Genes that increase fertility also shorten life – 12/10/2023 – Science

Why do we grow old and die?

In the 19th century, August Weismann argued that the mechanics of life inevitably wear out over time. Death evolved “out of necessity of the species”, declared the German biologist. She eliminated weak and elderly individuals so that they would not compete with the young.

This explanation never made sense to George Williams, an American evolutionary biologist. Natural selection acts only on genes that are transmitted from one generation to the next. What happens at the end of an animal’s life can have no effect on the course of evolution.

It occurred to Williams that aging might instead be an inevitable side effect of natural selection. In 1957, he proposed a new theory: genetic mutations that increased an animal’s fertility could also cause damage later in life. Over many generations, these mutations would create a burden that would ultimately lead to death.

A new study, published last Friday (7) in the journal Science Advances, reinforces Williams’ theory. Researchers found hundreds of mutations that could increase a young person’s fertility and that were later associated with damage to the body.

Smaller studies had already provided some support for Williams’ theory. In 2007, for example, a team of researchers studying a tiny worm found a pair of mutations that extended the creature’s lifespan but also reduced the average number of offspring.

However, evolutionary biologist Jianzhi Zhang of the University of Michigan was not satisfied with these experiments. “These are case studies,” he said. “We don’t know whether there are many mutations of this type throughout the genome.”

He accessed the UK Biobank, which houses the genetic material of half a million volunteers in Britain, as well as information about their health and life experiences. The database has allowed scientists to discover subtle links between genetic variations and thousands of traits, such as high blood pressure, schizophrenia and smoking.

Zhang and medical researcher Erping Long of the Chinese Academy of Sciences examined the database for information on reproduction and longevity. They found that genetic variations related to fertility, such as the number of children a volunteer had, were also linked to a shorter lifespan.

Furthermore, genetic variations that affected reproduction were almost five times more likely to influence longevity than those that had nothing to do with reproduction. And genetic variations that were good for reproduction were much more likely to be detrimental to long life.

The researchers also found that volunteers with a large number of genetic variations that promote reproduction were slightly less likely to survive to age 76.

Considering all these results, it is suggested that George Williams was correct and that aging is essentially a side effect of the impact of natural selection on fertility. “They all point in the same direction,” Zhang said.

He and Long also found evidence to suggest that this evolution did not stop at some point in our distant past. Volunteers who were born in 1965 carried a greater number of reproduction-boosting variants than people born in 1940.

The idea that fertility variants shorten life expectancy may seem paradoxical, given that people are living longer. In Britain, for example, average life expectancy is around 80 years, compared with 59 years a century ago.

Zhang noted that the mutations he and Long identified have a minimal influence on a person’s longevity. As variants have become more common, the environment has changed dramatically, with better foods and medicines reducing infant mortality and helping more people reach older ages.

Steven Austad, an aging expert at the University of Alabama at Birmingham who was not involved in the study, said that detecting the effect of these variants, even as life expectancy increases, makes the results even more impressive. “The pattern is so strong that it holds even in the face of these major changes in our life stories in modern times,” he said.