Mating System Evolution

Among angiosperm taxa, the ability to self-fertilize has evolved multiple times. Most research has attempted to explain the evolution of selfing as an adaptive response to selection under particular environmental conditions (e.g., where pollinators are scarce or unreliable). If selection for self-fertilization is occurring in outcrossing taxa in the extreme of their range, then we expect plants with the highest fitness to have traits that allow them to complete their lifecycle when water is plentiful such as higher rates of carbon gain, earlier flowering at smaller plant sizes, and lower water use efficiency. In this study, we measured above-ground plant biomass in an outcrossing population of Clarkia xantiana ssp xantiana to determine whether there is a significant correlation between their physiological processes (photosynthesis, transpiration, and water use efficiency) and above-ground plant biomass. Gas-exchange rates were measured on plants growing in the field. At plant senescence, above-ground stems were collected, dried and weighed. Our results show no significant correlation between photosynthesis, transpiration, or water use efficiency with plant biomass. All three relationships showed near zero slopes, small r2 values and large P-values. In conclusion, the current study did not support our prediction that outcrossing phenotypes growing at the edge their range should possess drought avoidant traits (low biomass with high gas-exchange rates). One explanation for this result may be that this study population grows at a high elevation with cooler temperatures, which may alleviate selection for drought avoidant traits.