Rising atmospheric CO2 may lead to natural selection for genotypes that exhibit greater fitness under these conditions. The potential for such evolutionary change will depend on the extent of within-population genetic variation in CO2 responses of wild species. We tested for heritable variation in CO2-dependent life history responses in a weedy, cosmopolitan annual, Raphanus raphanistrum. Progeny from five paternal families were grown at ambient and twice ambient CO2 using outdoor open-top chambers (160 plants per CO2 treatment). Elevated CO2 stimulated net assimilation rates, especially in plants that had begun flowering. Across paternal families, elevated CO2 led to significant increases in flower and seed production (by 22% and 13% respectively), but no effect was seen on time to bolting, leaf area at bolting, fruit set, or number of seeds per fruit. Paternal families differed in their response to the CO2 treatment: in three families there were no significant CO2 effects, while in one family lifetime fecundity increased by >50%. These genotype-specific effects altered fitness rankings among the five paternal families. Although we did not detect a significant genotype x CO2 interaction, our results provide evidence for heritable responses to elevated CO2. In a subset of plants, we found that the magnitude of CO2 effects on fecundity was also influenced by soil fertility.
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