Genes associated with having more children are also related to living shorter lives

Almost every animal experiences aging. As time passes, their bodies function less effectively, reproduction becomes more difficult, and the likelihood of mortality increases. From an evolutionary perspective, it may seem counterintuitive that natural selection doesn’t favor mutations that extend lifespan and the years of fertility. In 1952, scientist Peter Medawar proposed the theory of mutation accumulation to explain this. According to the theory, natural selection prioritizes the survival of organisms capable of reproducing. After reproduction, the evolutionary pressure to prevent bodily malfunctions decreases, resulting in the accumulation of harmful mutations. A child born with genetic variants causing death at 60 isn’t disadvantaged compared to one living trouble-free until 80 because their children should pass on their genes by then. In 1957, George Williams from the University of Michigan built on Medawar’s theory. He proposed that mutations related to aging, despite being fatal later in life, could still have positive selection effects if they cause the individual to reproduce earlier and more abundantly in his or her youth.

Humans have shown cases of this type of compensation. For instance, genetic variants that promote coronary heart disease tend to be associated with a higher number of offspring. In the Framingham heart study, a negative correlation was observed between the number of children and life expectancy in women. However, some studies question this hypothesis due to the complexity of establishing clear genetic links. Factors like life expectancy, number of children, and social aspects in humans are influenced by numerous genetic variants with minor effects.

Erping Long and Jianzhi Zhang from the University of Michigan recently published an article in the journal Science Advances. They conducted a study using genetic, reproductive and mortality data collected from the U.K. Biobank, which contains information on over 276,000 individuals. Long and Zhang found that people with high scores on polygenic factors favoring reproduction were less likely to live beyond age 76. Additionally, they observed an increase in genetic variants associated with a greater number of children and shorter life expectancy. This pattern aligns with the existence of positive selection, as predicted by the Williams hypothesis.

One possible explanation for this phenomenon, as described in the Science Advances article, is another theory about aging. According to this theory, since organisms have limited resources, they prioritize investing more in reproduction at the expense of DNA repair. This leads to the accumulation of mutations, which ultimately contributes to aging. A recent study published in Nature found a negative correlation between the rate of somatic mutations in 16 mammal species and their life expectancy. This finding lends support to the hypothesis that limited resources are a contributing factor to shorter lifespans in species with more offspring.

Long and Zhang’s study suggests that genetic factors promoting fertility may lower life expectancy. However, they recognize that social impacts are more significant than genetic impacts for humans, particularly in recent decades. There are instances where environmental and biological factors intertwine, leading to effects that don’t align with their hypothesis. For instance, a mutation impairing learning abilities can simultaneously reduce reproductive capacity and life expectancy. Additionally, different cultures show disparate outcomes. In China, women who have more children tend to have shorter lives, whereas the opposite is observed among the Amish. The study presents data that highlights the complexity of this phenomenon: women with two children lived longer than those with three, yet they also enjoyed longer lifespans than childless individuals.

Over the past few decades, life expectancy has greatly increased in almost every part of the world, now averaging over 72 years. This is nearly 30 years more than the average of 46 in 1950. During this time, birth rates have declined significantly due to various social and technological factors. These two trends counterbalance natural selection of genetic variants, as shown in the Science Advances study.

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