After reading this chapter, many students are curious to know if humans are still evolving. The answer is yes. As a species, we continue to respond to selective pressures biologically and culturally. This final section will focus on three contemporary examples of human evolution. Before beginning, let’s review the conditions necessary for natural selection to operate on a trait. First, the trait must be heritable, meaning it is transmitted genetically from generation to generation. There must also be variation of the trait within the population and the trait must influence reproductive success. Three examples of traits that meet these criteria are immunity to the Human Immunodeficiency Virus (HIV), height, and wisdom teeth (Andrews, Kalinowski, and Leonard 2011).
AIDS is a potentially fatal infectious disease caused by HIV, a zoonosis believed to be derived from Simian Immunodeficiency Viruses (SIVs) found in chimpanzees and monkeys and most likely transmitted to humans through the butchering of infected animals (Sharp and Hahn 2011). In total, 40 million people have died from AIDS-related illnesses since the start of the global epidemic in the 1980s. There were 38.4 million people around the world living with AIDS as of 2021, including 1.5 million new cases and 650,000 deaths in that year alone (UNAIDS 2021). A disease causing this level of morbidity and mortality represents a major selective pressure, especially given that infection can occur before birth (Goulder et al. 2016), thereby affecting future reproductive success.
The majority of people in the world are highly susceptible to HIV infection, but some are not. These latter individuals are homozygous for a rare, recessive allele at the CCR5 locus that makes them immune to HIV. Heterozygotes who inherit a single copy of this allele are more resistant to infection and, when infected, the disease takes longer to progress in the event that they are infected. The mechanism by which the allele prevents infection involves a 32-base pair deletion in the DNA sequence of the CCR5 gene, creating a nonfunctioning receptor on the surface of the cell that prevents HIV from infecting the cell. The allele is inherited as a simple Mendelian trait, and there is variation in its prevalence, ranging as high as 14% of the population in northern Europe and Russia (Novembre, Galvani, and Slatkin 2005; see Figure 2.19). What is interesting about the allele’s geographic distribution is that it does not map onto parts of the world with the highest rates of HIV infection (Figure 2.20), suggesting that AIDS was not the original selective pressure favoring this allele (see Figures 2.19 and 2.20).
Given its current geographic distribution, the bubonic plague, which ravaged Europe repeatedly from the 14th to the 19th centuries (Pamuk 2007), was initially proposed as the selective agent. Subsequent research suggests smallpox, which killed up to 400,000 people annually in 18th-century Europe (Hays 2005), was more likely the selective pressure (Novembre, Galvani, and Slatkin 2005). Given the mortality rates for smallpox (Crosby 2003), an allele that conferred immunity was highly advantageous, as it is for those faced with the threat of HIV infection today.
Height is another example of a trait experiencing selective pressure. If you have ever toured a historical site, you have likely hit your head on a doorframe or become claustrophobic trying to squeeze down a narrow hallway under a lower-than-average ceiling. It is not your imagination. Humans have gotten taller in recent centuries. In fact, the average height of people in industrialized nations has increased approximately 10 centimeters (about four inches) in the past 150 years. This increase has been attributed to improvements in nutrition, sanitation, and access to medical care (Hatton 2014). But this is only part of the story.
Height is highly heritable. Studies indicate 80% of variation in height within populations is due to genetics, with 697 different genetic variances identified as having an effect on adult stature (Devuyst 2014). Multiple studies also demonstrate a positive relationship between height and reproductive success for men (Andrews, Kalinowsky, and Leonard 2011). This is primarily due to sexual selection and nonrandom mating, namely women’s preferences for taller men, which may explain why height is one characteristic men often lie about on dating websites (Guadagno, Okdie, and Kruse 2012). Sexual selection also plays out with regard to economic success in Western cultures, with taller men more likely to be in higher-level positions that pay well. Research demonstrates an inch of height is worth an additional $789 per year in salary, meaning a man who is six feet tall will earn on average $5,525 more per year than an identical man who is five foot five purely due to heightism bias (Gladwell 2007). Over the course of a career, this can add up to hundreds of thousands of dollars, likely allowing taller men to attract more potential mates, increasing their reproductive success.
Wisdom teeth are also undergoing evolutionary pressure. Have you or anyone in your family had their wisdom teeth removed? While it can be a painful and expensive process, it is a common experience in Western nations. It begs the question as to why there is no longer room in our mouths for all of our teeth? Biological anthropologist Daniel Lieberman offers several reasons, including that modern humans are growing faster and maturing earlier, which could be leading to impaction if skeletal growth takes place faster than dental growth. He also argues that the soft diets many modern humans consume generate insufficient strain to stimulate enough growth in our jaws to accommodate all of our teeth. Lastly, as the human brain has expanded over the past hundreds of thousands of years, it is taking up more space in the skull, causing the jaw to shrink, leaving no room for third molars (Lieberman 2011).
Conversely, do you know anyone whose wisdom teeth never came in? That is fairly common in some populations, suggesting evolutionary pressure favoring the absence of wisdom teeth has been culturally influenced. The oldest fossil evidence of skulls missing third molars was found in China and is 300,000 to 400,000 years old, suggesting the earliest mutation selecting against the eruption of wisdom teeth arose in Asia (Main 2013). Since that time, jaws have continued to decrease in size to the point they often cannot accommodate third molars, which can become impacted, painful, and even infected, a condition physical anthropologist Alan Main argues might have interfered with the ability to survive and reproduce in ancestral populations (Main 2013). As we have learned, a mutation that positively influences reproductive success—such as being born without the trait to develop wisdom teeth—would likely be selected for over time. Evidence in modern humans suggests that this is the case, with 40% of modern Asians and 45% of Native Alaskans and Greenlanders (populations descended from Asian populations) lacking wisdom teeth. The percentage among those of European descent ranges from 10 to 25% and for African Americans is 11% (Main 2013). Later chapters of this textbook emphasize that directional selection progresses along a particular path until the environment changes and a trait is no longer advantageous. In the case of wisdom teeth, the ability of modern dentistry to preempt impaction through surgery may, in fact, be what is preventing natural selection from doing away with wisdom teeth altogether.