Past selection shaped phenological differentiation among populations at contrasting elevations in a Mediterranean alpine plant

Flowering phenology is an important life-history trait strongly influenced by the environment that directly affects plant fitness. Climate change is bringing about shifts in flowering time caused by adaptive evolution and phenotypic plasticity, but their relative contributions and effects are poorly understood. This is especially critical in Mediterranean alpine species, which experience steep environmental gradients at short distances characteristic of alpine environments and an intense summer drought period derived from the Mediterranean climate. Moreover, tracking climate change through migration to higher elevations is not always possible for alpine species since many already find their optimal niche at mountain summits. In this study, we aimed to determine if flowering phenology is genetically differentiated among populations inhabiting contrasting environmental conditions and ascertain if it has been subjected to past selection. This knowledge is crucial to understanding adaptation of alpine plants to current environmental gradients and to provide insight about what adaptations may be necessary to cope with future and ongoing climate warming. We used a common garden experiment to analyze genetic differentiation in phenological traits of nine populations of Silene ciliata Pourret (Caryophyllaceae) distributed at two environmentally differentiated areas (optimal and marginal habitat suitability) in three mountain ranges of Central Spain. Environmentally optimal areas for this species are close to the mountain tops, whereas marginal areas are found at the lower distribution edge. We also studied the relation between neutral genetic differentiation (F-ST) and quantitative trait differentiation (P-ST) to infer past selection on characters under contrasting environmental conditions. We found genetic differentiation for the onset, peak and end of flowering between populations in optimal and marginal areas in the presence of substantial gene flow. This finding highlights the strong diverging selection pressures between the different Mediterranean alpine environments. We also found evidence of past diversifying selection for flowering peak and end of flowering. This evidence of past adaptation in addition to adaptive phenotypic plasticity to advance flowering dates under warmer temperatures suggest that adaptation of flowering phenology to current and future warming should be feasible, especially for populations inhabiting optimal areas.