| Because evolution is the unifying theory of biology, understanding how evolution works is essential for explaining why we age. Ageing evolves because of the interaction of two effects. First, natural selection is stronger on the young than on the old; this explains why the autosomal dominant disease, Huntington's Disease, can persist even though it is inexorably lethal. Second, any genetic, developmental, or physiological effect that increases the reproductive performance of the young will evolve so long as the costs that it imposes on the old are not too great.[22] Or put another way, traits that benefit early survival and reproduction will be selected for even if they contribute to an earlier death. Such genetic effects are called antagonistic pleiotropy. "Antagonistic" refers to the impact on fitness in the young, which is positive and the negative effect on the old. Genetic pleiotropy refers to genes that have multiple effects. Antagonistic pleiotropy has been identified both in model organisms and in humans.[23] In humans, some of the genetic variants that increase fertility in the young are now known to increase cancer risk in the old. Such genes include p53[24] and BRCA1.[25] |
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