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.

Amphibians crossing the Bering Land Bridge: Evidence from holarctic treefrogs (Hyla, Hylidae, Anura)

“…Consistent with previous studies we date the MRCA of the genus Hyla into the Middle Eocene to Early Oligocene (Smith et al, 2005;Wiens et al, 2006) and confirm the origin of the genus being in North/Central America. Also, the occurrence of two dispersal events out of America as previously suggested by Hua et al (2009) is supported by the present analysis.…”

“…We can confirm monophyly of the genus Hyla as retrieved in several previous studies (Faivovich et al, 2005;Smith et al, 2005Smith et al, , 2007Wiens et al, 2005Wiens et al, , 2006Wiens et al, , 2010Hua et al, 2009). Differences from the study of Hua et al (2009) concern monophyly of the cinerea-and versicolor-groups as well as the phylogenetic relationships between these and the japonica-group.…”

“…16 Mya) fossil of Acris barbouri that was unearthed in North America, and which is probably the sister group of extant Acris species (Holman, 2003). As the genus Acris is in all previous phylogenies retrieved as the sister group of the genus Pseudacris (Smith et al, 2005(Smith et al, , 2007Wiens et al, 2006Wiens et al, , 2010, we set the minimum age for the divergence of Acris and Pseudacris to 16 Mya (5-95% interquantile range 19-16 Mya). (B) Fossil H. miofloridana was also described from Miocene Hemingfordian sediments in North America (20-16 Mya), which is supposed to be a close relative of extant H. gratiosa (Holman, 2003).…”

“…However, alternative trans-Atlantic routes (e.g., the DeGeer and Thulean routes, see above) might represent a possible alternative explanation. An estimate of the frequency of dispersal-route usage of 57 extant non-marine animal species suggested that transAtlantic routes (e.g., the DeGeer and Thulean routes, see above) were more common than trans-Pacific routes during the Paleogene (Early Tertiary) (Sanmartin et al, 2001), thus covering the period of the hypothesized divergence time of Asian and American species of Hyla 40-35 Mya (Smith et al, 2005;Wiens et al, 2006). However, previous studies lacked a complete taxon sampling and application of rigorous biogeographical methods to confirm this assumption and to investigate alternative dispersal routes.…”

Out of Asia: Natricine snakes support the Cenozoic Beringian Dispersal Hypothesis

“…Certainly, bidirectional dispersal across the Holarctic is possible for ectotherms, as seen in toads, ranid and hylid frogs, which crossed the between the OW and NW several times (Smith et al, 2005;Wiens et al, 2006Wiens et al, , 2009Pramuk et al, 2008). One possibility for the squamates is that return colonization from the NW was simply not possible due to degrading habitat at the end of Miocene in Beringia, an expectation of the CBDH.…”