Population history, mating system, and fitness variation in a perennial herb with a fragmented distribution

Patterns of genetic diversity are strongly influenced by the history of populations, which is often ignored when examining the viability of populations. We studied the relationship of population size, genetic variation, and fitness in the perennial herb Vincetoxicum hirundinaria (Asclepiadaceae) in the southwestern archipelago of Finland, where this species occurs commonly on many of the islands and its distribution is highly fragmented. We measured the magnitude and distribution of genetic variation in this species to examine how patterns of genetic diversity reflect population history, especially the colonization pattern of the islands and the mating system. Additionally, we used the genetic data to interpret previous results that suggest that fitness variation among populations is not directly related to population size. We found a high level of genetic variation (He = 0.396) and a low level of genetic differentiation (FST = 0.052) among the populations. In contrast to several rare plant species, the level of genetic variation (H e) was not correlated with population size or with male or female fitness of the plants. Our results also indicated that inbreeding is common in all populations regardless of their size (mean FIS = 0.460) but that it does not correlate significantly with male or female fitness traits. The observed patterns of genetic variation suggest that this species has a mixed mating system. Taken together, these results indicate that the studied populations are of recent common ancestry but may also have relatively high levels of current gene flow. The differences between our results and those from previous studies of rare plant species with a fragmented distribution pattern emphasize the importance of considering whether fragmentation is caused by recent human activities or whether it reflects a distribution pattern characteristic of the species when evaluating the conservation needs of the species.