Antagonistic pleiotropy hypothesis

The antagonistic pleiotropy hypothesis (APT) is a theory in evolutionary biology that suggests certain genes may confer beneficial effects early in an organism's life, enhancing survival or fertility, while also causing detrimental effects later in life, thereby contributing to the ageing process. APT provides an explanation of how some genes are not eliminated by natural selection even though they are associated with catastrophic health outcomes, especially in older age (e.g. Alzheimer's disease or sickle cell anaemia).

APT was first proposed in a 1952 paper on the evolutionary theory of aging by Peter Medawar and developed further in a paper by George C. Williams in 1957 as an explanation for senescence. Pleiotropy is the phenomenon where a single gene influences more than one phenotypic trait in an organism. It is one of the most commonly observed attributes of genes. A gene is considered to exhibit antagonistic pleiotropy if it controls more than one phenotypic trait, where at least one of these traits is beneficial to the organism's fitness and at least one is detrimental to fitness.

This line of genetic research attempts to answer the following question: if survival and reproduction should always be favoured by natural selection, why should ageing – which in evolutionary terms can be described as the age-related decline in survival rate and reproduction – be nearly ubiquitous in the natural world?" The antagonistic pleiotropy hypothesis provides a partial answer to this question. As an evolutionary explanation for ageing, the hypothesis relies on the fact that reproductive capacity declines with age in many species and that, therefore, the strength of natural selection also declines with age (because there can be no natural selection without reproduction). Since the strength of selection declines over the life cycles of human and most other organisms, natural selection in these species does not eliminate "alleles that have early beneficial effects, but later deleterious effects".

Antagonistic pleiotropy also provides a framework for understanding why many genetic disorders, even those causing life threatening health impacts (e.g. sickle cell anaemia), are found to be relatively prevalent in populations. Seen through the lens of simple evolutionary processes, these genetic disorders should be observed at very low frequencies due to the force of natural selection. Genetic models of populations show that antagonistic pleiotropy allows genetic disorders to be maintained at reasonably high frequencies "even if the fitness benefits are subtle". In this sense, antagonistic pleiotropy forms the basis of a "genetic trade-off between different fitness components."