The disparity in species richness between continents and oceans is one of the most dramatic biodiversity gradients on earth, with diversity on continents far exceeding that in oceans. I am interested in the evolutionary mechanisms that generated these disparate patterns of diversity. For example, using state-depenent diversification analyses, I investigated the evolutionary processes that have generated these disparate patterns using silverside fishes as a model system. I found that freshwater silversides (blue in figure below) have higher speciation (left panel) and extinction rates (right panel) than marine silversides (red in fig below). However, the overall faster net diversification rate in freshwater lineages best explains higher species richness found in freshwater (continental) habitats than oceans. Lineages have accumulated constantly over time in both marine and freshwater biomes, suggesting ecological limits are not regulating clade growth. Our study provides empirical evidence that differences in net diversification rate is the macroevolutionary mechanism that best explains the difference in diversity between continents and oceans. Ongoing research includes testing for differences in diversification rates between and freshwater fishes in other groups of fishes to determine the generality of the patterns shown below.
Figures from Bloom et al. 2013 Evolution.