A classic hypothesis to explain the high diversity found in tropical rain forests is that areas within the tropics served as climatic refuges during Pleistocene global climate fluctuations (e.g., ice ages). These refuges beget diversity because they face much lower extinction rates then non-refuges and they are older, allowing speciation events to accrue. This hypothesis has proven controversial as evidence has been circumstantial and circular (i.e., high diversity areas are taken as evidence of a refuge and the outcome of a refuge is high diversity.).
This conundrum has been solved by Ana Carnaval, a postdoctoral researcher in Craig Moritz's lab at UC Berkeley, and colleagues. They use patterns of diversity to identify probable refuges and then support several independent hypotheses about refuge effects on patterns of frog diversity. They show 1) that there is higher genetic diversity within and among refuge populations relative to non-refuges. 2) They show a multi-species pattern of recent population expansion in non-refuges from adjacent refuges. 3) The absence of isolating divergence in non-refuges because of a lack of time. Finally, 4) strong phylogenetic patterns of between refuge structure, indicating periods of isolation and divergence.
This paper reveals that hypotheses about the origin of species diversity in hotspots can be tested by using genetic divergence below the species level. Not only does this strongly support the spatial refuge hypothesis for tropical diversity patterns but it also elegantly intertwines microevolutionary processes with macroevolutionary patterns. There couldn't have been a more appropriate study published in the week before Darwin's birthday.
A. C. Carnaval, M. J. Hickerson, C. F. B. Haddad, M. T. Rodrigues, C. Moritz (2009). Stability Predicts Genetic Diversity in the Brazilian Atlantic Forest Hotspot Science, 323 (5915), 785-789 DOI: 10.1126/science.1166955
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