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Ecological niche divergence and evolution in western North American monkeyflowers Li, Qin

Abstract

The ecological niche is an essential concept for studies in ecology, evolution and biogeography. Geographic distributions are largely determined by species’ ecological niches. In turn, niches evolve via selection stemming from where species occur, which has implications for coexistence and the breadth of environmental tolerance. With modern comparative methods, we can improve our understanding of interactions among niche, range and diversification across spatial scales. To select variables for quantifying niche properties, I first applied generalized linear models with occurrence data of 71 western North American monkeyflowers (Mimulus sensu lato). Then I evaluated the relative importance of four bioclimatic variables by ranking them based on the magnitudes of model-averaged regression coefficients. Thus three out of four bioclimatic variables were identified as important predictors in determining geographic distributions of Mimulus species, while one variables was negligible due to its small effect. To determine how geographic overlap affects niche divergence, I quantified niche divergence for 16 closely related Mimulus species pairs. I found that macrohabitat niche divergence decreased with increasing range overlap, consistent with environmental filtering operating in sympatry and divergent selection operating in allopatry. For species pairs with partially overlapping ranges, greater microhabitat niche divergence was found in sympatry, consistent with competition driving divergence where species interact. Phylogenetic distance was positively related to niche divergence for two macrohabitat axes but negatively related for one microhabitat axis. This suggests increasing coarse-scale niche similarity with increasing sympatry following allopatric speciation, while greater local-scale niche divergence accumulates through time. Given differences in evolutionarily lability of niche axes across spatial scales, I next examined evolutionary trends in niche breadth. For 82 Mimulus species, I converted niche breadths into binary states, generalist or specialist. Then I tested whether niche breadth affected diversification rate and explored evolutionary transitions. My results showed higher diversification rates for generalists and weak generalist-to-specialist trends for three bioclimatic variables, but higher diversification rates for specialists and weak specialist-to-generalist trends for two microhabitat variables. Together, these results suggest that ecology plays an essential role in diversification processes, but underlying mechanisms might differ across spatial scales.

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