“Speciation and the latitudinal diversity gradient: insights from the global distribution of endemic fish”

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Patrick J. Hanly, Gary G. Mittelbach, and Douglas W. Schemske

Endemic fish show that speciation is faster at lower latitudes even after controlling for the effects of age and area

Top left: Characidae (Moenkhausia pittieri), Lake Valencia, Venezuela. Top right: Cichlidae (Haplochromis nyererei), Lake Victoria. Bottom left: Mochokidae (Synodontis grandiops), Lake Tanganyika. Bottom right: Poeciliidae (Lamprichthys tanganicanus), Lake Tanganyika.
(Photo credits: top left by Michael Palmer; top right by Kevin Bauman, CC-BY 1.0; bottom left by Wikimedia user Haps, CC-BY-SA 3.0; bottom right by Loury Cédric, CC-BY-SA 4.0)

The tropics are extraordinarily rich in species, but understanding the ecological and evolutionary causes of such diversity is challenging. Particularly controversial is whether tropical and temperate regions differ in how quickly new species arise. Analyses of DNA-based molecular phylogenies are increasing used to estimate latitudinal differences on speciation rates, but as yet there is little consensus in their conclusions (for example, see recent articles by Machac and Graham (2016) and Schluter (2016) in The American Naturalist). Here, three researchers from Michigan State University (Patrick Hanly, Gary Mittelbach, and Douglas Schemske) take a different approach to the question of whether latitude affects the process of speciation by examining the global distribution of endemism in freshwater fish. The world’s lakes and rivers contain tens of thousands of species fish – thousands of which are endemic (found in a single lake or in a single river basin). Because endemic species are unique evolutionary lineages confined to a single locale, they can be used to explore the conditions promoting the generation of new species.

Using a new compilation on the distribution of 1,933 single-lake endemic fish in the world’s largest lakes, Hanly and colleagues show that fish diversification is linked to latitude, lake age, and lake surface area. As expected from previous studies, the generation of endemic species occurs more frequently in both older and larger lakes. However, Hanly et al.’s analysis shows that there is an additional positive effect of low (tropical) latitude on the generation of species and this latitudinal effect is similar in magnitude to the effects of age and area. These results suggest that the evolution of biodiversity is not a consistent process across the globe and that factors such as climatic stability, the importance of biotic interactions, and the exploitation of unique niches may contribute to the evolution of the richness of life in the tropics. Read the Article