Why? It's because freshwater habitats limit dispersal, confining organisms to a well-defined space (most fishes cannot walk out of a river or lake). Therefore, their evolutionary relationships tend to closely reflect biogeographic history.
In a study published in Evolution, I showed that lineages in rivers (blue, left) tend to be old on average (lineages diversifying in river systems near the same place for millions of years, even if the river itself has changed course, split apart, etc). However, lineages in lakes (red, left) tend to have fast speciation rates (producing many species quickly even if not for a very long time). These patterns are repeated around the world, suggesting that freshwater systems can help us uncover "rules" of evolution.
In a study published in Evolution, I showed that lineages in rivers (blue, left) tend to be old on average (lineages diversifying in river systems near the same place for millions of years, even if the river itself has changed course, split apart, etc). However, lineages in lakes (red, left) tend to have fast speciation rates (producing many species quickly even if not for a very long time). These patterns are repeated around the world, suggesting that freshwater systems can help us uncover "rules" of evolution.
Scientists since Darwin have believed that bright colors are related to speciation, especially colors involved in courtship. Closely related fish species often differ only in color (red versus blue, for example). Meanwhile, many other fish groups are drab in color. Can this explain why some fish groups have tons of species, while other groups have few species? In a study published in The American Naturalist, we found the surprising result that the effect of sexual dichromatism (red, left) on speciation rates is inconsistent among fish clades (some clades show a strong effect, others have a negative or no effect). Can we predict when sexual dichromatism will matter? Freshwater habitats can help here too. For example, perhaps mating colors help separate species that live together in the same community, but matter less when species live in different rivers and never meet.
Key publications:
Miller, E. C. 2021. Comparing diversification rates in lakes, rivers, and the sea. Evolution, 75: 2055–2073.
Miller, E. C., S. L. Mesnick, and J. J. Wiens. 2021. Sexual dichromatism is decoupled from diversification over deep time in fishes. The American Naturalist, 198: 232–252.
Miller, E. C. and C. Román-Palacios. 2021. Evolutionary time best explains the latitudinal diversity gradient of living freshwater fish diversity. Global Ecology and Biogeography 30: 749–763.
Reeling in answers to the "freshwater fish paradox" by Amy McDermott (featuring an interview about this research)
MinuteEarth Youtube video explaining the freshwater fish paradox
Miller, E. C. 2021. Comparing diversification rates in lakes, rivers, and the sea. Evolution, 75: 2055–2073.
Miller, E. C., S. L. Mesnick, and J. J. Wiens. 2021. Sexual dichromatism is decoupled from diversification over deep time in fishes. The American Naturalist, 198: 232–252.
Miller, E. C. and C. Román-Palacios. 2021. Evolutionary time best explains the latitudinal diversity gradient of living freshwater fish diversity. Global Ecology and Biogeography 30: 749–763.
Reeling in answers to the "freshwater fish paradox" by Amy McDermott (featuring an interview about this research)
MinuteEarth Youtube video explaining the freshwater fish paradox