The spatial scale of speciation and patterns of diversity

Abstract

Many environmental factors and taxon traits have been studied as potential controllers of diversification, but there is still no consensus as to which are most important or how to link them into a general theory of diversification. I hypothesise that diversification is strongly controlled by the interaction between area and clades’ spatial scales of speciation, or the amount of area they require for speciation to occur. Furthermore, I hypothesise that the spatial scale of speciation is controlled by population genetic characteristics of clades, as speciation is ultimately a process of population divergence. In this thesis, I quantify taxonomic variation in the spatial scale of speciation, test whether it can be explained by variation in population genetics and evaluate whether it can explain taxonomic patterns of diversity. Using a survey of speciation events on isolated oceanic islands, I show that the spatial scale of speciation varies greatly between birds, lizards, snails, bats, carnivorous mammals, lepidoptera, angiosperms and ferns. I also use a meta-analysis of population genetic data collected from the literature to show that the minimum area for speciation of these groups correlate strongly with their average strength of gene flow. I then test the link between population genetics and diversification by comparing population genetic characteristics of sister clades of tropical orchids that differ greatly in species richness. Contrary to expectation, levels of gene flow, genetic drift and local adaptation do not correlate directly with rates of diversification. However, there is some evidence for an interaction between species range size and gene flow in controlling diversification. This thesis supports a framework based on the interaction between the spatial scale of speciation and area as a useful foundation for general theories of diversification. It also highlights the potential for using a comparative population genetics approach in macroevolutionary studies.