Ecological Marginalization Facilitated Diversification in Conifers

Species selection occurs when species traits influence speciation or extinction. But it is often difficult to demonstrate a net effect of traits on diversification, for example due to balancing effects of extinction and speciation. We tested if, since conifers lost their former status of dominant land plants to angiosperm trees, their extant diversity shows a signature of traits conferring resistance to extinction. We compared extant species richness across conifer genera in relation to phenotypic and ecological traits predicted to affect speciation and/or extinction, in two geographic regions that experienced distinct regimes of extinction and species turnover (the northern vs. southern hemispheres). Species richness had low phylogenetic signal across conifer genera, and species-rich genera were not older than species-poor ones, indicating intrinsic differences in diversification. High-altitude genera, which are ecologically more dissimilar to angiosperm trees, were more species-rich, suggesting that distinctiveness from angiosperm competitors facilitated conifer diversification. This effect seems more due to increased speciation than to reduced extinction, because it was unique to northern hemisphere genera, where rates of extinction and species turnover have been higher than in the south. We found no strong evidence that resistance to extinction was the main mechanism of species selection in conifers. Instead, having a marginal ecological niche relative to angiosperm competitors facilitated diversification in conifers, likely due to increased speciation. Resistance to extinction may nonetheless have been important in certain ancient lineages, and we discuss suggestive results for traits that may explain the endurance of such lineages.