Collaborative Research: Novel framework for estimating continuously-varying diversification rates

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

Determining the factors that have shaped the huge differences in the number of species among groups of organisms, such as insects versus mammals, has long been of interest to both biologists and paleontologists. Mathematical models for understanding differences in species diversity use two quantities, the rate of speciation (when one species becomes two) and the rate of extinction (when a species disappears). These quantities are similar to those used in studying human populations, the birth rate and death rate. Although paleontologists have proposed explanations for differences in the numbers of species, biologists, using mostly family trees of species based on the analysis of genetic data, have largely concluded that speciation is more important than extinction in explaining these patterns. The central theme of this research is that what drives modern species diversity is not differences in speciation, but rather the differences in "turnover rate," which is defined as the birth rate minus the death rate. Turnover rate measures how often extinction and speciation events happen over long periods of time. This research will expand existing mathematical models for estimating species diversity to include new models that use quantities, such as turnover rate, that are much more tied to biology. The project will also provide a training workshop in these computational skills in the southeastern U.S., an important and underrepresented part of the country. This workshop will train students and other early career researchers to become a computationally literate STEM workforce, which is important to the economic well-being of the nation. The project will also provide professional training opportunities for two postdoctoral researchers. Darwin's great insight was a rejection of typological thinking in favor of recognizing the variation across life. Yet, in diversification analyses, typological thinking is not just a convenient crutch made for methodological convenience. Finding the points at which species shift from one rate "type" instantly to another has received unjustified attention. This research will move away from this typological thinking, towards more biological realism by assuming that diversification rates continuously vary. Central to this framework is a theme of rate "inheritance," where diversification rates evolve and are inherited from ancestor to descendant, much in the same way as a trait would evolve on a tree. Rather than focusing on speciation and extinction, or even net diversification, this research will focus biological interpretations on estimates of turnover and extinction fraction. The overarching hypothesis is that extinction fraction is near but probably somewhat less than one. In other words, speciation events slightly outnumber extinction events over time. What drives diversification patterns, therefore, is not speciation rate varying independent of extinction, or even extinction rate varying, but rather the overall turnover rate. This research will develop a new likelihood-based framework that evaluates a large family of diversification models that more realistically treat rates as continuously varying on a tree and focuses estimation on a new and more mechanistic parameterization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.