues that all three features have an effect, with different processes becoming more dominant in different phases of evolution.

The conclusions come from a model developed by the authors. Writing in Nature Communications they say: “The model scales up from local ecological interactions and individual dispersal, integrates the evolution of prezygotic and postzygotic mechanisms of reproductive isolation, and represents geographical events on a slow timescale as an alternation of geographic isolation and contact between populations. Heritable traits of individuals, such as body size, influence their life history and ecological interactions and thus shape the ecological state of the community, which in turn generates selection driving trait evolution. The eco-evolutionary feedback between the population trait distribution and the state of the ecological system drives phenotypic evolution. Emerging eco-evolutionary theory emphasizes that phenotypic diversification itself can mold the ecological niches of interacting species; our model is designed to capture how feedbacks between ecological and evolutionary processes interact with geographic and other large-scale abiotic factors to shape speciation and extinction rates over a clade’s history.”

The model predicts that evolution for a clade runs in three phases. A clade starts by adapting to the geography open to it, diversifying in the process. Next comes a period of biotic interaction, through competition and hybridization. Extinction rates rise as the best-adapted species squeeze out their nearest competition. As competition becomes more intense, evolution enters a final phase where new populations cannot establish due to the presence of rivals. Now there is a negative diversity-dependence of speciation rates.

The authors say the model provides an argument against the usual explanation for negative diversity-dependence of speciation, that ecological niches get filled. They state the model also shows that there is a geographic element to the problem with niche filling causing the new populations to fail at spreading the geographical range of a species. Aguilée and colleagues say: “The emergence and shape of speciation negative diversity dependence thus depends on the interplay between the characteristics of competition (niche width) and the spatial scale of the landscape, which itself is determined by the minimal unit of spatial isolation relative to the organism’s characteristic dispersal distance.”

Understanding how these evolutionary phases play out could help develop new phylogenetic models and identify key stages of diversification in a clade’s history.