Evolutionary and Ecological Causes of the Latitudinal Diversity Gradient in Hylid Frogs: Treefrog Trees Unearth the Roots of High Tropical Diversity

Abstract

Why are there more species in the tropics than in temperate regions? In recent years, this long-standing question has been addressed primarily by seeking environmental correlates of diversity. But to understand the ultimate causes of diversity patterns, we must also examine the evolutionary and biogeographic processes that directly change species numbers (i.e., speciation, extinction, and dispersal). With this perspective, we dissect the latitudinal diversity gradient in hylid frogs. We reconstruct a phylogeny for 124 hylid species, estimate divergence times and diversification rates for major clades, reconstruct biogeographic changes, and use ecological niche modeling to identify climatic variables that potentially limit dispersal. We find that hylids originated in tropical South America and spread to temperate regions only recently (leaving limited time for speciation). There is a strong relationship between the species richness of each region and when that region was colonized but not between the latitudinal positions of clades and their rates of diversification. Temperature seasonality seemingly limits dispersal of many tropical clades into temperate regions and shows significant phylogenetic conservatism. Overall, our study illustrates how two general principles (niche conservatism and the time-for-speciation effect) may help explain the latitudinal diversity gradient as well as many other diversity patterns across taxa and regions.

Mean family age of angiosperm tree communities and its climatic correlates along elevational and latitudinal gradients in eastern North America

“…Results of previous studies are mixed in terms of which of these two thermal factors play a more important role in driving species distributions (e.g. Qian et al., ; Wang et al., ; Wiens et al., ). However, few studies have assessed which of the two have played a more important role in driving community assembly, and all used latitudinal gradients as study systems (e.g.…”

“…Lastly, cold tolerance is considered a key trait under the tropical conservatism explanation for species distributions (Futuyma, ; Hawkins et al., ; Latham & Ricklefs, ), but both minimum temperature and temperature seasonality have been considered stressful thermal factors driving species distributions of plants and animals (e.g. Currie, ; Qian, Wang, Wang, & Li, ; Wang, Fang, Tang, & Lin, ; Wiens, Graham, Moen, Smith, & Reeder, ). Results of previous studies are mixed in terms of which of these two thermal factors play a more important role in driving species distributions (e.g.…”

“…However, few studies have assessed which of the two have played a more important role in driving community assembly, and all used latitudinal gradients as study systems (e.g. Wiens et al., ), across which minimum temperature and temperature seasonality can co‐vary strongly. We hypothesize that minimum temperature, which is closely related with cold tolerance, plays a more important role than temperature seasonality in shaping MFA patterns (prediction 4); we evaluate this hypothesis by assessing the relative strengths of the relationships of the two temperature variables with MFA for both latitudinal and elevational gradients, along the latter of which the two temperature variables co‐vary much less strongly (Jump et al., ; Körner, ).…”

Evolutionary history of tree squirrels (Rodentia, Sciurini) based on multilocus phylogeny reconstruction

“…Distribution of Sciurini squirrels exhibits latitudinal gradient in species richness (Wiens et al. 2006; Mittelbach et al.…”

“…In Sciuridae, the high species richness in equatorial regions on different continents is probably a result of higher diversification rate in the tropics based on higher lineage ‘birth’ and ‘death’ rates (Roth & Mercer 2008). Other hypotheses that explain the species richness gradient from tropics to the poles assume that lineages in the tropics are older or occupy larger areas facilitating lineage diversification through historical and spatial processes that influence populations (Stebbins 1974; Rosenzweig 1995; Chown & Gaston 2000; Fine 2001; Stephens & Wiens 2003; Wiens & Donoghue 2004; Wiens et al. 2006).…”