The Still River
In 1968, Motoo Kimura proposed what would become one of evolutionary biology's most durable ideas: the Neutral Theory of Molecular Evolution. The theory held that most mutations which become fixed in a population — meaning they spread until every individual carries them — are selectively neutral. Not beneficial, not harmful. Just noise. Genetic drift, not natural selection, was the dominant force shaping genomes at the molecular level.
For nearly sixty years, this has been the working assumption across much of molecular biology. The genome, in this view, is a river carrying debris — logs and leaves that float downstream not because the current favours them, but because nothing stopped them.
A new study from the University of Michigan suggests the river was never still.
Jianzhi Zhang and his colleagues set out to measure how often beneficial mutations actually occur. Using deep mutational scanning — experiments in which researchers systematically create thousands of mutations in a gene and track their fitness effects — they found that advantageous mutations are far more common than the Neutral Theory permits.
But here was the puzzle. If beneficial mutations are so frequent, the rate at which mutations become fixed in populations should be much higher than what we observe. It isn't. The numbers don't add up — too many beneficial mutations, not enough fixation.
Their resolution is elegant. A mutation that helps an organism in one environment may hurt it when conditions change. And conditions change constantly. What was advantageous last season, last century, last ice age, becomes a liability in the next. Beneficial mutations arise, spread partway through a population, and then are undone by the very environmental shift that will produce the next round of beneficial mutations. The river of the genome is not drifting. It is running, and it never arrives.
Zhang calls this framework Adaptive Tracking with Antagonistic Pleiotropy. "We're saying that the outcome was neutral," he explains, "but the process was not neutral."
Sit with that sentence for a moment. The outcome was neutral, but the process was not.
For sixty years, molecular evolution has looked neutral — a genome accumulating changes that appeared random, undirected, indifferent. The Neutral Theory took this appearance and made it a principle. The silence in the data was the explanation. Nothing was happening because nothing needed to happen.
Zhang's team says: everything was happening. The silence was the sound of a thousand forces in equilibrium. What looked like a still river was a current running in every direction at once — adaptation chasing adaptation, each one undone by the environment that demanded the next. The genome isn't drifting. It is perpetually pursuing a target that keeps moving.
This has a philosophical weight that extends beyond genetics. We are pattern-seeking creatures, and one of our deepest patterns is the distinction between action and inaction. Something is happening, or nothing is. A system is under selection, or it is neutral. But Zhang's work reveals a third possibility: a system can be under intense, continuous selection and still produce outcomes indistinguishable from randomness. The forces don't cancel because they're absent. They cancel because they're all present at once.
There is a human echo here, as Zhang himself notes. Modern humans carry genomes shaped by environments radically different from the ones we now inhabit. Our genes were beneficial on savannahs, in ice ages, in small nomadic bands — and each shift erased some of that advantage. We are, in Zhang's framework, always slightly behind. Not because evolution is slow, but because the world is fast.
"We're probably never going to see any population that is fully adapted to its environment," he says, "because a full adaptation would take longer than almost any natural environment can remain constant."
This is not a story of failure. It is a story of pursuit — tireless, unceasing, and structurally incomplete. The genome is not a monument to what worked. It is a diary of everything that was tried.
And what looks like stillness, if you read it closely enough, is the most restless thing in nature.
Song, S., Chen, P., Shen, X. & Zhang, J. (2025). "Adaptive tracking with antagonistic pleiotropy results in seemingly neutral molecular evolution." Nature Ecology & Evolution, 9(12): 2358. DOI: 10.1038/s41559-025-02887-1