As just another species of animal subject to changes in the environment, humans are not detached from natural selection. But with modern advances in technology, and the huge movements of people mixing things about a bit, many have suggested that perhaps we have broken free of its impacts. But new research claims to have found traces of evidence that humans may indeed still be evolving.
The study looked at the DNA of over 200,000 people living across both the United States and Britain, to see if they could tease out any changes in genetic variation in these populations over time. They report that their analysis of these genomes, published in PLOS Biology, shows natural selection is weeding out gene variants most often associated with Alzheimer’s disease and heavy smoking.
Their results suggest that one variant of the ApoE4 gene, which is thought to increase the risk of late-onset Alzheimer’s has dropped slightly. This might seem counterintuitive as you would think that any adult would have passed their genes on before the disease sets in, but the researchers think that as people are choosing to have children in their 40s and 50s, when those with the variant are at most risk of dying, the gene is being selected against.
While the researchers did find some changes in the frequency of certain genes, it is important to note that these changes were incredibly small. For example, they found that one gene associated with heavy smoking, known as CHRNA3, declined by 1 percent between generations when people over the age of 80 were compared with those over the age of 60.
But as they did not screen people under the age of 40, the researchers were actually unable to prove this shift definitively. “It's a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations,” explained study co-author Joseph Pickrell.
The problem here is scale. While there are indeed different selection pressures occurring on different populations and groups, the long generation time of people means that the changes these pressures may be favoring take a long time to be reflected in the variation in our genes.
That’s not to say it can’t happen, though. One of the most well-studied examples of natural selection occurring in humans in recent times is that of the evolution to be able to digest milk into adulthood. Obviously, milk is usually only drunk by infants, and yet most Europeans, along with a few small groups in Africa and India, have evolved within the last 10,000 years the ability to continue digesting the white stuff way into adulthood, after the spread of agriculture.