Men who have children later in life will end up with fewer grandchildren than younger dads, according to a new study that combed through centuries of census data. The findings suggest that because children born to older fathers tend to have more potentially harmful genetic mutations than those with younger dads, they’re less likely to pass on their genes. It seems natural selection is working against older dads, weeding out their descendants.
Geneticists have shown that children born to older fathers tend to have more new genetic mutations than those with younger dads. Most of those mutations are harmless, but a few might increase the risk for certain diseases.
For the daughter of a 17th century farmer, a mutation causing a weakened immune system or a malfunctioning heart might have been an early death sentence—or at least prevented her from having her own children. Had she been born in the 21st century, however, the chances of her living a long and productive life would be dramatically higher, thanks to modern medicine. So are older fathers still the evolutionary burden they once were?
To find out, Ruben Arslan, a psychologist at the University of Göttingen in Germany, and colleagues analyzed data from census records from 17th and 18th century Germany, Canada, and Sweden, as well as a national population registry from 20th century Sweden, looking at more than 1.3 million people in total. In both the modern and preindustrial populations, children born to older fathers had fewer kids of their own that survived past age 5. For each decade that a father aged, his children had somewhere between 5% (in modern Sweden) and 13% (in the early German population) fewer children of their own, the team reports this month in a preprint available on bioRxiv.org.
That seems like a small price for any one father, Arslan says, but when magnified by the hundreds of thousands of dads in a population, it has a real effect.
By having fewer kids of their own, an older father’s children are less likely to pass on new mutations to future generations, possibly preventing those mutations from piling up in the population, says Martin Fieder, an anthropologist at the University of Vienna who was not involved in the work. “If children of older fathers don’t reproduce, harmful mutations are removed from the gene pool.”
That’s not to say modern medicine isn’t having an impact. More than 99% of Swedish babies now survive to 1 year of age; 200 years ago, fewer than 90% did. That could help explain the slightly smaller effect of paternal age in the modern Swedish population: Babies born prematurely or with birth defects are now more likely to live.
So why does the father’s age still have an effect at all? At-risk kids might be surviving longer now, but that doesn’t necessarily mean they’re able to reproduce when they reach adulthood, Arslan says—they could struggle with infertility or have a condition like autism that makes it harder for them to find a partner. Or for some, it could be deliberate: Prospective parents who know their children would be likely to inherit a certain mutation might decide to adopt instead.
Arslan emphasizes that the findings carry weight for genetic makeup of the population as a whole over many generations, but that they shouldn’t dissuade an individual older prospective father from having kids. “When we need at least 10,000 people in the samples to detect the effects, you can imagine that this is a fairly low-risk situation.”