Bridging Time Scales in Evolutionary Biology

Evolution is, fundamentally, a tug-of-war around biological diversity in which two main types of opposing processes are involved. Diversity is generated by means of mutation, migration, and recombination; diversity is reduced by natural selection of fitter individuals over less-fit ones among a population. An additional component, known as genetic drift, is commonly acknowledged as the main random force in evolution. The diversity resulting from this tug-of-war can be observed at two different time and biological scales: between individuals of the same species at the population-genetic scale and between different species at the phylogenetic scale. The goal of this chapter is to formally describe the bridge between these two biological scales by defining diversity within each and by relating the underlying evolutionary processes between them. To this aim, we will describe the neutral and nearly neutral theories of evolution, setting the mathematical framework that will allow us to link the mutation rate at the population-genetic scale, which defines the amount of diversity present within a population, with the substitution rate, which defines the divergence between species at the phylogenetic scale. We here review how the neutral theory not only allows, conceptually, to establish a null model to study evolution but also actually sets clear connections between two sub-disciplines, namely population genetics and phylogenetics, which study manifestations of the same phenomena at different time scales. We discuss some consequences of bridging these time scales in current research in evolutionary biology.