Adaptive Species Differentiation and Population Uniformity in Viola Species Sharing Similar Geographical Distribution but Differing Habitat Preferences

Abstract

Selection of favorable alleles is sufficient to maintain species cohesion even with low levels of gene flow. Here, in the study of two sister species growing in contrasting restricted ecological areas, we assess whether their divergent traits have been subject to uniform selection among populations. Two different analyses were performed to evaluate the relative importance of selection and drift. First, we compared FST and PST indexes, analogous to QST, in eleven populations each of Viola eizanensis and V. chaerophylloides. PST was computed for two reproductive traits, two leaf traits and two allocation traits. Second, we compared the observed PST with the expected distribution of PST under neutrality generated by phylogeny-based simulation assuming the Brownian motion model for trait evolution. These analyses indicated that spring leaf mass per area and seed number per capsule were divergent between species and uniform among populations. We suggest that these candidate traits are associated with ecological speciation. We also show that uniform, not divergent, selection among populations is probably common in both species. This result suggests that uniform selection maintaining similarity of traits among populations is a more dominant force than divergent selection in species growing in restricted ecological areas.