What explains patterns of species richness? The relative importance of climatic-niche evolution, morphological evolution, and ecological limits in salamanders.

John J. Wiens,Kenneth H. Kozak

doi:10.1002/ece3.2301

Abstract

A major goal of evolutionary biology and ecology is to understand why species richness varies among clades. Previous studies have suggested that variation in richness among clades might be related to variation in rates of morphological evolution among clades (e.g., body size and shape). Other studies have suggested that richness patterns might be related to variation in rates of climatic‐niche evolution. However, few studies, if any, have tested the relative importance of these variables in explaining patterns of richness among clades. Here, we test their relative importance among major clades of Plethodontidae, the most species‐rich family of salamanders. Earlier studies have suggested that climatic‐niche evolution explains patterns of diversification among plethodontid clades, whereas rates of morphological evolution do not. A subsequent study stated that rates of morphological evolution instead explained patterns of species richness among plethodontid clades (along with “ecological limits” on richness of clades, leading to saturation of clades with species, given limited resources). However, they did not consider climatic‐niche evolution. Using phylogenetic multiple regression, we show that rates of climatic‐niche evolution explain most variation in richness among plethodontid clades, whereas rates of morphological evolution do not. We find little evidence that ecological limits explain patterns of richness among plethodontid clades. We also test whether rates of morphological and climatic‐niche evolution are correlated, and find that they are not. Overall, our results help explain richness patterns in a major amphibian group and provide possibly the first test of the relative importance of climatic niches and morphological evolution in explaining diversity patterns.

Highlights

Explaining patterns of species richness among clades is a major goal of evolutionary biology and ecology
25.2547 28.7885 25.8266 phenotypic evolution among them (Table 1), we found no evidence that declines in diversification rates over time within clades are associated with measures of niche filling
The relationship between gamma and size rate approaches significance, more negative values of gamma are associated with faster rates of size evolution (Fig. 2). This is the opposite of what would be expected if slowing rates of diversification were associated with slowing rates of phenotypic evolution, as predicted under the ecological limits hypothesis

Summary

Introduction

Explaining patterns of species richness among clades is a major goal of evolutionary biology and ecology. An important approach for answering this question is to test whether particular ecological, morphological, behavioral, or genetic traits are correlated with patterns of diversification (e.g., speciation and extinction over time) and species richness. Individual studies often tend to focus on a single variable, rather than comparing the relative importance of multiple traits on diversification or species richness. Two traits that might be broadly important in explaining patterns of diversification and species richness are climatic-niche evolution and the evolution of body shape and size. These traits might be broadly important for several reasons.

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