CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

12. Diversity Within Species and Population Genetics

 

12.2. The Biological Species Concept

A species is a population of organisms that share a gene pool and are reproductively isolated from other populations. This definition of a species is often called the biological species concept; it involves the understanding that organisms of different species do not interchange genetic information—that is, they don’t reproduce with one another. An individual organism is not a species but, rather, is a member of a species; some people refer to the “male” or “female” species. This is an incorrect understanding of the species concept. A correct statement would be, “the male of the species.” A clear understanding of the concept of species is important as we begin to consider how genetic material is passed around within populations as sexual reproduction takes place. It will also help in considering how evolution takes place.

If we examine the chromosomes of reproducing organisms, we find that they are equivalent in number and size and usually carry very similar groups of genes. In the final analysis, the biological species concept assumes that the genetic similarity of organisms is the best way to identify a species, regardless of where or when they exist. Individuals of a species usually are not evenly distributed within a geographic region but, rather, occur in clusters as a result of barriers that restrict movement or the local availability of resources. Local populations with distinct genetic combinations may differ quite a bit from one place to another. There may be differences in the kinds of alleles and the numbers of each kind of allele in different populations of the same species. Note in figure 12.2 that, within the gene pool of the species, there are 3 possible alleles for color (C+, C, and c). However, how often these alleles appear in the population—the frequencies of these alleles—is different in the four local populations, and the difference in how often an allele occurs is reflected in the colors seen in the individuals of the population.

FIGURE 12.2. Genes, Populations, and Gene Pools

Each individual shown here has a specific combination of alleles that constitutes its genotype. The frequency of a specific allele varies from one local population to another. Each local population has a gene pool that is somewhat different from the others. Notice how differences in the frequencies of particular alleles in local populations affect the appearance of the individuals. Assume that T = long tail; t = short tail; C+ = gray color; C = brown color; c = white color; S = large size; and s = small size.

Gene and Allele Frequencies

Gene frequency and allele frequency are defined as how often an allele is found in a population. In general, the term gene frequency is used when describing the idea that there are genetic differences between populations. The term allele frequency is used when specifically discussing how common a particular form of a gene (allele) is compared, with other forms. For example, the frequency of the blond hair allele is high in northern Europe but low in Africa.

Allele frequency is commonly stated in terms of a percentage or a decimal fraction (e.g., 10%, or 0.1; 50%, or 0.5). It is a mathematical statement of how frequently an allele is found in a population. It is possible for two populations of the same species to have all the same alleles, but with very different frequencies.

As an example, all humans are of the same species and, therefore, constitute one, large gene pool found on Earth. There are, however, many distinct, local populations scattered around the world. These, more localized populations show many distinguishing characteristics, which have been perpetuated from generation to generation. In Africa, alleles for dark skin, tightly curled hair, and a flat nose have very high frequencies. In Europe, the allele frequencies for light skin, straight hair, and a narrow nose are the highest (Outlooks 12.1). People in Asia tend to have moderately colored skin, straight hair, and broad noses (figure 12.3). All three of these populations have alleles for dark skin and light skin, straight hair and curly hair, narrow noses and broad noses. The three differ, however, in the frequencies of these alleles. Once a mixture of alleles is present in a population, that mixture tends to maintain itself, unless something changes the frequencies. In other words, allele frequencies do not change without reason. With the development of transportation, more people have moved from one geographic area to another, and human allele frequencies have begun to change. Ultimately, as barriers to interracial marriage (both geographic and sociological) are leveled, the human gene pool will show fewer and fewer geographically distinct populations.

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FIGURE 12.3. Allele Frequency Differences Among Humans

Different physical characteristics displayed by people from different parts of the world are an indication that allele frequencies differ as well.

People think that allele frequency has something to do with dominance and recessiveness, but this is not true. Often in a population, recessive alleles are more frequent than their dominant counterparts. Straight hair, blue eyes, and light skin are all recessive characteristics, yet they are quite common in the populations of certain European countries. See table 12.1 for other examples. What really determines the frequency of an allele in a population is the allele’s value to the organisms possessing it. Dark-skin alleles are valuable to people living under the bright sun in tropical regions. These alleles are less valuable to those living in the less intense sunlight of the cooler European countries. This idea of the value of alleles and how it affects allele frequency will be dealt with more fully when the process of natural selection is discussed in chapter 13.

TABLE 12.1. Recessive Traits with a High Frequency of Expression

Many recessive characteristics are extremely common in some human populations. The corresponding dominant characteristic is also shown here.

Recessive

Dominant

Light skin color

Dark skin color

Straight hair

Curly hair

Five fingers

Six fingers

Type O blood

Type A or B blood

Normal hip joints

Dislocated hip birth defect

Blue eyes

Brown eyes

Normal eyelids

Drooping eyelids

No tumor of the retina

Tumor of the retina

Normal fingers

Short fingers

Normal thumb

Extra joint in the thumb

Normal fingers

Webbed fingers

Ability to smell

Inability to smell

Normal tooth number

Extra teeth

Presence of molars

Absence of molars

OUTLOOKS 12.1

Your Skin Color, Gene Frequency Changes, and Natural Selection

For centuries we have classified humans into "races" based on superficial traits. One of the most obvious is skin color. Fill out any survey and you will probably find a category asking you to identify yourself according to race. Skin color is almost always what comes to mind when you decide if you are Caucasian, African-American, Hispanic, or mixed. But are there genes involved in this trait? When did humans develop different skin colors? What factors may have led to and stabilized the existence of different-colored groups of people?

Yes, skin color is regulated by your genes. In fact, several genes are involved in the polygenic inheritance of pigment production. Scientists had thought that beginning about 40,000 years ago, modern humans in Europe began to grow paler as they migrated farther north. They hypothesized that pale skin allows more sunlight to penetrate the skin. This allowed more UV light to stimulate the production of vitamin D, used in bone growth and many other essential pathways. One of the genes that apparently causes pale skin, known as SLC24A5, has been identified in many Europeans (but not in Asians). There are two forms of this gene that control the production of proteins used for skin pigmentation. The proteins produced by these two alleles differ by only one amino acid. Almost all Africans and East Asians carry the "dark" form of the gene, whereas 98% of Europeans have the other—the "pale" gene. Careful analysis of DNA now suggests that the source of the pale gene was a mutation and that it increased in frequency in European populations more recently than previously thought, most likely between 6,000 and 12,000 years ago.

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Subspecies, Breeds, Varieties, Strains, and Races

Within a population, genetic material is repackaged into new individuals from one generation to the next. Often, there is very little adding or subtracting of genetic material from a local population of organisms, and a widely distributed species consists of a number of more or less separate groups, known as subspecies (or breeds, varieties, strains, or races). All of these terms are used to describe distinct populations within a species. However, certain terms are used more frequently than others, depending on one’s field of interest. For example, dog breeders use the term breed, horticulturalists use the term variety, microbiologists use the term strain, and anthropologists use the term race (Outlooks 12.2). The most general and most widely accepted term is subspecies. Look again at figure 12.2. The gene pool of the species consists of all the alleles of all individuals of the 4 separate populations. A local that shows differences from other local populations is considered a subspecies. For example, the American robin is found throughout North America from southern Canada to central Mexico (figure 12.4). Of the seven subspecies of this common thrush (Turdus = thrush), the San Lucas robin, Turdus migratorius confinis is found in the highlands of Baja California. It is distinct from the others in having a pale gray-brown under belly instead of the ‘robin red breast.’

Most robins have alleles for red breast coloration; very few individuals have alleles for gray-brown. Since the San Lucas robins are geographically isolated from the main gene pool, they only mate with one another. Thus, the different color patterns result from a higher incidence of gray-brown color alleles in the Baja California populations and a high frequency of the red breast alleles in the other populations.

FIGURE 12.4. Subspecies of the American Robin (Turdus migratorius).

(a) While six of the subspecies of this thrush have the distinctive red breast, the San Lucas subspecies (b), located in the highlands of the Cape District of Southern Baja California, Mexico, has an under belly that is a pale gray-brown.

OUTLOOKS 12.2

Biology, Race, and Racism

The concept of racial difference among groups of people must be approached carefully. Three distortions can occur when people use the term race. First, the designation of race focuses on differences, most of which are superficial. Skin color, facial features, and hair texture are examples. Although these characteristics are easy to see, they are arbitrary, and an emphasis on them tends to obscure the fact that humans are all fundamentally the same, with minor variations in the frequency of certain alleles. A second problem with the concept of race is that it is very difficult to separate genetic from cultural differences among people. People tend to equate cultural characteristics with genetic differences. Culture is learned and, therefore, is an acquired characteristic not based on the genes a person inherits. Cultures do differ, but these differences cannot be inherited and therefore used as a basis for claiming genetic distinctions. Third, a study of the human genome has revealed that there are usually more genetic differences within so-called racial groups than between them. Because of such distortions, the concept that humans can be divided into racial groups is no longer popular among scientists.

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Changing attitudes

12.2. CONCEPT REVIEW

4. Describe the biological species concept.

5. What is meant by the terms gene frequency and allele frequency?

6. How are the terms gene and allele frequency used differently?

7. Give an example of a human characteristic that has a high frequency in Europe and a low frequency in Africa.