How diverse can rare species be on the margins of genera distribution?

Many rare or narrowly distributed species show low levels of genetic diversity, whilst species with larger geographic ranges show increased diversity. Species that inhabit the border of the distribution of their respective genera usually have smaller and more fragmented populations and can have fixed locally adapted alleles. Different genetic patterns have been demonstrated for these narrowly distributed taxa, many of them linking rarity to evolutionary history and historical climate change. Quite a few species in young genera are endemic and have several populations that present low variability, sometimes attributed to geographical isolation or dispersion processes. Assessing the genetic diversity and structure of such species may be important for protecting them and understanding their diversification history.

In a recent Editor’s Choice article in AoBP, Backes et al. used microsatellite markers and plastid sequences to characterize the levels of genetic variation and population structure of two endemic and restricted species (Petunia mantiqueirensis and Calibrachoa elegans) in high altitude Brazilian forests. Both species grow in isolated areas on the margin of the distribution of their respective genera. Plastid and nuclear diversities were low and weakly structured in their populations. Evolutionary scenarios for both species are compatible with open-field expansions during the Pleistocene interglacial periods and genetic variability supports founder effects to explain diversification. Both species are suffering from habitat loss and further changes in the environment could lead these species towards extinction. The authors conclude by suggesting urgent conservation strategies to save these two threatened species. In situpreservation is necessary however in an extreme situation ex situ intervention (i.e. creation of a seed bank) could be carried out. Further studies of these two species will help to create an efficient conservation plan.