Genetic and Phenotypic Variation in Relation to Population Size in Two Plant Species: Salvia pratensis and Scabiosa columbaria

Due to human activities many populations have become small, fragmented and isolated and consequently more sensitive to genetic drift and inbreeding, resulting in loss of genetic variation and fixation of deleterious alleles. This process often involves a decrease in viability (inbreeding depression) and therefore may significantly affect the probability of extinction of populations. To assess the relevance of these predictions, the amount of genetic variation was measured in relation to population size for two plant species, Salvia pratensis and Scabiosa columbaria. Both species are endangered in The Netherlands and show a considerable decline in number of populations during the last 30 years. For allozymes significant correlations were observed between population size and both the proportion of polymorphic loci and the mean observed number of alleles, the large populations being more variable than the small populations. In addition, substantial genetic differentiation was observed between populations, the differentiation being more extensive among small populations than among large populations. Concerning morphological characters also a positive relationship between the total amount of phenotypic variation and population size was observed. The results suggest that, predominantly due to genetic drift, small populations are depauperate in genetic variation. Although small populations, at least for Salvia, showed some indications of decreased viability, much more research is needed to decide whether these low levels of genetic variation influence the probability of extinction significantly.