Genes related to having more children are also related to living less

Almost all animals age. As time goes by, bodies function worse, it becomes harder to reproduce, and the chances that the next day will be your last increase. From an evolutionary point of view, an explanation is needed, because it seems that natural selection should favor mutations that prolong life expectancy and fertile years. In 1952, the scientist Peter Medawar proposed his theory of the accumulation of mutations. Since the main goal of an organism is reproduction, natural selection does everything it can to keep us alive until we are able to reproduce. Then, the evolutionary pressure to prevent faults from appearing in the organism decreases and harmful mutations begin to accumulate. A child born with genetic variants that will surely kill him at age 60 has no evolutionary disadvantages compared to one who lives without problems until he is 80, because by then his children should already be looking for a way to perpetuate their genes. In 1957, George Williams, then at the University of Michigan, added in another article an adaptive aspect to Medawar's approach. Aging-related mutations, which are fatal in old age, could have positive selection if they cause the individual to reproduce earlier or in greater numbers during their youth.

Cases of this type of compensation have been seen in humans. For example, genetic variants that favor the appearance of coronary heart disease are usually associated with a greater number of offspring, and a negative correlation has also been observed between the number of children and life expectancy in women within the Framingham studio from the heart. However, there are studies that question the hypothesis and it is not easy to establish clear genetic links to prove it, because life expectancy and the number of descendants, in addition to social aspects in the case of humans, are influenced by many genetic variants. which have small effects.

Recently, Erping Long and Jianzhi Zhang, from the University of Michigan, published an article in the magazine Science Advances in which they have tested the hypothesis using genetic, reproductive and mortality data collected in the United Kingdom Biobank, where this type of information can be accessed from more than 276,000 people. With that sample, the authors observed that, in terms of traits, those who scored high on polygenic factors that favor reproduction were less likely to survive to age 76. Furthermore, looking at the genetic variants associated with a greater number of descendants and shorter life expectancy, they observed that they have increased, something that, according to the authors, is a pattern consistent with the existence of positive selection, as predicted by the Williams hypothesis.

One of the explanations for this phenomenon can be found, as explained in the article by Science Advancesin another theory about aging. This suggests that organisms have limited resources and a greater investment in reproduction is made at the cost of investing less in DNA repair, something that facilitates the accumulation of mutations that end up causing aging. Recently, an article published in the magazine Naturefound an inverse correlation between the rate of somatic mutations of 16 species of mammals and their life expectancy, something that may support this hypothesis of limited resources as an explanation for the fact that more offspring are related to a shorter life.

Although the authors consider that their analysis supports the hypothesis that what promotes fertility reduces life expectancy, they recognize that in the human case—and particularly during recent decades—the social effect is much greater than the genetic one. Although there are cases in which the environmental and the biological are intertwined and produce effects that do not fit well with the hypothesis. For example, a mutation that impairs a person's learning abilities can simultaneously reduce their reproductive capacity and life expectancy. Furthermore, there are situations in which different effects are observed in different cultures: in China, women with more children live shorter lives, but the opposite is true among the Amish. And the study itself includes data that shows the complexity of this phenomenon: those who had two children were also more likely to survive than those who had three, but they lived longer than those who had none.

During the last decades, life expectancy has multiplied in almost all regions of the planet and now exceeds 72 years on average, almost three decades more than the 46 in 1950. In that time, there has also been a significant decrease in birth rate, for a wide range of reasons, from social to technological. Both trends together represent a push in the opposite direction of the natural selection of genetic variants detected by the study by the University of Michigan researchers.

You can follow EL PAÍS Health and Wellbeing in Facebook, x and instagram.

_