Natural selection drives evolutionary change through the differential survival and reproduction of individuals based on their heritable traits. Organisms with characteristics better suited to their current environment are more likely to survive and pass those traits to the next generation. A common misconception is that this process stopped for humans when civilization and technology began to shield us from environmental hardship. However, evolution is an ongoing consequence of having a population that reproduces, and recent genetic evidence confirms that the human species is still adapting to the world we have created.
The Mechanics of Modern Human Evolution
For natural selection to occur, three conditions must be met: variation in a trait, heritability of that trait, and differences in reproductive success resulting from that variation. These three conditions are fully present in the modern global human population. Genetic variation persists across human groups, and a vast number of human traits, from height to metabolism, are passed from parent to offspring.
The concept of fitness in modern humans centers on who successfully reproduces and contributes the most genes to the next generation. While medicine and hygiene have dramatically increased survival rates, they have not eliminated the differences in the number of children people have. Even a slight, statistically measurable advantage in reproductive output can lead to rapid evolutionary change over a few generations.
The selection pressures acting on humanity today are often subtle, operating across enormous population sizes and over short time scales. This process is statistical, favoring certain genetic variants that lead to a small but consistent reproductive edge. The resulting changes are detectable by analyzing shifts in gene frequencies across populations over time.
Observable Examples of Recent Selection
One of the most widely cited examples of recent human evolution is lactase persistence, the ability to digest milk sugar into adulthood. Historically, the gene responsible for producing the lactase enzyme would switch off after infancy. In populations that began domesticating dairy animals and relying on milk as a consistent food source, a genetic mutation allowing lactase production to continue was strongly favored by selection.
This trait is a clear example of gene-culture coevolution, where dairying created a new selective pressure that drove a biological change. The mutation for lactase persistence arose and spread independently in multiple human populations across Europe, the Middle East, and Africa within the last 10,000 years. This rapid spread confirms the strong selective advantage of being able to tap into a reliable, calorie-rich food source.
Other adaptations relate to challenging environments, such as high-altitude regions. Populations living at extreme elevations, such as the Tibetans, Andeans, and Ethiopian highlanders, have independently evolved unique physiological adaptations to cope with low oxygen levels. For instance, Tibetans possess a unique gene variant that allows them to use oxygen more efficiently without increasing their red blood cell count, which is a common but dangerous response to high altitude.
The battle against infectious disease continues to be a powerful, ongoing selective force. The high mortality rates of diseases like malaria have driven the rapid spread of protective genetic traits in affected populations. The sickle-cell trait, which causes mild anemia in carriers but provides resistance to malaria, is a classic example of this trade-off. A more recent example is the rapid evolution of malaria resistance in the Cabo Verde archipelago over just 20 generations, where a specific mutation that prevents the $Plasmodium \ vivax$ parasite from invading red blood cells surged in frequency.
A final, more subtle example of ongoing selection is the change in human reproductive timing. Analysis of historical records shows that certain genetic variants associated with a lower age at first birth are being selected for in some populations. This occurs because those who start reproducing earlier tend to have more offspring over their lifetime. Conversely, in low-mortality environments, selection may favor later reproduction, as delaying the onset of childbearing allows for greater parental investment and resource accumulation, ultimately improving offspring success.
How Culture and Technology Alter Selective Pressures
The rise of advanced medicine, improved hygiene, and global food security has not halted evolution, but it has dramatically altered the selective landscape. Medical advances have relaxed many traditional pressures, such as high infant mortality from infectious diseases. Traits that might have been severely disadvantageous in the past, like poor eyesight or a genetic predisposition to certain diseases, are no longer a reproductive death sentence.
The modern environment has simultaneously introduced an array of new selective pressures. The shift toward diets rich in processed foods, sugar, and fat, coupled with a sedentary lifestyle, has created a mismatch with our ancestral biology. This has led to new selection pressures on genes related to metabolism, with some variants potentially offering protection against chronic conditions like type 2 diabetes and obesity.
Technology also creates evolutionary dilemmas, such as the increasing reliance on Cesarean-section deliveries (C-sections). For millions of years, the size of the fetal head and the width of the female birth canal were balanced by selection. By removing the fatal risk of obstructed labor, C-sections have relaxed this pressure, potentially allowing genetic variants that contribute to a larger fetal head size to persist and increase in the population.
Increasing global travel and urbanization influence human gene flow and mating patterns. Greater mobility and the breakdown of geographic isolation are leading to a more mixed global gene pool, which increases genetic variation. At the same time, cultural factors like assortative mating—the tendency to choose partners with similar characteristics—can create new, non-random patterns of selection within modern societies.