Scientists say evolution works differently than we thought
A major evolutionary theory says most genetic changes don’t really matter, but new evidence suggests that’s not true. Researchers found that helpful mutations happen surprisingly often. The twist is that changing environments prevent these mutations from spreading widely before they become useless or harmful. Evolution, it turns out, is less about reaching perfection and more about endlessly chasing a moving target.
For decades, many evolutionary biologists have believed that most genetic changes shaping genes and proteins are neutral. Under this view, mutations are usually neither helpful nor harmful, allowing them to spread quietly without being strongly favored or rejected by natural selection.
A new study from the University of Michigan challenges that long-standing assumption and suggests evolution may work very differently than once thought.
Rethinking the Neutral Theory of Evolution
As species evolve, random genetic mutations arise. Some of these mutations become fixed, meaning they spread until every individual in a population carries the change. The Neutral Theory of Molecular Evolution argues that most mutations that reach this stage are neutral. Harmful mutations are quickly eliminated, while helpful ones are assumed to be extremely rare, explains evolutionary biologist Jianzhi Zhang.
Zhang and his colleagues set out to test whether this idea holds up when examined more closely. Their results pointed to a major problem. The researchers found that beneficial mutations occur far more often than the Neutral Theory allows. At the same time, they observed that the overall rate at which mutations become fixed in populations is much lower than would be expected if so many helpful mutations were taking hold.
Why Beneficial Mutations Often Do Not Last
To explain this contradiction, the team proposed a new explanation rooted in environmental change. A mutation that provides an advantage in one setting may become harmful once conditions shift. Because environments change frequently, many beneficial mutations never spread widely enough to become fixed.
The study, which was supported by the U.S. National Institutes of Health, was published in Nature Ecology and Evolution.
"We're saying that the outcome was neutral, but the process was not neutral," said Zhang, a professor of ecology and evolutionary biology at U-M. "Our model suggests that natural populations are not truly adapted to their environments because environments change very quickly, and populations are always chasing the environment."
Zhang calls this framework Adaptive Tracking with Antagonistic Pleiotropy. The idea helps explain why organisms are rarely perfectly matched to their surroundings.
What This Means for Humans and Other Species
Zhang believes the findings have wide-ranging implications, including for humans. Modern environments differ dramatically from those our ancestors experienced, which may help explain why certain genetic traits no longer serve us as well as they once did.
"I think this has broad implications. For example, humans. Our environment has changed so much, and our genes may not be the best for today's environment because we went through a lot of other different environments. Some mutations may be beneficial in our old environments, but are mismatched to today," Zhang said.