Assessing the biological relevance of edaphic heterogeneity: Genetic differentiation and phenotypic plasticity in an invasive annual plant, Erodium cicutarium

Environmental heterogeneity is ubiquitous in natural systems, and in part determines the distributions of plant species both on landscape and local (0.5 to 10m) spatial scales. Broadly distributed plant species can respond to environmental heterogeneity via genetic differentiation between populations occurring in distinct environmental patches, so long as dispersal is rare relative to the strength of diversifying selection. Individuals experiencing heterogeneity on local spatial scales may persist by morphological or physiological changes in response to the environment (phenotypic plasticity). We conducted a full reciprocal transplant experiment between two non-serpentine and four serpentine sites in an area of the UC McLaughlin reserve with substantial edaphic heterogeneity. Previous field experiments revealed a surprising amount of small-scale edaphic heterogeneity both within and between these sites. Phenotypic traits at both early and late life stages were measured on all experimental plants. Analyses of the multivariate phenotype revealed plasticity in response to environmental heterogeneity on multiple spatial scales. Additionally, genotypes collected in serpentine and non-serpentine sites responded differentially to heterogeneity among destination sites and small-scale heterogeneity within a site. These results indicate that edaphic variation may drive distinct patterns of selection within and between populations of E. cicutarium. Phenotypic plasticity is favored in response to heterogeneity experienced by successive generations within the spatial scale of dispersal, but differential selection in distinct adjacent sites results in genetic differentiation between populations.