My work seeks to understand the role of evolutionary rate and history in promoting the formation of the latitudinal diversity gradient in New World birds. Faster evolutionary rates in the tropics could promote a rapid buildup of species richness, resulting in the latitudinal diversity gradient. I test this by looking at latitudinal differences in the rate at which reproductive isolation evolves. Contrary to expectations of faster rates in the tropics, evolutionary rates in traits important for reproductive isolation evolved faster at high latitudes. These results are mirrored by phylogenetic estimates of rates of cladogenesis and extinction, both of which are elevated at high latitudes, and slowest in the tropics. Despite slower rates of reproductive isolation and cladogenesis, the tropics may have elevated rates of net diversification (the difference between the rate of cladogenesis and extinction), which could explain the gradual accumulation of high diversity there. Another important factor in generating high species richness in the New World tropics is biotic interchange. Using calibrated phylogenies, I show that despite their ability to fly, bird interchange between North and South America was limited until the completion of the Central American Landbridge. Following landbridge completion, avian and mammalian faunas that evolved in isolation in each continent merged rapidly at tropical latitudes, resulting in a steeper latitudinal diversity gradient than observed in the Old World.

”‹Sponsored by the UMMZ Norman E. Hartweg Fund