For a long time, many evolutionary biologists have viewed most molecular evolution as surprisingly calm. The idea was that many genetic changes that spread through populations are neither helpful nor harmful. They simply flow through nature without attracting much attention from natural selection.
A University of Michigan study challenges this image. Led by evolutionary biologist Jianzi Zhang, the research suggests that helpful mutations could also be more common than long-held theory predicts. But there’s a catch. Many of those helpful mutations may not last long enough to turn into everlasting.
An essential evolutionary theory faces a brand new test.
During evolution, mutations arise by likelihood. Something missing. Others spread until everyone within the population takes them with them, a process generally known as fixation.
For greater than half a century, some of the influential theories in molecular evolution has been the neutral theory of molecular evolution. First proposed within the Sixties, the idea holds that the majority fixed genetic changes on the gene and protein level are neutral. In this view, deleterious mutations are often removed by natural selection, while truly helpful mutations are so rare that the majority long-lasting molecular changes are expected to be neutral.
Zhang and his colleagues got down to test a very important hypothesis behind this theory. Are helpful mutations really that rare?
Their findings suggest the reply could also be no.
Contributory mutations might be surprisingly common.
Using large deep mutational scanning datasets from their very own lab and others, the team checked out the results of many mutations in model organisms similar to yeast and E. coli. In deep mutational scanning, scientists create many mutations in a gene or region of the genome after which measure how these changes affect the organism.
The researchers tracked the organisms over several generations and compared them to the wild type, or the version normally present in nature. By measuring growth, they’ll gauge whether a change helped, hurt, or had little effect.
They found that greater than 1% of the amino acid-changing mutations were helpful. This may sound small, but in evolutionary theory it is big. If this many mutations are helpful, the team calculated that greater than 99 percent of amino acid substitutions needs to be compatible. Gene evolution must also occur much faster than what scientists have seen in nature.
This similarity forced the researchers to rethink considered one of their hypotheses. They concluded that the issue could also be that the atmosphere shouldn’t be static.
Evolution follows a moving goal.
A change might be helpful in a single setting and detrimental in one other. If the environment changes before a helpful mutation spreads throughout the population, that mutation may lose its profit and even turn into a liability.
“We’re saying the result was neutral, but the process was not neutral,” said Zhang, a UM professor of ecology and evolutionary biology. “Our model suggests that natural populations do not adapt to their environment because the environment changes so rapidly, and populations are always chasing the environment.”
The team calls this framework Adaptive Tracking with Antagonistic Pleiotropy. Simply put, which means that a population can reply to a consistently changing environment, while many variables have trade-offs that rely upon the environment.
A mutation that increases fitness today may decrease fitness tomorrow. As a result, evolution might be full of helpful changes which might be never everlasting.
Experiments with yeast show what happens when conditions change.
To test this concept, Zhang’s team compared two groups of yeast over 800 generations. A gaggle evolved in a stable environment. The second was developed in a changing environment consisting of 10 different growth media.
The variable-environment group spent 80 generations in the primary medium, then 80 in the subsequent, and so forth, until it had accomplished 800 generations. (Each race lasted for 3 hours)
The researchers found little helpful variation within the group exposed to the changing conditions. Helper mutations still appeared, but they often didn’t have time to spread through the population before conditions modified again.
“This is where asymmetry arises. While we observe many beneficial mutations in a given environment, these beneficial mutations don’t have a chance to be fixed because as their frequency increases to a certain level, the environment changes,” Zhang said. “Those beneficial mutations in the old environment may be detrimental in the new environment.”
Why Perfect Adaptation May Be Out of Reach
The results point to a more pessimistic view of evolution. Instead of moving steadily toward an ideal fit between organisms and their environment, populations can often get stuck chasing conditions that keep changing.
This idea has broad implications for living things, including humans, Zhang said.
“I think this has broader implications. For example, humans. Our environment has changed a lot, and our genes may not be optimal for today’s environment because we’ve been through many other different environments. Some mutations may have been beneficial in our old environment, but not compatible with today’s,” Zhang said.
He added that the degree of adaptation seen in any given population may rely upon how recently its environment has modified.
“Any time you observe a natural population, depending on when the last major environmental change occurred, the population may be very poorly adapted or it may be relatively well adapted. But we will probably never see a population that is completely adapted to its environment, because complete adaptation would take longer than almost any natural environment.”
A significant change in how scientists study mutations
The neutral theory emerged at a turning point in biology. Before the Sixties, scientists often studied evolution by examining an organism’s shape, structure, and physical traits. As researchers began to sequence proteins and later genes, they might study evolution on the molecular level.
This variation revealed patterns that were well explained by the neutral theory, including why many genetic differences appear to build up steadily over time. The Michigan study doesn’t erase that history. Instead, it offers a technique to reconcile two seemingly contradictory observations.
On the one hand, many molecular changes that turn into fixed still appear neutral when scientists compare genomes. On the opposite hand, experiments show that helpful mutations might be very large in a given environment. Zhang’s team argues that each could also be true if helpful mutations are sometimes transient.
Recent research in evolutionary genetics has emphasized the importance of fixing environments. Oh 2026 Assessment of Adaptation to Rapidly Changing Conditions Highlighted how allele frequencies and changes in traits are highly depending on available genetic variation. Other yeast research has also shown that adaptations might be shaped by environmental stress and that helpful mutations in a single sequence might be costly in others.
Together, these findings reinforce a growing topic in evolutionary biology. The effect of mutation cannot all the time be understood in isolation. This may rely upon the environment, the organism’s history and the speed at which conditions change.
Warning and next query
Zhang notes a very important limitation. Most of the information utilized in the study got here from yeast and single-celled organisms, which makes it easy to measure the fitness effects of mutations. Deeper mutational scanning data from multicellular organisms can be needed to see if the identical patterns apply to animals, plants, and humans.
The team also plans to research why it takes so long for organisms to totally adapt even when the environment stays constant.
The study was supported by the US National Institutes of Health and published in Nature Ecology and Evolution. Other authors include former UM graduate students Silyang Song and Xukang Shen and former UM postdoctoral researcher Piaopiao Chen.
For now, the work points to a beautiful prospect. Evolution can turn into less like a gentle climb toward perfection and more like chasing after a world that keeps on going.