What is evolutionary mismatch theory?

Evolutionary mismatch, also called mismatch theory or an evolutionary trap, describes a situation in which a trait that was once advantageous becomes maladaptive because the environment has changed, especially when the change is rapid. Biological evolution is slow, so organisms can be left carrying traits tuned to conditions that no longer exist. [1] [2]

One tutorial describes mismatch as a state of disequilibrium in which an organism that evolved in one environment develops a phenotype that harms its fitness or well-being in another. It separates three elements: the ancestral environment, the novel environment, and the trait under study. The harm is usually measured as reduced evolutionary fitness, but it can also mean reduced welfare more broadly. A late-onset disease with little effect on fitness would still count if it is worth curing. [3]

A paper in Evolution, Medicine, and Public Health gives a genetic version. Mismatch is the phenomenon in which previously adaptive alleles are no longer favoured in a new environment, because the environment has changed but the genetics have not yet caught up. [4]

The tutorial notes that related terms such as adaptive lag and adaptive trap are subtypes of mismatch, since they assume the past trait was an adaptation. Mismatch is broader, because a trait that was originally neutral or even harmful can acquire new harmful consequences. Traits that turn out neutral or beneficial in the new setting are excluded from the term. [3]

Birds on remote islands often evolve to be fearless of mammalian predators. That becomes highly disadvantageous when mammals arrive. In many cases the birds cannot be trained to be fearful, and the options for preventing extinction include removing the predators or selecting for more fearful strains. [3]

A textbook illustration imagines a finch with a fine, pointed beak suited to extracting insects from tree bark. If the trees disappear and are replaced by small bushes, the beak no longer helps it find food. [5]

The most common human example concerns food. Ancestral environments varied in food availability and quality, favouring efficient energy extraction and storage. In industrialized settings with plentiful, calorie-dense food and low energy expenditure, those traits promote an energy imbalance that contributes to weight gain and obesity. [4]

Mismatch can also arise in development. Children in Romanian orphanages grew up in impoverished cognitive and emotional conditions outside the range their ancestors experienced, and showed reduced brain activity, lower IQ and weaker attachment and language development. [3]

Psychologists apply the idea to the mind. Li, van Vugt and Colarelli argue that psychological mechanisms evolved to turn environmental inputs into behaviour that on average aided survival or reproduction. Modern inputs differ in quantity and intensity, or no longer carry the same fitness associations, so some mechanisms produce maladaptive output. [6]

In principle, demonstrating mismatch requires showing that a trait was adaptive, neutral or deleterious in its ancestral environment and is relatively detrimental in the new one. The tutorial cautions that real cases are often subtler than the classic one: the trait itself may change with the environment through gene-environment interactions, or the harm may be to health rather than to fitness. [3]