As we discussed above, it is important that instructors expose students to multiple examples of a concept (Catrambone and Holyoak 1989). This section of the paper (and the supplemental materials described in Table 2) provide the background information necessary to present these examples of human evolution to students, as well as numerous citations an instructor could use to find additional information.
HIV Resistance and the CCR5 Locus
AIDS is a disease of the human immune system caused by the human immunodeficiency virus (HIV) that kills over 2 million people each year (Joint United Nations Programme on HIV/AIDS 2009). Most people in the world are highly susceptible to HIV infection, but individuals who are homozygous for a rare allele at the CCR5 locus are essentially immune to the disease (Samson et al. 1996). Simply put, HIV enters a white blood cell by binding to the CCR5 protein. A rare resistant allele, called CCR5-Δ32, has a 32-base pair deletion in the DNA sequence of the CCR5 gene. This deletion causes a frame shift, creating a non-functional receptor and preventing HIV from infecting the cell (Samson et al. 1996).
Is There Variation in the Population?
CCR5-Δ32 has a frequency of around 10% in many European countries and in Russia (Samson et al. 1996; Stephens et al. 1998), but this mutated allele is essentially absent in Asia and Africa (Samson et al. 1996). Students often believe that mutations occur because they are needed, and if that were true, the CCR5-Δ32 mutation should be most common in Africa where HIV is more prevalent.
The reason why European populations have high frequencies of the CCR5-Δ32 allele is not well understood. Mathematical models suggest that a random drift of a neutral allele cannot explain the high frequency of CCR5-Δ32 in European populations (Stephens et al. 1998), meaning that selection was likely responsible. However, debate remains about what may have caused this selection pressure. Some researchers suggest that outbreaks of the bubonic plague, which killed 25–33% of Europeans about 650 years ago, are the most likely source of strong selective pressure for this mutation (Stephens et al. 1998). Other researchers argue that the plague would not have provided sufficient selective pressure to create the current frequency and distribution of the CCR5-Δ32 allele (Galvani and Slatkin 2003). Studies have also shown that the CCR5-Δ32 allele does not confer resistance to the plague in mice (Mecsas et al. 2004). Instead, Galvani and Slatkin (2003) suggest it is more likely that the CCR5-Δ32 allele conferred resistance to smallpox and was therefore strongly selected. Finally, one hypothesis proposes that selective pressure from outbreaks of both smallpox and hemorrhagic plague explains the current frequency and distribution of the mutated CCR5 allele (Duncan et al. 2005).
Is This Trait Heritable?
The immunity conferred by CCR5-Δ32 is inherited as a simple Mendelian trait, so it is heritable. We use this example to emphasize to students that the ability of organisms to survive and reproduce is influenced by genotypes present at a specific loci. This should help students connect natural selection with Mendelian genetics (two of the most important concepts in biology). We also show students the DNA sequence of CCR5 and CCR5-Δ32 alleles in order to provide a concrete example of how DNA sequences influence phenotypes (Kalinowski et al. 2010). Later, we use CCR5-Δ32 allele frequencies as an example to illustrate Hardy–Weinberg proportions.
Does Having This Trait Affect the Ability of an Individual to Survive or Reproduce?
Two copies of CCR5-Δ32 (homozygosity) confer a high level of resistance to HIV infection (Samson et al. 1996). Even one copy of CCR5-Δ32 provides protection from AIDS (Stewart et al. 1997), most likely by prolonging the transition from HIV infection to AIDS. As long as HIV affects an individual’s reproductive success in the human population, there will be selection for the CCR5-Δ32 allele. Globally, only 42% of individuals in need of treatment for AIDS are being treated (Joint United Nations Programme on HIV/AIDS 2009), suggesting that if CCR5-Δ32 exists in a population, it will be selected for.
Students frequently suggest that humans are evolving to be taller, and human height provides an ideal example to illustrate some of the complexities of natural selection. As students suspect, human height has increased substantially over the past three decades (Smith and Norris 2004; Freedman et al. 2000). However, only some of that change in certain populations seems to be due to evolution rather than improved nutrition and medical care (Mueller and Mazur 2001).
Is There Variation Within the Population?
Human height is clearly variable, and a histogram shows human height has a “bell”-shaped distribution. We have provided height data collected by Karl Pearson (Table 2) to illustrate this point, but a similar figure could be made from students’ heights. Pearson’s data are from the early twentieth century, and as many students will note, people in most countries are taller now. Average adult height has increased about one inch between 1960 and 2002 (Ogden et al. 2004).
Is Height Heritable?
Human height is highly heritable, and in fact, the first studies of heritability examined human height. Sir Francis Galton started this work and his younger colleague, Karl Pearson, developed the statistical method of correlation to analyze father–son height data. Current studies estimate heritability of height in humans to be 0.8, meaning that about 80% of the variation in height within populations is due to genetics (Visscher 2008).
Height is a quantitative trait, which means that it is controlled by many genes of small effect. At least 20 genes have been found that contribute 0.2–0.6 cm to height per allele (Weedon et al. 2007, 2008). These genes explain only about 3% of the variation in human height (Weedon et al. 2008), which suggests that many more genes of small effect will be found.
Twin studies are an interesting method of understanding heritability. Studies show that after birth, monozygotic (identical) twins grow to be more similar in height than dizygotic (fraternal) twins. Monozygotic twins reared apart are more different in stature than monozygotic twins reared together, but are still more similar than dizygotic twins who grew up together (Chambers et al. 2001). In dizygotic twins aged 14–36 months, 61–82% of variation in height can be attributed to genes (Chambers et al. 2001).
Does Being Taller (or Shorter) Affect an Individual’s Ability to Survive or Reproduce?
Several studies have shown a positive relationship between height and reproductive success—in particular for men. For example, height was positively related to number of children in a sample of Polish men (after controlling for other factors that affected height in this sample, such as locality of residence; Pawlowski et al. 2000). A study of West Point Cadets (class of 1950) also showed that taller men had more children (Mueller and Mazur 2001). This study did not control for potential environmental differences, but used a highly homogeneous sample—mostly middle-class men of European descent who came from rural backgrounds and had parents who had at least a high school degree. Finally, a study of British men born in 1958 found that taller men were less likely to be childless than shorter men, and men who were taller than average were more likely to find a long-term partner and to have several long-term partners (Nettle 2002b). This study controlled for socioeconomic status and serious health problems. Together, this research suggests that—in some populations—men are evolving to be taller, but it is likely that in other populations, male height is not evolving; selection could even be moving height in the other direction.
Selection for taller men is likely due to sexual selection, meaning that the increase in reproductive success is mediated by opportunities to mate. Women frequently prefer taller men for dates, sexual partners, or husbands (Buss and Schmitt 1993; Ellis 1992; De Backer et al. 2008). For example, a study of personal ads showed that 80% of women advertised for men six feet or taller, even though the average American male is five feet nine inches. Interestingly, studies of reproductive success do not show that taller men have more children within any single marriage, but instead are more likely to remarry and have a second family (Mueller and Mazur 2001).
Female preference for tall men is not likely to lead to unconstrained directional selection. Extremely tall men (those in the top decile) are slightly more likely to be childless. They are also more likely to have a work-impairing, long-standing illness, and they have a slightly higher mortality (Nettle 2002b). Additionally, mating partners who are more similar in height are more likely to have non-induced labor and have higher numbers of live-born children (Nettle 2002a, b).
The relationship between a woman’s height and fitness is more complicated. In developed countries such as America and England, the average woman is five feet four inches. In these countries, shorter women have the highest reproductive success and are least likely to be childless (Nettle 2002a). In contrast, in less developed countries such as Guatemala and Gambia, a woman’s height is positively related to reproductive success. In these countries, tall women are more likely to have healthier children (Sear et al. 2004; Pollet and Nettle 2008). In all studies, the effect of height on reproductive success of women is less drastic than in men.
What Else Affects Human Height?
As students are likely to note, human height is strongly affected by nutrition and health care as well as by genes. Because of this, the average height and weight of children is often used to monitor the health of populations worldwide. For example, several studies have shown that North Koreans are shorter than South Koreans (see Schwekendiek and Pak 2009 for a meta-analysis), and researchers attribute these differences to nutrition. Similarly, height increased in the Japanese population in the generation born after World War II (Ali et al. 2000). Height also tends to vary by socioeconomic status within countries; children from more well-off families are taller than children from poorer families (even in developed countries like the U.S.; Eveleth and Tanner 1990). Both nutrition and childhood illness are oft-cited sources of growth limitations. These two forces can form a positive feedback loop. Infections cause nutritional status to deteriorate, and malnourished children are more susceptible to illness (Eveleth and Tanner 1990).
In summary, height is highly heritable in ideal conditions, but the effects of childhood illness and malnutrition can have large and lasting effects on overall height. This point is both important and challenging for many students. Understandably, they have a hard time imagining the mechanisms through which genes could have some effect but not complete control, and instead often consider a trait a result of either nature or nurture, but not both.