Given the current global burden of non-communicable diseases like heart disease, cancer, and diabetes discussed in this chapter, many students ask why humans have not yet evolved in response. First, the health conditions highlighted here do not typically have repercussions on reproductive success, meaning natural selection cannot act to favor one genotype over another to protect against them. There also may not have been sufficient time for natural selection to act (Stearns et al. 2008). The cultural transformation of our food supply and lifestyle came about quickly. The transition from foraging to farming took place beginning 12,000 years ago, industrial food production came about in the last 200 years, and technologies like television, the internet, and social media that promote sedentarism are less than 75 years old—one human lifetime. Even under the strongest selective pressure, evolution takes many generations. For example, the mutation that led pastoral populations to be able to digest fresh milk likely took 8,000 years, or 325 generations, to reach a frequency of 90% (Crow and Kimura 1970). This does not mean, however, that humans have stopped evolving. As a species, we continue to respond to selective pressures biologically and culturally. This portion of the chapter will focus on contemporary examples of human evolution.
Before beginning, let’s remind ourselves of the modern definition of evolution, which is a change in allele frequencies across generations in a given population for a particular trait. We must also 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. Non-inherited traits are learned and include things like cultural preferences for certain types of foods or who we think it is best to marry. Not only must a trait be heritable, there must also be variation of the trait in human populations and the trait must influence reproductive success. Three examples of traits that meet these criteria are immunity to HIV, height, and wisdom teeth (Andrews et al. 2011).
AIDS is a potentially fatal infectious disease caused by the Human Immunodeficiency Virus (HIV), a zoonosis believed to be derived from Simian Immunodeficiency Viruses (SIVs) found in chimpanzees and monkeys, most likely transmitted to humans through the butchering of infected animals (Sharp and Hahn 2011). In total, 35 million people have died from AIDS-related illnesses since the start of the global epidemic in the 1980s. There were 36.7 million people around the world living with AIDS as of 2016, including 1.8 million new cases and 1 million deaths in that year alone (UNAIDS 2018). 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), having an affect on future reproductive success.
The majority of people in the world are highly susceptible to HIV infection, but some are not. There are individuals who are homozygous for a rare, recessive allele at the CCR5 locus who are immune. Heterozygotes who inherit a single copy of this allele are more resistant to infection and the disease takes longer to progress in the event 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 non-functioning 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 et al. 2005; see Figure 16.8). 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 (UNAIDS 2018), suggesting that AIDS was not the original selective pressure favoring this allele (Figure 16.10 and Figure 16.11).
Figure \{\PageIndex{1}\}: Map of CCR5-delta32 allele distribution.Figure \{\PageIndex{2}\}: Map of global HIV infection.
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 et al. 2005). Given the mortality rates for smallpox and the selective pressure it has exhibited on humanity for centuries (Crosby 2003), an allele that conferred immunity was highly advantageous, as it is for those faced with the threat of HIV infection today. There are efforts to employ the natural immunity this mutation provides in the creation of an AIDS vaccine (Lopalco 2010).
Height is another example of a trait currently 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 et al. 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 et al. 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 (Gladwell 2007). Over the course of a thirty-year career, this adds up to hundreds of thousands of dollars, likely allowing the taller man to attract more potential mates and increase his reproductive success.
Wisdom teeth are also undergoing evolutionary pressure. Have you or anyone in your family had their wisdom teeth removed? It can be a painful and expensive process, and it is a common experience in Western nations. Conversely, do you know anyone whose wisdom teeth never came in? That is fairly common in other populations, suggesting evolutionary pressure favoring the absence of wisdom teeth has been culturally influenced. According to research by physical anthropologists, 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 in an environment where food was cooked before eating—would likely be selected for over time. Evidence in modern humans suggests 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). Earlier chapters in this text 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.
Figure \{\PageIndex{1}\}: Map of CCR5-delta32 allele distribution..png?revision=1)
Figure \{\PageIndex{2}\}: Map of global HIV infection.