nationalhumanitiescenter | First, some terminology and background, especially for the nonspecialist. “Evolution” has different meanings to different scientists; a population geneticist, for example, views evolution simply as changes in allele frequencies (that is, the frequencies of the variant forms of a gene) over time. Such changes are usually random, reflecting the fact that not everybody leaves offspring, so by chance some alleles increase in frequency and others decrease in frequency over time. These random fluctuations, known as genetic drift, occur more rapidly in small populations than in large ones. Genetic drift results in loss of genetic variation within populations and increases in genetic differences among populations over time, and is countered by migration among populations, which restores genetic variation within populations and decreases genetic differences among populations. Thus, to a population geneticist, since allele frequencies are always changing because of drift and migration, by definition evolution is always happening, and it therefore makes no sense to say that humans are no longer evolving.
But to most people who are not population geneticists, biological evolution means natural selection, in the Darwinian sense: increase in the frequency of an inherited trait which enhances the survival and/or reproductive success of individuals with that trait, also referred to as genetic adaptation. Often, this is expressed as a response to a change in the environment, which in turn leads to a change in those traits that confer enhanced survival/reproduction. Familiar examples of genetic adaptations that resulted in human evolution include bipedality, increased brain size, loss of body hair, and variation in skin pigmentation. To say that humans have stopped evolving, then, is to say that such inherited traits no longer matter when it comes to how humans respond to their environment. This is the view that I often hear: culture acts as a barrier or a buffer between us and the environment, thereby preventing human evolution.
However, if culture is a buffer, it is an imperfect one. For example, humans are plagued by a variety of infectious diseases, and for every success story (e.g., eradication of smallpox and polio) there are diseases that resist our efforts at finding vaccinations or cures (e.g., malaria and AIDS). And you can be sure that if our culture is unable (or unwilling) to do what it takes to prevent or cure a disease, then genetic resistance will indeed occur and will increase in frequency. Some classic examples of natural selection in humans involve genetic variants that increase resistance to malaria, such as sickle-cell anemia. Genetic variants that increase resistance to AIDS have been identified, and it is a safe bet that such variants will increase in frequency if there is no cure/vaccination for AIDS – but such increase comes at the expense of individuals who do not carry such genetic variants. Evolution in response to infectious disease is thus an ongoing story in humans.
But there is an alternative view to that of culture as a (leaky) barrier to human evolution, which can be expressed as follows: humans have been evolving and continue to evolve, not just in spite of culture, but because of culture. That is, cultural practices have actually caused humans to evolve, and a classic example is lactose tolerance. The story goes as follows: lactose is the major sugar present in mammalian milk, and most mammals stop making lactase, the enzyme that digests lactose, shortly after weaning because they are never again exposed to lactose in their diet. This, incidentally, is a nice example of the evolutionary principle of “use it or lose it”: there is no need to continue making lactase if there is no lactose in the diet. Some humans are weird, however, in that they retain the ability to digest lactose into adulthood. It turns out that the frequency of this trait, known as lactose tolerance (or lactase persistence), is highly correlated with milk-drinking populations in Europe and Africa, and was apparently driven to high frequency by natural selection in those populations. Thus, a human cultural trait – domestication of cattle, thereby providing cow’s milk as a new source of nutrition – resulted in human evolution (namely, an increase in lactose tolerance).