Abstract
Recent evidence has questioned whether the Latitudinal Diversity Gradient (LDG), whereby species richness increases towards the Equator, results in higher rates of speciation in the tropics. Allowing for time heterogeneity in speciation rate estimates for over 60,000 angiosperm species, we found that the LDG does not arise from variation in speciation rates because lineages do not speciate faster in the tropics. These results were consistently retrieved using two other methods to test the association between occupancy of tropical habitats and speciation rates. Our speciation rate estimates were robust to the effects of both undescribed species and missing taxa. Overall, our results show that speciation rates follow an opposite pattern to global variation in species richness. Greater ecological opportunity in the temperate zones, stemming from less saturated communities, higher species turnover or greater environmental change, may ultimately explain these results.
Highlights
Biodiversity on Earth is very unevenly distributed
We found that tip-specific speciation rates were consistently smaller in the tropics for
Using correlation tests that controlled for phylogenetic pseudoreplication (Rabosky & Huang 2016), we found that tropical species had smaller mean speciation rates (λ) than temperate species
Summary
The Latitudinal Diversity Gradient (LDG), whereby species richness increases towards the Equator, is the most prominent example of this unevenness. A major class of explanations proposes that elevated rates of speciation generate the higher diversity in the tropics and so primarily create the LDG (Mittelbach et al.2007). This increase in speciation is generally attributed to higher environmental energy in the tropics, which in turn can hasten evolution through shorter generation times and higher mutation rates (Dowle et al 2013), and is known as the “evolutionary speed hypothesis” (Rensch 1959; Rohde 1992). Recent speciation rates should be less affected by extinction (Nee et al 1994) and more accurate than deep-time estimates
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