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Evolution of the latitudinal species diversity gradient of New World birds and mammals Weir, Jason Tyler

Abstract

The latitudinal diversity gradient in which species diversity is highest near the equator and declines toward the poles is well characterized in most higher level taxonomic groups and is strongest in the New World. However, the underlying causes of this gradient are poorly understood. By sampling New World birds and mammals, we found that the distribution of the evolutionary ages of sister species adheres to a latitudinal gradient. The time to divergence for sister species is shortest at high latitudes and longer in the tropics. Birth-death models fitting these data estimate that the highest recent speciation and extinction rates occur at high latitudes and decline toward the tropics suggesting that rates of species turnover are greatest where diversity is lowest. A pattern of endemism in boreal superspecies plausibly links the recent divergence of high latitude species to the fragmentation of the North American boreal forest by climatic oscillations during the Pleistocene ice ages. While all boreal superspecies tested date to the Pleistocene, only 56% of sub-boreal superspecies members date to this period. Similarly, montane tropical faunas, which were also directly impacted by buildup of glaciers during ice age cycles, were composed of younger species than low elevation faunas. Together these results suggest that faunas directly fragmented by expanding and retracting ice sheets experienced rapid rates of species turnover contributing to the lower species diversity of these regions. In contrast, the older ages of faunas at lower latitudes and altitudes suggest that long-term climatic stability promoted the gradual accumulation of high species diversity in these regions. Faunal interchange between continents may also have contributed to the exceptionally high species diversity of the New World tropics. Molecular dating suggests interchange rates between the distinctive avifaunas of North and South America were significantly faster after completion of the Central American landbridge three million years ago. Composition of tropical bird faunas were directly impacted by a relatively recent burst of faunal mixing which had the potential to elevate Neotropical species diversity over other tropical regions.

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