People learning about evolution often ask if humans are still evolving. The answer is almost certainly "yes". However, one must remember that natural selection works across generations, so animals that reproduce in their twenties (for example, humans) will require centuries to millennia for evolution to be evident. With that in mind, here are four examples.
One example of recent human evolution is the development of lactose tolerance in certain populations of humans. All mammals generate a lactase enzyme when young, so that they can digest lactose, the principal form of carbohydrate in milk. That makes sense, because mammalian young by definition are suckled by their mothers. Almost all mammals have genes that then suppress production of lactase, once the young reach the age that they are done nursing and thus after they no longer need to be able to digest milk. Suppression of production of lactase after infancy makes sense evolutionary, because any natural mammal would be wasting its resources to make lactase when it no longer drank its mother's milk.
Then along came some populations of humans that had domesticated cattle in the last few thousand years. In these populations, individuals who aberrantly continued to produce lactase would have an advantage, because they could consume cow's milk, and survive on dairy products, when their more "normal" cousins could not. Recent studies have shown that at least four different genes (or more correctly, four different alleles) allowing production of lactase throughout the human lifespan have arisen by mutation in at least four different human populations that had domesticated cattle. These lactose-tolerate "mutants", who include most Europeans and many Tanzanians, Kenyans, and Sudanese, are thus an example of recent human evolution. They're also an example of convergent evolution, in that these four groups have arrived at the same capability via independent pathways of mutation and evolution.
Genetics and the high-carb diet
A second example of recent human evolution is similarly related to digestion. Human digestion of starch, or carbohydrate, involves the enzyme amylase, and production of amylase depends on a gene called AMY1. AMY1 is a gene that shows copy-number varation between individuals, which means that any two of us are likley to have different numbers of copies of this gene (and so have different total lengths of DNA). Recent work has shown that people with more copies of AMY1 produce more amylaze and thus are more efficient metabolizers of starches. The researchers found that people in populations that consume more carbohydrate in their diets have more copies of AMY1, whereas people in populations that eat more protein have fewer copies of AMY1. It appears that, in populations eating a lot of carbohydrate, having more copies of AMY1 and thus being able to metabolize carbohydrate more effectively has allowed greater survival. Such populations have thus drifted toward possession of more copies of AMY1.
Genetics and a remarkable low-carb high-fat diet
A third example of recent human evolution is also related to digestion. The Inuit people of Greenland eat a distinctive diet consisting mostly of the meat of whales, seals, and fish, and thus remarkably rich in protein and fatty acids, and especially omega-3 polyunsaturated fatty acids. Researchers examining the genomes of people of largely Inuit ancestry found an exceptional abundance of alleles that promote processing of fatty acids. It appears that, in this population eating a great abundance of fatty acids, natural selection favored individuals with alleles favoring more extensive processing of fatty acids, so that the population as a whole moved toward a genome rich in those alleles.
Living the high life - or not
In a fourth example from Tibet, the genetic basis of evolution is less clear, but the effects of natural selection are more so. At the high elevations typical of Tibet, the air is so thin that a human can easily suffer oxygen deprivation. Researchers working there found that women in the native population can be divided into two distinct groups with differing concentrations of oxygen in their blood. The difference in oxygen concentration appears to be controlled by difference in alleles of one gene, although that gene is yet to be identified. The result, in terms of natural selection and evolution, is nonetheless clear: the researchers found that women in the group with greater concentations of oxygen in their blood give birth to children who are more likely to survive infancy than the children of women with lesser concentrations of oxygen in their blood. Thus natural selection is at work at this moment, because the preferential survival of the children of women from the high-oxygen group means that the population is evolving toward a greater proportion of individuals with the allele for greater oxygen concentrations.
Beall, C.M., et al., 2004, Higher offspring survival among Tibetan women with high oxygen saturation genotypes residing at 4,000 m: Proceedings of the National Academy of Sciences, v. 101, p. 14300-14304.
Beall, C.M., et al., 2007, Two routes to functional adaptation: Tibetan and Andean high-altitude natives: Proceedings of the National Academy of Sciences, v. 104, p. 8655-8660.
Fumagalli, M., et al., 2015, Greenlandic Inuit show genetic signatures of diet and climate adaptation: Science, v. 349, p. 1343-1347.
Perry, G.H., et al., 2007, Diet and the evolution of human amylase gene copy number variation: Nature Genetics . . . .(doi:10.1038/ng2123)
Shadan, S., 2007, You are what you ate: Nature, v. 449, p. 155.
Tishkoff, S.A., et al., 2006, Convergent adaptation of human lactase persistence in Africa and Europe: Nature Genetics, v. 39, p. 31 - 40.
Troelsen, J.T., 2005, Adult-type hypolactasia and regulation of lactase expression: Biochimica et Biphysica Acta, v. 1723, p. 19-32.
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