Selection, drift and gene flow shape genetic variation within and among natural populations and their study is important for conservation and evolutionary biology. Habitat destruction and fragmentation affect these processes by reducing the size and increasing the isolation of populations. Plants in small and isolated populations often have a lower performance than those in larger, interconnected populations due to increased inbreeding. They have lower levels of genetic variation, limited evolutionary potential and they are more strongly threatened by random environmental fluctuations. As a consequence, fragmented populations have a higher risk of becoming extinct.
A recent paper in Annals of Botany presents the results of a study of the quantitative genetic variation within and among Saxifraga sponhemica populations and the performance of plants in a common garden to address the following questions. (1) What has been the relative importance of drift and selection in shaping the distribution of quantitative genetic variation? In particular, is there evidence for adaptive differentiation along climatic gradients for the rare S. sponhemica? (2) Do the fitness of plants, quantitative genetic variation, molecular genetic variation and population size correlate positively? (3) Is the differentiation of a trait among populations and its evolvability positively correlated?
Walisch, T.J., Colling, G., Bodenseh, M., & Matthies, D. (2015) Divergent selection along climatic gradients in a rare central European endemic species, Saxifraga sponhemica. Annals of Botany 115 (7): 1177-1190.
The effects of habitat fragmentation on quantitative genetic variation in plant populations are still poorly known. Saxifraga sponhemica is a rare endemic of Central Europe with a disjunct distribution, and a stable and specialized habitat of treeless screes and cliffs. This study therefore used S. sponhemica as a model species to compare quantitative and molecular variation in order to explore (1) the relative importance of drift and selection in shaping the distribution of quantitative genetic variation along climatic gradients; (2) the relationship between plant fitness, quantitative genetic variation, molecular genetic variation and population size; and (3) the relationship between the differentiation of a trait among populations and its evolvability. Genetic variation within and among 22 populations from the whole distribution area of S. sponhemica was studied using RAPD (random amplified polymorphic DNA) markers, and climatic variables were obtained for each site. Seeds were collected from each population and germinated, and seedlings were transplanted into a common garden for determination of variation in plant traits. The findings suggest that studies of molecular and quantitative genetic variation may provide complementary insights important for the conservation of rare species. The strong differentiation of quantitative traits among populations shows that selection can be an important force for structuring variation in evolutionarily important traits even for rare endemic species restricted to very specific habitats.