maladaptation

Maladaptation is a state in which a trait, behavior, or plastic response reduces an organism’s fitness in its current environment relative to alternative phenotypes or to ancestral conditions. It occurs when a phenotype that was once favored is no longer well suited, or when a trait trades off one function for another in a way that lowers overall reproductive success.

Maladaptation can arise for several reasons. Environments can change faster than populations can adapt, leaving previously advantageous traits mismatched to current conditions. Traits with beneficial effects in one context may incur costs in another, creating trade-offs that reduce net fitness. Genetic constraints, such as pleiotropy or limited variation, can prevent optimal solutions. Gene flow can introduce alleles that are locally suboptimal, and imperfect plasticity can lead to maladaptive responses to rare or novel stimuli.

Examples include the costs associated with antibiotic resistance in bacteria when the antibiotic is absent, which can reduce growth rate; and in humans, hypotheses that alleles advantageous in historical, resource-scarce environments (the so-called thrifty genes) may predispose modern individuals to obesity or type 2 diabetes in high-calorie settings. Maladaptation can also arise from mismatches between species’ evolved traits and rapidly changing climates, habitats, or community interactions.

Maladaptation is distinct from neutral variation or simple perpetual imperfection; it entails lower fitness relative to alternatives. It can be temporary or persistent, depending on whether selection, further environmental change, or new trade-offs shift the balance toward better-fit phenotypes.