Abstract
The degree of phenotypic divergence and reproductive isolation between taxon pairs can vary quantitatively, and often increases as evolutionary divergence proceeds through various stages, from polymorphism to population differentiation, ecotype and race formation, speciation, and post-speciational divergence. Although divergent natural selection promotes divergence, it does not always result in strong differentiation. For example, divergent selection can fail to complete speciation, and distinct species pairs sometimes collapse (‘speciation in reverse’). Widely-discussed explanations for this variability concern genetic architecture, and the geographic arrangement of populations. A less-explored possibility is that the degree of phenotypic and reproductive divergence between taxon pairs is positively related to the number of ecological niche dimensions (i.e., traits) subject to divergent selection. Some data supporting this idea stem from laboratory experimental evolution studies using Drosophila, but tests from nature are lacking. Here we report results from manipulative field experiments in natural populations of herbivorous Timema stick insects that are consistent with this ‘niche dimensionality’ hypothesis. In such insects, divergent selection between host plants might occur for cryptic colouration (camouflage to evade visual predation), physiology (to detoxify plant chemicals), or both of these niche dimensions. We show that divergent selection on the single niche dimension of cryptic colouration can result in ecotype formation and intermediate levels of phenotypic and reproductive divergence between populations feeding on different hosts. However, greater divergence between a species pair involved divergent selection on both niche dimensions. Although further replication of the trends reported here is required, the results suggest that dimensionality of selection may complement genetic and geographic explanations for the degree of diversification in nature.
Highlights
The ecological niche is a key concept in ecology [1,2,3,4,5,6,7,8,9,10], and plays a central role in evolutionary divergence
We found that the species pair examined here was more phenotypically and evolutionarily divergent than previously studied ecotype pairs, and that the species pair was subject to divergent selection on a greater number of niche dimensions
The findings suggest that selection on a greater number of niche dimensions promotes evolutionary divergence
Summary
The ecological niche is a key concept in ecology [1,2,3,4,5,6,7,8,9,10], and plays a central role in evolutionary divergence. Recent years have seen numerous examples of this process in a wide range of taxa [17,18,19]. Another increasingly realized factor is the often continuous nature of evolutionary divergence (even if the end point of the process is the development of a discontinuity) [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. The key point is that different taxon pairs may, at any point in time, exhibit different degrees of phenotypic, reproductive, and genetic divergence
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