Directional epistasis is common in morphological divergence

Epistasis is often portrayed as unimportant in evolution. While random
patterns of epistasis may have limited effects on the response to
selection, systematic directional epistasis can have substantial effects
on evolutionary dynamics. Directional epistasis occurs when allelic
substitutions that change a trait also modify the effects of allelic
substitutions at other loci in a systematic direction. In this case, trait
evolution may induce correlated changes in allelic effects and effective
genetic variance (evolvability) that modify further evolution. Although
theory thus suggests a potentially important role for directional
epistasis in evolution, we still lack empirical evidence about its
prevalence and magnitude. Using a new framework to estimate systematic
patterns of epistasis from line-crosses experiments, we quantify its
effects on 197 size-related traits from diverging natural populations in
24 animal and 17 plant species. We show that directional epistasis is
common and tends to become stronger with increasing morphological
divergence. In animals, most traits displayed negative directionality
toward larger size, suggesting that epistasis constraints reducing
evolvability toward larger size may be common. Dominance was also common
and did not systematically alter the effects of epistasis.