Selection Effects and Longevity Genes

How is it possible for evolutionary selection to favor genetic variants beneficial in human old age, long after reproduction is impossible? An open access paper examines this question: "Evidence points towards the existence of a strong heritable component of human longevity. Around a quarter to a third of the variability of lifespan can be attributed to the action of genes. One of the best examples of a gene affecting survival in old age is the apolipoprotein E gene APOE. ... One of the major ideas in the evolutionary theory of ageing is the suggestion that, because the force of natural selection declines with age, alleles with deleterious effects seen only at older ages can reach higher frequencies than those that have their effects earlier in life. Therefore, if a gene exerts an effect only after the end of the reproductive phase of the lifespan it has been thought unlikely that it could have been subject to significant direct selection pressure ... It is often claimed that genes affecting health in old age, such as cardiovascular and Alzheimer diseases, are beyond the reach of natural selection. We show in a simulation study based on known genetic (apolipoprotein E) and non-genetic risk factors (gender, diet, smoking, alcohol, exercise) that, because there is a statistical distribution of ages at which these genes exert their influence on morbidity and mortality, the effects of selection are in fact non-negligible."

Link: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2848859/

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