The role of deleterious mutations in the adaptation to a novel environment

Abstract

Organisms adapt by accumulating beneficial mutations. Yet sometimes these beneficial mutations are not directly accessible, and organisms may have to cross a fitness valley before further adaptation is possible. A few recent works have shown that crossing of fitness valleys, as evidenced by fixation of deleterious mutations, may be surprisingly common in adaptation, and may be an important contributor to long-term fitness increase. Here we ask how important crossing of fitness valleys is for organisms that have reached a local fitness peak in one environment and are then placed into a new environment. We compare two treatments of evolving digital organisms, one in which organisms are exposed to deleterious mutations and thus can freely explore fitness valleys, and one in which they are prevented from experiencing deleterious (but not lethal) mutations and thus cannot. We find that organisms that are exposed to deleterious mutations always do at least as well as organisms that are not. Whether organisms exposed to deleterious mutations do better depends on the relative similarity and complexity of the old and new environment. We conclude that crossing of fitness valleys is important for successful adaptation to certain types of novel environments.