Climate and the evolution of serpentine endemism in California

Abstract

We asked whether evolutionary transitions to serpentine endemism are associated with transitions to more favorable environments. Theory and observation suggest that benign (e.g., high rainfall and less extreme temperatures) climates should favor the evolution of habitat specialism, both because such climates may facilitate persistence of small populations with novel adaptations, and because competition with non-specialists may be stronger in benign climates. Non-climatic factors, such as habitat availability, should also be associated with transitions to habitat specialism. We examined phylogenetic transitions to serpentine endemism in 23 Californian plant taxa. We contrasted transitions from serpentine-intolerant ancestors, where speciation entails novel adaptations to serpentine, with transitions from serpentine-tolerant ancestors, where the formation of a new serpentine-endemic taxon may result from an altered competitive environment. We found that transitions to endemism were strongly associated with transitions to regions with more benign climates, but only in the case of endemics arising from intolerant ancestors. In contrast, transitions to endemism from both types of ancestor were associated with transitions to regions with greater habitat availability. These results are consistent with the expectation that benign climates promote the persistence of small populations with novel adaptations both before and after speciation.