Background and Aims Natural selection and genetic drift are important evolutionary forces in determining genetic and phenotypic differentiation in plant populations. The extent to which these two distinct evolutionary forces affect locally adaptive quantitative traits has beenwell studied incommonplant and animal species.However,we knowless about howquantitative traits respond to selection pressures and drift in endangered species that have small population sizes and fragmented distributions. To address this question, this study assessed the relative strengths of selection and genetic drift in shaping population differentiation of phenotypic traits in Psilopeganum sinense, a naturally rare and recently endangered plant species.
Methods Population differentiation at five quantitative traits (QST) obtained fromacommongarden experimentwas compared with differentiation at putatively neutral microsatellite markers (FST) in seven populations of P. sinense. QST estimates were derived using a Bayesian hierarchical variance component method.
Key Results Trait-specific QST values were equal to or lower than FST. Neutral genetic diversity was not correlated with quantitative genetic variation within the populations of P. sinense.
Conclusions Despite the prevalent empirical evidence forQST . FST, the results instead suggest a definitive role of stabilizing selection and drift leading to phenotypic differentiation among small populations. Three traits exhibited a significantly lower QST relative to FST, suggesting that populations of P. sinense might have experienced stabilizing selection for the same optimal phenotypes despite large geographical distances between populations and habitat fragmentation. For the other two traits, QST estimates were of the same magnitude as FST, indicating that divergence in these traits could have been achieved by genetic drift alone. The lack of correlation between molecular marker and quantitative genetic variation suggests that sophisticated considerations are required for the inference of conservation measures of P. sinense from neutral genetic markers.
