A Quantitative Survey of Cogradient and Countergradient Variation in Nature

AbstractGradient variation evolves when environmental and genotypic effects on a phenotype covary positively (cogradient variation) or negatively (countergradient variation) across locations, whereas gene-by-environment interactions (G × E) reflect nonadditive genetic and environmental influences on phenotypes. Spatial covariance in environmental and genotypic effects (Cov_GE) shapes variation in quantitative traits, facilitates local adaptation, and provides insights into eco-evolutionary dynamics. Yet several debates regarding gradient variation remain unresolved, including whether qualitative patterns of reaction norms accurately reflect Cov_GE, whether cogradient or countergradient variation occurs more frequently than G × E, and whether general patterns emerge according to taxonomic groups, forms of environmental gradient, or trait types. We conducted a quantitative survey of 556 phenotypes and measured Cov_GE and G × E across various phenotypes, taxa, and environmental gradients. We found that the qualitative assessment of reaction norms was unreliable for identifying Cov_GE and that Cov_GE occurred as frequently as G × E. No distinct patterns in Cov_GE emerged across environmental, taxonomic, or trait-based groups. Our results challenge prevailing views regarding Cov_GE and suggest that gradient variation can evolve under any environmental condition, taxonomic grouping, or trait type. We suggest that broader application of quantitative methods for Cov_GE across diverse systems will enhance our understanding of Cov_GE in nature.