Race

B

2

Natural Selection

Natural selection is the principal way that organisms adapt to their environment. It is the very basis of the evolution of all living organisms. In natural selection, a trait that provides individuals with greater evolutionary fitness (reproductive success) will increase in frequency over generations, and one that makes individuals less fit will decrease. An initially rare gene resulting from a single mutation will become common in a population if it produces an effect that enables individuals to better adapt to their environment. Those with the beneficial gene will survive longer and produce more offspring than those without the gene. Offspring who inherit the favorable gene will also leave more offspring, and individuals with the gene will soon outnumber those without it.

Natural selection thus automatically sorts out and preserves useful changes in the gene pool, the total of all genes in a population. Successful genes—or more specifically, certain alleles—become widespread in the gene pool of a population, and therefore the characteristics of individuals with successful genes also spread. How quickly an adaptive trait spreads through a population depends on the magnitude of the difference in fitness between individuals with the trait and those without it.

Scientists have shown that many of the differences in human populations—such as skin color, body type, and susceptibility to specific diseases—evolved through natural selection as adaptations to local environmental conditions. These adaptations are explained in the Variation and Environmental Adaptation section of this article.

B

3

Random Genetic Drift

Besides mutation and natural selection, pure chance factors may change the frequencies of genes present in a population. Most genes occur in two or more forms, or alleles. For each gene, an individual inherits one allele from the mother and one from the father. Even though a parent may have two different alleles of the gene, only one will be passed down to a child. Which allele is passed down is determined entirely by chance at the time of conception. Random genetic drift refers to the change in a population’s gene frequencies resulting from this chance factor. Due to genetic drift, certain alleles will disappear from a population—even if they confer evolutionary fitness—simply because they occur in very low frequencies. At the same time, other alleles will become widespread because they occur in higher frequencies.

---

---

Random genetic drift does not usually have a major effect on a population’s gene pool if the size of the population is large. If the population is small, however, genetic drift can dramatically influence gene frequencies. The founder effect illustrates this phenomenon well. The founder effect refers to the founding of a new settlement by a small group of individuals. The sudden establishment of a small, isolated population, whether intentional or accidental, creates a situation in which the alleles of a very few individuals will become predominant as the population grows. For example, in 1790 the Pacific Ocean island of Pitcairn, which had previously been uninhabited, was occupied by a few mutineers from the British merchant ship HMS Bounty, along with a small group of Polynesian men and women whom the sailors had brought from Otaheite (now Tahiti) and other islands. This low number of settlers passed on a very small sample of genetic variation to the next generations of Pitcairn islanders—a strong founder effect. The genes of these settlers—including alleles that were formerly very rare—would have a high likelihood of passing on to large numbers of descendants, becoming more prevalent in each new generation. In a similar way almost all island populations have unusually high frequencies of certain genes.

Epidemic diseases, wars, and destructive natural events, such as volcanic eruptions, earthquakes, and floods, can create a similar situation, known as a population bottleneck. The calamity leaves a reduced population in which the genes of only a few survivors remain.

B

4

Gene Flow and Migration

Another source of genetic change in human populations is gene flow, the exchange of genes between populations. Gene flow occurs directly when individuals from one population mate with members of another population, thereby introducing their genes into the population. Increased gene flow between populations generally makes them more alike than they had been previously. Gene flow also occurs indirectly. For example, if population A interbreeds with population B, and population B interbreeds with population C, some genes from population A will pass to population C. In this way, gene flow occurs across vast geographic regions and connects distant populations. In fact, global gene flow maintains the unity of the human species, ensuring that people from any two populations in the world can successfully mate. If a human population became isolated and no longer shared gene flow with other populations, it might, over hundreds of thousands of years, lose the ability to breed successfully with other human populations. At that point the isolated population would be considered a new species.

In humans, gene flow often occurs as a result of migration. Migrations most frequently occur on a small scale, as when individuals or families move to a neighboring village, town, or city. Small-scale migration usually takes place at short distances and is reciprocal—that is, members of neighboring populations each migrate to the region of the other population. Large-scale or mass migrations occur when a large group of people moves to a new region, often because of the effects of war or natural disaster.

Mass migration and major population resettlements dramatically increase gene flow. For example, Africans who were brought to the United States as slaves, as well as their descendants, intermixed with white populations. Today the gene pool of those who identify themselves as African American is intermediate between that of American whites and African blacks. On average, African Americans in the United States have 30 percent European ancestry. Those African Americans in the northern United States may have up to 50 percent European ancestry while those in the Southern states—where laws and cultural values long prohibited racial mixing—may have as little as 10 percent European ancestry. This difference illustrates the power that psychological and cultural barriers can have in decreasing gene flow. People who feel deeply rooted in a particular racial or ethnic group may have some animosity toward the mating of people with different physical appearances or from different cultural backgrounds. Religious and socioeconomic differences can also act as barriers to gene flow. However, people are highly social by nature. Even with the effects of racism and ethnocentrism (a belief in the superiority of one’s own social or cultural group), people have always intermarried and interbred with members of neighboring groups.

Historically, natural barriers such as large rivers, seas, deserts, and mountain ranges have prevented migration and reduced gene flow between certain regions. Geographic distance also impeded migrations; people preferred to migrate only short distances. Over the course of the past several centuries, technological improvements in transportation have reduced the influence of geography and distance. For instance, people now can travel easily from one side of the world to the other within a day by airplane. In general, however, populations tend to be more similar to their neighbors and more different from populations that live far away.

C

Variation and Environmental Adaptation

The differences among people most easily and commonly noticed include those in skin color, body shape, the shape of the face and facial features, and hair color and texture. Many of these variations evolved as simple adaptations to the environments in which our ancestors lived. As discussed earlier, current research supports the idea that the modern human species, Homo sapiens, evolved first in Africa within the last 200,000 years and that all living people are descended from a relatively small population. This original population shared a similar climate, and its members probably looked very similar to each other. However, within the last 100,000 years, humans expanded out of Africa and eventually settled a wide range of climates, from hot, humid rainforests to frigid tundras. In order to survive, populations were forced to adapt to extreme conditions both biologically, by the process of natural selection, and culturally, by producing innovations such as clothing.

Variations due to climatic adaptation generally affect exposed parts of the body rather than internal structures. The reason involves a basic fact about the human body: its temperature must stay very close to 37ºC (98ºF), regardless of the external temperature. Overheating of the body to above 42ºC (105ºF) is especially dangerous for the brain and may cause death. Thus, people have evolved a variety of external physical characteristics that work to increase or decrease the exchange of heat between the interior and the exterior of the body. For instance, people indigenous to the Arctic have short, stout bodies adapted to retain heat, while people indigenous to equatorial savannas have tall, lean bodies adapted to dissipate heat. However, these physical distinctions do not mean that all Arctic peoples or all equatorial savanna peoples have identical bodies. To the contrary, the same feature can and often does vary broadly among members of the same population.

Because climatic adaptations affecting skin color and body shape are outwardly visible, populations that evolved in the same type of climate tend to appear similar, and populations that evolved in different climates tend to appear different from each other. However, genetic research has shown that the seemingly large amount of physical variation among people has resulted from very few biological changes. All people differ very little biologically.

Although most scientists agree that variation in physical features most often results from environmental adaptation, other factors may also contribute to differences. Darwin suggested that a type of natural selection known as sexual selection accounts for some of the differences we observe among human populations. According to this hypothesis, some physical traits evolved the way they did because of competition among individuals for mates; traits that provided an advantage in attracting a mate tended to persist. Others believe that a certain amount of variation in human traits is nonadaptive—that is, some human features simply occur in a broad range of variation for no particular reason.

C

1

Skin Color

Skin color, perhaps the most conspicuous human trait, is determined largely by the amount of the pigment melanin in the skin. People with large amounts of melanin have dark skin, and those with little melanin have light skin. The function of melanin is to absorb ultraviolet radiation from the sun. Thus, many scientists have proposed that dark skin, with its high amount of melanin, is an environmental adaptation that evolved to protect people in areas of high solar radiation from sunburn and skin cancer.

In support of this argument, many populations of tropical areas—where solar radiation is most intense—do have dark skin. For example, indigenous people in tropical Africa, Australia, and parts of India and the South Pacific have very dark skin. However, inhabitants of tropical Central and South America have much lighter skin than these populations. Although the reason for this difference is debated, native Central and South American populations usually live in forest areas, where shadows from trees considerably reduce their exposure to solar radiation. Moreover, forests are very humid, a factor that also decreases the intensity of ultraviolet light. Thus, African Pygmies and, to a lesser extent, other Africans who also live in forest areas are lighter in skin color than Africans who live in unforested areas. Darker skin is more often observed in tropical savannas and deserts.

Ultraviolet light has another effect on the human body—it converts certain molecules in the skin to vitamin D, a nutrient necessary for absorption of calcium into the bones. Too little vitamin D leads to rickets, a debilitating disease that causes bones to soften and deform. As African populations migrated north into Europe, where the level of ultraviolet radiation is lower than at tropical latitudes, darker-skinned people would have developed rickets if their diet did not contain enough vitamin D. Women with rickets would have suffered deformation of the pelvis and a high risk of death during childbirth. The European diet of 5,000 to 10,000 years ago, prevalent in wheat and other cereals produced by agriculture, was very poor in vitamin D. But their diet contained a precursor of vitamin D that could be converted to vitamin D through exposure to ultraviolet radiation. Over time, therefore, natural selection favored lighter-colored skin, which maximizes absorption of whatever sunlight is available so that the body can produce enough vitamin D.

Two other observations support the idea that skin color is an adaptation to the level of ultraviolet radiation. First, people with darker skin have a lower incidence of certain types of skin cancer that are most likely produced by exposure to ultraviolet light. In addition, the existence of sun tanning, which darkens skin when it is exposed to ultraviolet radiation, demonstrates the importance of having darker skin in areas of intense solar radiation.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain