Genetic polymorphism in heterogeneous environments: The age of genomics

The selective mechanisms for maintaining polymorphism in natural populations has been the subject of theory, experiments, and review over the past half century. Advances in molecular genetic techniques have provided new insight into many examples of balancing selection. In addition, new theoretical developments demonstrate how diversifying selection over environments may maintain polymorphism. Tests for balancing selection in the current generation, the recent past, and the distant past provide a comprehensive approach for evaluating selective impacts. In particular, sequenced-based tests provide new ways to evaluate the long-term impact of selection on particular genes and the overall genome in natural populations. Overall, there appear to be many loci exhibiting the signal of adaptive directional selection from genomic scans, but the present evidence suggests that the proportion of loci where polymorphism is maintained by environmental heterogeneity is low. However, as more molecular genetic details become available, more examples of polymorphism maintained by selection in heterogeneous environments may be found.