A recent study by researchers at the Wellcome Sanger Institute and their collaborators has uncovered how sperm evolve within the human body as men age, leading to an accumulation of harmful genetic mutations that may raise disease risk in their children. Published in Nature (October 2025), the research provides a new understanding of how so-called “selfish” sperm cells gain an advantage, multiplying over time while carrying mutations that can disrupt normal development.

Human sperm are produced continuously throughout life, giving rise to billions of cells from a pool of stem cells within the testes. Each division of these cells introduces small errors in DNA, and over decades of sperm production, mutations accumulate. The study reveals that certain mutations give sperm-producing stem cells a competitive edge, allowing them to expand at the expense of normal cells. This evolutionary process means that as men age, their sperm population becomes dominated by these “selfish” lineages, increasing the likelihood of passing on disease-related mutations to offspring.

Using single-cell genetic sequencing, the team analyzed testicular tissue to map how these mutated cells evolve and spread. The findings show clear patterns of clonal expansion—small pockets of sperm-producing stem cells carrying identical mutations—indicating that natural selection is occurring at the cellular level. Many of these mutations affect genes involved in cell signaling and development, including those linked to conditions such as achondroplasia, Apert syndrome, and potentially autism spectrum disorders.

This research highlights an underappreciated aspect of reproductive biology: that natural selection operates not only at the organismal level but also within individual tissues. As men age, this internal competition reshapes the genetic landscape of their sperm, increasing both diversity and the potential for transmitting harmful variants.

The findings may influence future fertility counseling and genetic screening strategies, helping clinicians better understand age-related reproductive risks. Researchers are now exploring whether interventions could one day slow or counteract this selfish evolution, protecting sperm integrity as men age.

Sources

28 October 2025. Wellcome Sanger Institute

Hidden evolution in sperm raises disease risk for children as men age

28 October 2025. Nature

Sperm sequencing reveals extensive positive selection in the male germline

28 October 2025. New Scientist

Selfish sperm see older fathers pass on more disease-causing mutations