Ever since Darwin proposed his theories in On the Origin of Species, scientists have been able to observe and explain many biological phenomena in which certain mutations start off as a rare occurrence in a population and then balloon into a widely possessed trait. But a widely accepted theory that explains this on a molecular level may have been disproven in a recent study by geneticists at the University of Chicago and the University of Oxford.
Generally, the spread of mutations in a population can be explained by natural selection. Favorable traits or characteristics in individuals or populations confer improved fitness to the trait’s owner. As a result, these individuals are able to reproduce more effectively and increase the number of individuals that have the mutation. Eventually, most in the population will have the trait.
This phenomena can be observed on a larger scale. For example, during the Industrial Revolution, moth populations with darker pigmentation were better camouflaged in the high soot environment, which increased their chance of avoiding predators. Because of the improved fitness conferred by the mutation, dark pigmentation quickly spread through the moth population.
One example in humans is sickle cell anemia. Because the gene for the disease is co-dominant, if one copy of the gene is positive for sickle cell and the other is not, half of the individual’s blood cells will be sickle-shaped and half will be normal. In Africa, where malaria is more prevalent, there is a much higher proportion of individuals who are heterozygous for the sickle cell trait. The sickle-shaped blood cells confer resistance to malaria, while the normal blood cells prevent full-scale sickle cell anemia from developing.
Though such phenomena can be observed on a larger scale, researchers have been developing theories to explain them from a molecular standpoint. One such theory, widely accepted in the scientific community, is the “classic selective sweep” model.
This model states that if a certain genetic mutation is favorable, the DNA adjacent to it, even though it does not play a role in the mutation, would also be preserved and passed down. The reasoning behind this is based on the assumption that the mutation would be so beneficial that it would be passed down and disseminated very quickly, preventing variations in the surrounding DNA from developing.
Therefore, if the model is correct, researchers should be able to observe low genetic diversity in DNA sequences around beneficial mutations. In this study, published in Science last month, they analyzed the genomes of 179 humans, looking to see whether this actually occurred.
They compared mutations in which changes in the nucleotide sequence would alter an amino acid in the encoded protein to mutations in which nucleotide changes would not alter it. Because a mutation in which an amino acid is changed is more likely to confer an evolutionary advantage compared to a mutation in which nothing is different, the diversity in the adjacent DNA in the first case would be lower than in the second.
But after analyzing these genetic sequences, the researchers actually found little difference in the level of diversity between the two groups. This suggests that mutations in human populations may not have come about via the selective sweep model.
However, this does not imply that the model is incorrect, or that humans are not as heavily shaped by diversity from an evolutionary standpoint. Instead, it is possible that classic selective sweeps occurred too infrequently to have had noticeable effects on genomic diversity. Furthermore, other scientists have suggested that because the human population is spread widely across the globe, any mutation that followed this model would have to confer a strong advantage to affect a significant proportion of the population.
Regardless, the researchers emphasize in the paper that future studies should rely less heavily on the selective sweep model to explain human adaptations. Even for a principle as well-established and accepted as natural selection, its molecular explanations are still in flux, and the most common mechanisms have yet to be discovered.