A Phylogenetic Perspective On The Dynamics Of Speciation And Extinction During Evolutionary Radiations

Abstract

One of the most striking features of the natural world is that some groups of organisms are stunningly diverse while many other groups are species-poor. Both intrinsic, lineage-specific traits as well as extrinsic ecological factors can influence species richness through their influence on speciation and extinction rates. Here I investigate intrinsic and extrinsic influences on the diversification process, using a joint theoretical and empirical approach. I focused on reconstructing patterns of species diversification from time-calibrated molecular phylogenetic trees, which provide a valuable window into macroevolutionary tempo and mode for the large number of groups with inadequate fossil records. I review the manner in which species traits influence the dynamics of speciation and extinction, and I relate the literature on "species selection" to the emerging perspective on diversification rates from modern molecular phylogenetic studies. I argue that this literature demonstrates the effects of species selection on both species richness and the distributions of phenotypic traits in higher taxa. I develop tools for analyzing variation in speciation and extinction both over time and among lineages, including a new analytical approach for modeling rates that vary continuously through time. Using several real phylogenies and simulations, I show that, under the birth-death model, the phenomenon of early rapid diversification in phylogenies can only be explained by decreasing speciation through time in conjunction with low background extinction. I develop an alternative model that postulates constant diversity and a balanced speciation-extinction process that can also explain the rapid accumulation of lineages during the early stages of many radiations. I found evidence for an explosive increase in diversification in a lineage of Australian scincid lizards, suggesting that the evolution of traits associated with climate tolerance may have played a role in shaping patterns of diversity in this group. Using multiple published datasets, I tested three hypotheses for the absence of an age-diversity relationship in higher taxa, including (1) among-clade rate heterogeneity, (2) clade volatility, and (3) ecological limits on clade growth. Using a new modeling framework, I found that only ecological limits are capable of explaining the observed patterns.