Reconstruction of State-Dependent Diversification: Integrating Phenotypic Traits into Molecular Phylogenies

The relative contribution of speciation and extinction into current diversity is certainly unknown, but mathematical frameworks that use genetic information have been developed to provide estimates of these processes. To that end, it is necessary to reconstruct molecular phylogenetic trees which summarize ancestor-descendant relationships as well as the timing of evolutionary events (i.e., rates). Nevertheless, diversification models show poor fit when assuming that single rate of speciation/extinction is constant over time and across lineages: species exhibit such a great variation in features that it is unlikely they give birth and die at the same pace. The state-dependent diversification framework (SSE) reconciles the species phenotypic variation with heterogeneous rates of diversification observed in a clade. This family of models allows testing contrasting hypotheses on mode of speciation, trait evolution, and its influence on speciation/extinction regimes. Although microbial species richness outnumbers diversity in plants and animals, diversification models are underused in microbiology. Here, we introduce microbiologists to models that estimate diversification rates and provide a detailed description of SSE models. Besides theoretical principles underlying the method, we also show how SSE analysis should be set up in R. We use pH evolution in Thaumarchaeota to explain its evolutionary dynamic in the light of SSE model. We hope this chapter spurs the study of trait evolution and evolutionary outcomes in microorganisms.