The rate and tempo of adaptation is a fundamental question in evolutionary biology. Evolution is generally considered to be a gradual process and it is unclear if populations can adapt rapidly to environmental selection pressures. Temporal changes in environmental selection pressures across seasonal time may results in traits associated with fitness to vary as a function of season; this may result in seasonal oscillations of traits associated with high fitness that are favorable for reproduction and population expansion (summer) and those that are not and must be endured (winter). Long-term datasets can be used to investigate the association between environmental variance, fitness-related phenotypes and changing allele frequencies that are driven by natural selection and therefore reflect the adaptive process. This provides a long-term empirical investigation of seasonal life history patterns in natural populations of Drosophila melanogaster using isofemale lines collected from a temperate orchard over five consecutive years. There are pronounced and repeated changes in phenotypic performance based on temporal change in the genetic composition of the population from spring to fall; the variance in traits across seasonal time is equivalent to measurements across broad geographic distances over 20º latitude. Changes in both the phenotypic and genetic correlations from spring to fall counters the basic assumption of stable covariance over time and suggests that selection acts rapidly to change the genetic architecture of a population. Whole-genome resequencing of the population across seasonal time reveals thousands of SNPs that cycle as a function of season. Seasonal SNPs are associated with the fitness traits are identified based on genome-wide association and their function is evaluated. Environmental data from the study site is used to identify environmental parameters that correlate with the direction and magnitude of observed seasonal phenotypic and allele variation. The results suggest that selection acts in a rapid fashion with cyclic alternating selection by different environmental parameters between winter and summer phenotypes and genes. The fluctuating-stabilizing selection created by alternating selection pressures across seasons may be integral in the maintenance of phenotypic and genetic variation in natural populations.