Fighting change with change: adaptive variation in an uncertain world

Organisms live in an ever-changing world. Most of evolutionary theory considers one solution to this problem: population-level adaptation. In fact, empirical studies have revealed an enormous variety of mechanisms to cope with environmental fluctuations. Some organisms use behavioral or physiological modifications that leave no permanent trace in the genes of future generations. Others withstand environmental change through the regular production of diverse offspring, in which the diversity can be either genetic or nongenetic. Evolutionary theorists now have the opportunity to catch up with the empirical evolutionary biology, and to integrate the diverse forms of ‘adaptive variation’ into a single conceptual framework. Here, we propose a classification according to the level at which the adaptive variation occurs and discuss some of the mechanisms underlying the variation. This perspective unites independent lines of research in molecular biology, microbiology, macroevolution, ecology, immunology and neurobiology, and suggests directions for a more comprehensive theory of adaptive variation.