Differential adaptation drives ecological speciation in campions (Silene): evidence from a multi-site transplant experiment

In order to investigate the role of differential adaptation for the evolution of reproductive barriers, we conducted a multi-site transplant experiment with the dioecious sister species Silenedioica and S.latifolia and their hybrids. Crosses within species as well as reciprocal first-generation (F-1) and second-generation (F-2) interspecific hybrids were transplanted into six sites, three within each species' habitat. Survival and flowering were recorded over 4yr. At all transplant sites, the local species outperformed the foreign species, reciprocal F-1 hybrids performed intermediately and F-2 hybrids underperformed in comparison to F-1 hybrids (hybrid breakdown). Females generally had slightly higher cumulative fitness than males in both within- and between-species crosses and we thus found little evidence for Haldane's rule acting on field performance. The strength of selection against F-1 and F-2 hybrids as well as hybrid breakdown increased with increasing strength of habitat adaptation (i.e. the relative fitness difference between the local and the foreign species) across sites. Our results suggest that differential habitat adaptation led to ecologically dependent post-zygotic reproductive barriers and drives divergence and speciation in this Silene system.