Broad geographic sampling reveals predictable, pervasive, and strong seasonal adaptation in Drosophila

To advance our understanding of adaptation to temporally varying selection pressures, we identified signatures of seasonal adaptation occurring in parallel among Drosophila melanogaster populations. To study these evolutionary dynamics, we estimated allele frequencies genome-wide from flies sampled early and late in the growing season from 20 widely dispersed populations. We identify parallel seasonal allele frequency shifts across North America and Europe, demonstrating that seasonal adaptation is a general phenomenon of temperate fly populations. The direction of allele frequency change at seasonally variable polymorphisms can be predicted by weather conditions in the weeks prior to sampling, linking the environment and the genomic response to selection. The extent of allele frequency fluctuations implies that seasonal evolution drives substantial (5-10%) allele frequency fluctuations at >1% of common polymorphisms across the genome. Our results suggest that fluctuating selection is an important evolutionary force affecting the extent and stability of linked and functional variation.