A latitudinal phylogeographic diversity gradient in birds

Abstract

High tropical species diversity is often attributed to evolutionary dynamics over long timescales. It is possible, however, that latitudinal variation in diversification begins when divergence occurs within species. Phylogeographic data capture this initial stage of diversification in which populations become geographically isolated and begin to differentiate genetically. There is limited understanding of the broader implications of intraspecific diversification because comparative analyses have focused on species inhabiting and evolving in restricted regions and environments. Here, we scale comparative phylogeography up to the hemisphere level and examine whether the processes driving latitudinal differences in species diversity are also evident within species. We collected genetic data for 210 New World bird species distributed across a broad latitudinal gradient and estimated a suite of metrics characterizing phylogeographic history. We found that lower latitude species had, on average, greater phylogeographic diversity than higher latitude species and that intraspecific diversity showed evidence of greater persistence in the tropics. Factors associated with species ecologies, life histories, and habitats explained little of the variation in phylogeographic structure across the latitudinal gradient. Our results suggest that the latitudinal gradient in species richness originates, at least partly, from population-level processes within species and are consistent with hypotheses implicating age and environmental stability in the formation of diversity gradients. Comparative phylogeographic analyses scaled up to large geographic regions and hundreds of species can show connections between population-level processes and broad-scale species-richness patterns.

Genomic divergence in allopatric Northern Cardinals of the North American warm deserts is linked to behavioral differentiation

“…We dated the divergence between the Sonoran Desert and Chihuahuan Desert Northern Cardinals at approximately 990,000 years. This divergence is similar to estimates for bird species ages in the region, which typically date to the Pleistocene (Smith, Seeholzer, Harvey, Cuervo, & Brumfield, ). Divergence dates across the Cochise Filter Barrier, for various taxa, range from 500,000–5,000,000 years (Bryson, García‐Vázquez, & Riddle, ; Bryson, Jaeger, Lemos‐Espinal, & Lazcano, ; Klicka, Kus, & Burns, ; Leaché & Mulcahy, ; Myers et al, ; O'Connell, Streicher, Smith, & Fujita, ; Pyron & Burbrink, ; Smith & Klicka, ; Weyandt & Van Den Bussche, ; Wilson & Pitts, ; Zink & Blackwell‐Rago, ), suggesting that population connectivity and isolation have been dynamic in the region.…”

Latitudinal biodiversity gradients at three levels: Linking species richness, population richness and genetic diversity

“…On one hand, because tropical species tend towards smaller geographic ranges, one could envision lower genetic diversity in these groups. On the other hand, tropical species tend to be older and inhabit more stable environments, and so some authors have suggested that genetic diversity could be maintained/accumulated throughout time (Adams & Hadly, ; Smith et al, ). Complicating expectations further, temperate species have a longer history of fragmentation, bottlenecks and founder effects, which all may contribute to a sharp decline in genetic diversity at high latitudes.…”

A positive association between population genetic differentiation and speciation rates in New World birds

“…We examined previously published population-level mitochondrial datasets of New World birds, including a subset that we generated for this and related projects (22). We restricted our sampling to those datasets containing at least 10 samples (mean = 101) and range-wide coverage.…”

“…Population differentiation in temperate areas, for example, may be more ephemeral than in tropical regions (20)(21)(22), which could loosen its association with speciation rates. Alternatively, if differentiated populations form readily at low latitudes, but species formation is limited by the availability of vacant niches or the evolution of novel ecologies (23,24), population differentiation may be a poorer predictor of speciation rate in the Tropics.…”