Why does the magnitude of genotype-by-environment interaction vary?

Genotype-by-environment interaction (GxE), that is, genetic variation in phenotypic plasticity, is a central concept in ecology and evolutionary biology. GxE has wide-ranging implications for trait development and for understanding how organisms will respond to environmental change. Although GxE has been extensively documented, its presence and magnitude vary dramatically across populations and traits. Despite this, we still know little about why GxE is so evident in some traits and populations, but minimal or absent in others. To encourage synthetic research in this area, we review diverse hypotheses for the underlying biological causes of variation in GxE. We extract common themes from these hypotheses to develop a more synthetic understanding of variation in GxE and suggest some important next steps.