Genes that promote fertility also shorten our lives, research suggests
Why do we grow old and die?
In the 19th century, German biologist August Weismann argued that the machinery of life would inevitably become worn out over time. Death had evolved “for the needs of the species,” he declared. It cleared out weak, old individuals so that they would not compete with young people.
That explanation never made sense to George Williams, an American evolutionary biologist. Natural selection only works on the genes that are passed from one generation to the next. What happens at the end of an animal’s life cannot influence the course of evolution.
It occurred to Williams that growing old might be an inevitable side effect of natural selection. In 1957 he made suggested a new theory: genetic mutations that increase an animal’s fertility can also cause damage later in life. Over many generations, these mutations would carry a burden that would ultimately lead to death.
A new study, published Friday in the journal Science Advances, supports Williams’ theory using a trove of human DNA. Researchers have found hundreds of mutations that can increase a young person’s fertility and may be linked to physical damage later in life.
Smaller past studies had already provided some support for Williams’ theory. In 2007For example, a team of researchers studying a small worm found a pair of mutations that extended the creature’s lifespan while simultaneously reducing the average number of offspring.
But Jianzhi Zhang, an evolutionary biologist at the University of Michigan, was not satisfied with these experiments. “These are case studies,” he said. “We don’t know whether many of these types of mutations occur throughout the genome.”
Dr. Zhang took advantage of the British biobank, a database of genetic material from half a million volunteers in Britain, along with information about their health and life experiences. The biobank has enabled scientists to uncover subtle links between genetic variations thousands of properties such as high blood pressure, schizophrenia and smoking habit.
Together with Dr. Erping Long, a medical researcher now at the Chinese Academy of Sciences, Dr. Zhang scoured the database for information on reproduction and longevity. The scientists found that the genetic variations linked to fertility, such as the number of children a volunteer had, were also linked to a shorter lifespan.
Furthermore, variants that affected reproduction were almost five times more likely to affect lifespan than variants that had nothing to do with reproduction. And variants that are good for reproduction were much more likely to be bad for longevity.
Dr. Zhang and Dr. Long also found that volunteers with a high number of reproductively beneficial variants had slightly lower chances of surviving to age 76. Taken together, these results suggest that George Williams was right, and that aging is essentially a side effect of natural selection’s influence on fertility. “They’re all pointing in the same direction,” said Dr. Zhang.
He and Dr. Long also found evidence indicating that this evolution did not stop sometime in our distant past. People in the database who were born in 1965 had a greater number of reproduction-enhancing variants than people born in 1940.
The idea that fertility variants shorten lifespan may seem like a paradox, given how much longer people live today. In Great Britain, for example, the average lifespan expectation is about 80 years, compared to 59 years a century ago.
Dr. Zhang noted that the mutations he and Dr. Long identified each had a small impact on a person’s lifespan. As the variants become more prevalent, the environment has changed dramatically, with better nutrition and medicine reducing infant mortality and helping more people live to older ages.
Steven Austad, an expert on aging at the University of Alabama at Birmingham who was not involved in the study, said detecting the effect of these variants even though life expectancy had increased made the results all the more impressive.
“The pattern is so strong that it arises from these major changes in our life history in modern times,” he said.