Genetic and plastic responses in geographic differentiation of Bromus rubens populations

Populations of B. rubens were sampled in the northern and central Coastal Ranges and northern Central Valley of California to study the geographic variation patterns. Quantitative estimates of variation were obtained on plants collected in nature, and their progenies were grown under controlled environments. Discriminant function analysis was used to estimate the variation among populations in both natural and controlled environments for four quantitative characters. These variations appear approximately proportional to the geographical distances among localities and to the measures of genetic differentiation. It was found that for these four quantitative characters, about 12–27% of the total geographic variation among natural populations of B. rubens was contributed by genetic differentiation, while the rest (73–88%) was contributed by the so-called plastic response (nongenetic variation) to different local environments. This finding suggested that phenotypic plasticity is important for the wide adaptability of this species.The similarity between the dendrogram patterns of both genetic similarity and phenotypic similarity for esterases and quantitative characters, respectively, suggested that the observed variation may be adaptive and correlated with environmental components of climate and habitat diversity. Such comparisons of different character sets may provide a useful way of deriving the adaptive basis of geographic variation.