Functional traits in parallel evolutionary radiations and trait-environment associations in the Cape Floristic Region of South Africa
Evolutionary radiations present a unique opportunity to investigate the relationships between plant functional traits and the physical environment. If climate-driven adaptation plays a role in shaping morphological diversification, we expect to find close associations between traits and the environments where they are found. The Cape Floristic Region (CFR) of South Africa is an environmentally heterogeneous biodiversity hotspot, characterized by high levels of endemism and over 9000 plant species, most of which are found in only a few recent radiations. We ask (1) whether we can detect trait-environment associations consistent with adaptation and (2) whether relationships between two drastically different radiations display signatures of similar adaptive responses. Protea L. and Pelargonium L’Her ex Aiton are both species-rich genera in the CFR and overlap in their geographic distribution, yet they differ in growth form and life history attributes. We made field-intensive measurements of functional traits on individuals across the region and characterized the site-level climate with environmental variables that capture the major gradients in this region. We use a novel Bayesian multiple-response generalized linear mixed effects model with a phylogenetic component to relate whole-plant responses to multiple environmental covariates, and sampled from the posterior distributions to compare relationships between the two genera.
We examined 24 possible trait-environment associations in each genus, and we detected 9 in Protea and 13 in Pelargonium. Thus, in just under half (22) of the 48 cases we detected a relationship. Our ability to detect these associations at the within-biome level, while taking into account multiple forms of covariation, suggests a role for environmental adaption in trait evolution. Comparing the two lineages, we found only two cases with strong evidence for associations in opposite directions, while six show strong support for being in the same direction (either negative or positive). For 12 of these pairwise comparisons, the highest posterior probability is for the two genera having opposite signs, although the support for these was weak. Cases where the two genera align might point to generalizable adaptations to specific factors, while cases of discordance may reflect alternative life history strategies or other lineage-specific factors not taken into account in our analysis. Our results suggest that patterns in worldwide datasets may not hold at finer taxonomic and geographic scales, but they also suggest patterns of adaptation that may be shared across lineages within this biome.