University of Michigan study suggests beneficial mutations more common than assumed
A University of Michigan study published in Nature Ecology and Evolution challenges a long-standing assumption in evolutionary biology. Researchers led by evolutionary biologist Jianzhi Zhang found that beneficial mutations may be far more common than the Neutral Theory of Molecular Evolution predicts. However, the study suggests these advantageous mutations rarely become permanent because environments change before they can spread through a population. Using deep mutational scanning data from yeast and E. coli, the team observed that more than 1% of amino acid changing mutations were beneficial, a figure described as enormous in evolutionary theory. The team then tested their hypothesis by comparing yeast groups over 800 generations in stable versus shifting environments. The group in changing conditions showed fewer beneficial mutations, as helpful changes often lost their advantage when conditions shifted. Zhang described the framework as “Adaptive Tracking with Antagonistic Pleiotropy,” meaning populations constantly chase changing surroundings while mutations have tradeoffs depending on the environment.
What’s reported
The study challenges the Neutral Theory of Molecular Evolution, which holds that most fixed genetic changes are neutral.
Researchers used deep mutational scanning datasets from yeast and E. coli to measure the effects of many mutations.
Over 1% of amino acid changing mutations examined were beneficial.
The team calculated that if beneficial mutations are that common, more than 99% of amino acid substitutions should be adaptive, yet observed rates do not match.
The researchers concluded that environments change too quickly for beneficial mutations to fix.
The team compared two groups of yeast over 800 generations: one in a stable environment, one in a shifting environment of 10 different growth media (each lasting 80 generations).
The changing environment group had fewer beneficial mutations that became fixed.
The study was supported by the U.S. National Institutes of Health and published in Nature Ecology and Evolution.
Open questions
It remains unclear whether the same patterns of beneficial mutation abundance apply to multicellular organisms such as animals, plants, and humans, as the data came mainly from single-celled organisms.
The team plans to investigate why organisms take so long to fully adapt even in constant environments.
Key figures
Jianzhi Zhang, evolutionary biologist and professor of ecology and evolutionary biology at the University of Michigan (lead author)
Siliang Song, former University of Michigan graduate student (co-author)
Xukang Shen, former University of Michigan graduate student (co-author)
Piaopiao Chen, former University of Michigan postdoctoral researcher (co-author)
Sources: ScienceDaily
