Ecological contexts of balancing selection in nature

Abstract

How genetic variation is maintained in the face of persistent natural selection is a central question in evolutionary biology. Here, I leverage a focal polymorphism, leaf chemical profile in a perennial wildflower (Boechera stricta, Brassicaceae) to investigate the ecological and genetic mechanisms that may influence the maintenance of variation in this trait. In the first chapter, I present data from a suite of common garden and greenhouse experiments showing that the alleles underlying variation in chemical profile have contrasting fitness effects across environments. I identify two putative selective drivers on chemical profiles, and utilize phenotype-environment associations and molecular genetic analyses to test for additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, likely caused by pleiotropic effects of genes that influence secondary chemical biosynthesis on herbivore defense and drought response. In the second chapter, I utilize a multi-year, manipulative field experiment to test for the effects of variation in selective drivers on genotypes conferring contrasting chemical profiles. I integrate variable effects of the environment on fitness components across life history and across environments to assess patterns of lifetime fitness. These data suggest that environmental conditions in which contrasting genotypes can both persist may be widespread. Together, these chapters provide complementary perspectives on the question of persistent natural variation, suggesting that variation in secondary metabolic profiles in B. stricta may persist at present due to balancing selection, and may continue to persist in the future under variable environmental scenarios.