The role of standing genetic variation in adaptive evolution remains unclear. Although there has been much progress in identifying candidate genes that underlie adaptive traits, we still lack direct evidence that natural allelic variation in these genes can actually mediate adaptive evolution. In this study, we investigate the role of natural allelic variation in two candidate flowering time genes, in response to selection for early flowering in Arabidopsis thaliana: FRIGIDA (FRI) and FLOWERING LOCUS C (FLC). We performed artificial selection for early flowering under 'spring-' and 'winter-annual' growth conditions using an outbred population of A. thaliana produced by intermating 19 natural accessions. FRI and FLC are involved in A. thaliana's response to winter conditions, and nonfunctional and weak alleles at these loci are know to reduce flowering time, particularly under spring-annual conditions. Our results provide direct evidence that natural allelic variation in FRI can provide rapid and predictable adaptive evolution in flowering time under spring-annual conditions. We observed a strong response to selection, in terms of reducing flowering time, in both growth conditions (approximately 2 standard deviation reduction). Concomitantly, the frequency of functional FRI alleles under spring-annual conditions was reduced by 68%, in agreement with predicted changes. No significant changes in allele frequencies were observed in FRI in the winter-annual growth condition or in FLC for either growth conditions. These results indicate that changes in flowering time are mediated by different genetic factors under spring- and winter-annual growth conditions, and that other loci must also be contributing to the response to selection.