Comparing diversification rates in lakes, rivers, and the sea.

The diversity of species inhabiting freshwater relative to marine habitats is striking, given that freshwater habitats encompass <1% of Earth's water. The most commonly proposed explanation for this pattern is that freshwater habitats are more fragmented than marine habitats, allowing more opportunities for allopatric speciation and thus increased diversification rates in freshwater. However, speciation may be generally faster in sympatry than in allopatry, as illustrated by lacustrine radiations such as African cichlids. Such differences between rivers and lakes may be important to consider when comparing diversification broadly among freshwater and marine groups. Here I compared diversification rates of teleost fishes in marine, riverine and lacustrine habitats. I found that lakes had faster speciation and net diversification rates than other aquatic habitats. However, most freshwater diversity arose in rivers. Surprisingly, riverine and marine habitats had similar rates of net diversification on average. Biogeographic models suggest that lacustrine habitats are evolutionarily unstable, explaining the dearth of lacustrine species in spite of their rapid diversification. Collectively, these results suggest that strong diversification rate differences are unlikely to explain the freshwater paradox. Instead, this pattern may be attributable to the comparable amount of time spent in riverine and marine habitats over the 200-million-year history of teleosts.