Where is my sister?: Habitat use, range overlap, and co-occurrence in the California Floristic Province
A debate continues on the relative importance of history, evolutionary constraints, and current ecological processes in organizing species into communities. The study of closest relatives (i.e., sister taxa) lies at the nexus of these processes, as historical and ecological forces combine to influence their geographic ranges and their degree of ecological similarity. Speciation mode can leave an imprint on the geographic distribution of sister taxa, with high range overlap expected in cases of sympatric speciation and low range overlap in cases of allopatric speciation. Due to their genetic similarity and shared ancestry, sister taxa may also be generally more ecologically similar in traits and environmental preferences than non-sisters. We examined the geographic distribution and niche attributes of 72 sister pairs in the California flora, including 19 genera for which there were complete phylogenies. We asked if range overlap among sister taxa is common, if time since divergence is related to range overlap, and if there is evidence for strong niche conservatism or character displacement. Species-level attributes examined included climate niche, habitat affinity, growth form, plant height, and flowering time.
We found that most sister taxa had narrowly overlapping, parapatric distributions, but this limited range overlap was significantly higher than overlap between non-sister pairs. Sisters were more similar in climate niche, habitat use, growth form, plant height, and flowering time than based on comparison with randomly selected congeners, indicating conservatism of these traits. We found no evidence for character displacement in habitat use; range overlap was actually highest among species in the same habitats and unrelated to contrasts of growth form, height, and flowering time. We also found that range overlap was greater in younger sister pairs. Our results combine to suggest that sister taxa co-occurrence is less related to ecological or habitat segregation than expected. Rather, speciation mechanisms may be most important in determining distribution patterns among sister taxa, highlighting the importance of historical and evolutionary explanations for local community diversity.