Abstract
Species may respond to variation in environmental conditions by
modifying their phenotype to match local levels of resource
availability. This phenotypic response can be driven by plastic
physiological change, or by adaptive genetic change. Here we useLemna minor (lesser duckweed), a small aquatic macrophyte that is
increasingly used as a model in ecology and evolution, to investigate
the source and maintenance of phenotypic variation in natural
environments. We found substantial phenotypic variation in L.
minor in the field, with its frond area and root length changing
predictably over natural environmental gradients of resource
availability. Separating environmental and genetic variation in these
traits in a common garden, we attribute the majority of phenotypic
variation we observed in the field to phenotypic plasticity. Despite
this, there was substantial within-site genetic variation. We found
evidence of strong purifying selection in the field, that is necessarily
balanced by mutation and migration. Using measures of environmental and
genetic variation in phenotype and fitness, we estimate rates of
evolution of fitness, and dispersal necessary to sustain the observed
levels of genetic variation.