_{Evolution on}
a Small Scale

Chapter

15

Outline

15.1 Microevolution

• Microevolution occurs when allele frequencies change from one generation to the next.233

• The Hardy-Weinberg principle describes a nonevolving population in terms of allele frequencies.234

• The raw material for microevolutionary change is mutations.235

• Mutations, gene flow, nonrandom mating, genetic drift, and natural selection can cause allele frequency changes in a population.
235–38

15.2 Natural Selection

• Natural selection causes changes in allele frequencies in a population due to the fitness of certain phenotypes to reproduce.238

• Natural selection results in adaptation to the environment. The three types of natural selection are directional selection, stabilizing selection, and disruptive selection.239–40

You might think that fitness means keeping in shape, but to an evolutionist it means having more fertile offspring than other individuals. Think about it—only if an animal reproduces can that animal’s genes be passed on and become prevalent in the next generation. Adaptation to the environment increases the chance of reproducing, but so does sexual selection, which occurs because of an advantage that helps an animal get a mate.

Such advantages as increased size, resplendent feathers, evolution of horns, or enlarged canines help males fight for and attract females. Females, in turn, must choose carefully. Perhaps to a female, the showier male is healthier or more appealing. If so, the same characteristics will be advantageous for her sons! Sexual selection increases the chances of reproducing, but can shorten the life span. A large, showy male that fights a lot most likely doesn’t live as long as a small, inconspicuous male.

Do male competition and female choice occur among humans? Some think so. They point out that human males tend to be larger and more aggressive than females, and that statistically males have a shorter life span. Also, wealthy and successful males are more apt to be attractive to women, and sometimes older men marry younger women who are still fertile, thereby increasing their own fitness.

Sexual selection is one of the factors that influences evolution on a small scale (microevolution), which is the subject of this chapter.

_{15.1 Microevolution}

Thus far, it may seem to you as though individuals evolve, but this is not the case. As evolution occurs, genetic and therefore phenotypic changes occur within a population. A population is all the members of a single species occupying a particular area and reproducing with one another. Micro-evolution pertains to evolutionary changes within a population.

Darwin stressed that the members of a population vary as in Figure 15.1, but he did not know how variations come about and how they are transmitted. Today, DNA sequencing in a number of plants and animals shows that each gene in sexually reproducing organisms has many alleles. The reshuffling of alleles during sexual reproduction can result in a range of phenotypes among the members of a population (see Chapter 10). Even so, as we shall see, sexual reproduction in and of itself cannot bring about evolution. Evolution is influenced by several other circumstantial factors.

Evolution in a Genetic Context

It was not until the 1930s that population geneticists were able to apply the principles of genetics to populations and thereafter to develop a way to recognize when evolution has occurred. In population genetics, the various alleles at all the gene loci in all individuals make up the gene pool of the population.

To simplify matters, we will describe the gene pool of a population in terms of gene frequencies, assuming just two alleles per gene locus. Suppose that in a Drosophila population, 36% of the flies are homozygous dominant for long wings, 48% are heterozygous, and 16% are homozygous recessive for short wings. Therefore, in a population of 100 individuals, we have

36 LL, 48 Ll, and 16 ll

What is the number of the L allele and the l allele in the population?

Number of L alleles: Number of l alleles:

LL (2 L 3 36) 5 72 LL (0 l) 5 0 l

Ll (1 L 3 48) 5 48 Ll (1 l 3 48) 5 48 l

ll (0 L) 5 0 ll (2 l 3 32) 5 32 l

120 L 80 l

To determine the frequency of each -allele, calculate its percentage from the total number of alleles in the population. In each case, for the dominant allele L, 120/200 5 0.6; for the recessive allele l, 80/20050.4. The sperm and eggs produced by this population will also contain these alleles in these frequencies. Assuming random mating (all possible gametes have an equal chance to combine with any other), we can calculate the ratio of genotypes in the next generation by using a Punnett square, as you first learned in Chapter 10.

There is an important difference between a Punnett square that represents a cross between individuals and the following one. Below, the sperm and eggs are those produced by the members of a population—not those produced by a single male and female. As you can see, the frequency of the allele in the next generation is the product of the frequencies of the parental generation. The results of the Punnett square indicate that the frequency for each allele in the next generation is still 0.6 for L and 0.4 for l:

Therefore, sexual reproduction alone cannot bring about a change in allele frequencies. Also, the dominant allele need not increase from one generation to the next. Dominance does not cause an allele to become common (Fig. 15.2). Microevolution requires that allele frequencies within a population change, and influences aside from sexual reproduction are required to bring this about.

The potential constancy, or equilibrium state, of gene pool frequencies was independently recognized in 1908 by G. H. Hardy, an En-glish mathematician, and W. Weinberg, a German physician. They used the binomial equation (p² 1 2pq 1 q²) to calculate the genotypic and allele frequencies of a population. Figure 15.3 shows how this is done. The Hardy-Weinberg principle states that an equilibrium of allele frequencies in a gene pool, calculated by using the expression p² 1 2pq 1 q², will remain in effect in each succeeding generation of a sexually reproducing population as long as five conditions are met:

1. No mutations: Allelic changes do not occur, or changes in one direction are balanced by changes in the opposite direction.

2. No gene flow: Migration of alleles into or out of the population does not occur.

3. Random mating: Individuals pair by chance and not according to their genotypes or phenotypes (Fig. 15.4).

4. No genetic drift: The population is very large, and changes in allele frequencies due to chance alone are insignificant.

5. No selection: No selective agent favors one genotype over another.

Often these conditions are rarely, if ever, met, and allele frequencies in the gene pool of a population do change from one generation to the next. Therefore, microevolution does occur. The significance of the Hardy-Weinberg principle is that it tells us what factors cause evolution—those that violate the conditions listed. Evolution can be detected by noting any deviation from a Hardy-Weinberg equilibrium of allele frequencies in the gene pool of a population.

For a change in allele frequencies to be subject to natural selection, it must result in a change of phenotype frequencies. Industrial melanism provides us with an example (Fig. 15.5). Before soot was introduced into the air due to industry, the original peppered moth population in Great Britain included only 10% dark-colored moths. When dark-colored moths rest on light trunks, they are seen and eaten by predatory birds. With the advent of industry, the trunks of trees darkened, and the light-colored moths became visible and were eaten. The birds acted as a selective agent, and microevolution occurred; the last observed generation of peppered moths had 80% dark-colored moths.

Causes of Microevolution

Any conditions that deviate from the list of conditions for allelic equilibrium cause evolutionary change. Thus, these five conditions can cause a divergence from the Hardy--Weinberg equilibrium: mutation, gene flow, nonrandom mating, genetic drift, and natural selection.

Genetic Mutations

Mutations are the raw material for evolutionary change. Without mutations, there could be no new variations among members of a population. Prokaryotes reproduce asexually and therefore are dependent on mutations alone to introduce variations. All mutations that occur and result in phenotypic differences can be tested by the environment. However, in sexually reproducing organisms, mutations, if recessive, do not immediately affect the phenotype.

In a changing environment, even a seemingly harmful mutation that results in a phenotypic difference can be the source of an adaptive variation. For example, the water flea Daphnia ordinarily thrives at temperatures around 20°C, but a mutation exists that requires Daphnia to live at temperatures between 25°C and 30°C. The adaptive value of this mutation is entirely dependent on environmental conditions.

If a trait is polygenic, with many alleles for each gene locus, certain combinations of these alleles might be more adaptive than others in a particular environment. The most favorable genotype may not occur until just the right combination of alleles are grouped in a single individual. For example, if alleles K and T are present in one parent, and alleles Q and R are present in the other parent, an offspring could inherit all of these alleles, and the result might be a protein with a different structure that enables the offspring to tolerate a warmer environment than its parents did.

Gene Flow

Gene flow, also called gene migration, is the movement of alleles between populations by migration of breeding individuals. Gene flow can increase the variation within a population by introducing novel alleles that were produced by mutation in another population. Constant gene flow can occur between adjacent animal populations due to the migration of organisms. Continued gene flow makes gene pools similar and reduces the possibility of allele frequency differences between populations now and in the future. Indeed, gene flow among populations can prevent speciation from occurring. Due to gene flow, the snake populations featured in Figure 15.6 are subspecies instead of separate species.

Nonrandom Mating

Random mating occurs when individuals pair by chance and not according to their genotypes or phenotypes. Inbreeding, or mating between relatives to a greater extent than by chance, is an example of nonrandom mating. Inbreeding does not change allele frequencies, but it does gradually increase the proportion of homozygotes, because the homozygotes that result must necessarily produce only homozygotes.

Assortative mating occurs when individuals tend to mate with those that have the same phenotype with respect to a certain characteristic. For example, in humans, tall people tend to mate with each other. Assortative mating causes the population to subdivide into two phenotypic classes, between which gene exchange is reduced. Homozygotes for the gene loci that control the trait in question increase in frequency, and heterozygotes for these loci decrease in frequency.

Sexual selection favors characteristics that increase the likelihood of obtaining mates, and in this way it promotes nonrandom mating. In most species, males that compete best for access to females and/or have a phenotype that pleases females are more apt to mate and have increased fitness (see the introduction to this chapter.)

Genetic Drift

Genetic drift refers to changes in the allele frequencies of a gene pool due to chance. Although genetic drift occurs in both large and small populations, a larger population is expected to suffer less of a sampling error than a smaller population. Suppose you had a large bag containing 1,000 green balls and 1,000 blue balls, and you randomly drew 10%, or 200, of the balls. Because there is a large number of balls of each color in the bag, you can reasonably expect to draw 100 green balls and 100 blue balls, or at least a ratio close to this. It is extremely unlikely that you would draw 200 green or 200 blue balls. But suppose you had a bag containing only 10 green balls and 10 blue balls, and you drew 10%, or only 2 balls. You could easily draw two green balls or two blue balls, or one of each color.

When a population is small, there is a greater chance that some rare genotype might not participate at all in the production of the next generation. Suppose in a small population of frogs, certain frogs by chance do not pass on their traits. Certainly, the next generation will have a change in allele frequencies (Fig. 15.7). When genetic drift leads to a loss of one or more alleles, other alleles over time become fixed in the population.

In an experiment involving brown eye color, each of 107 Drosophila populations was kept in its own culture bottle. Every bottle contained eight heterozygous flies of each sex. There were no homozygous recessive or homozygous dominant flies. From the many offspring, the experimenter chose at random eight males and eight females. This action, which represented genetic drift, continued for 19 generations. By the nineteenth generation, 25% of the populations contained only homozygous recessive flies, and 25% contained only homozygous dominant flies with the allele for brown eye color.

Genetic drift is a random process, and therefore it is not likely to produce the same results in several populations. In California, there are a number of cypress groves, each a separate population. The phenotypes within each grove are more similar to one another than they are to the phenotypes in the other groves. Some groves have longitudinally shaped trees, and others have pyramidally shaped trees. The bark is rough in some colonies and smooth in others. The leaves are gray to bright green or bluish, and the cones are small or large. Because the environmental conditions are similar for all the groves and no correlation has been found between phenotype and environment across groves, scientists hypothesize that these variations among populations are due to genetic drift.

Bottleneck EffectSometimes a species is subjected to near extinction because of a natural disaster (e.g., earthquake or fire) or because of overharvesting and habitat loss. It is as though most of the population has stayed behind and only a few survivors have passed through the neck of a bottle (Fig. 15.8). This so-called bottleneck effect prevents the majority of genotypes from participating in the production of the next generation.

The extreme genetic similarity found in cheetahs is believed to be due to a bottleneck. In a study of 47 different enzymes, each of which can occur in several different forms in other types of cats, all the cheetahs studied had exactly the same form. This demonstrates that genetic drift can cause certain alleles to be lost from a population. Exactly what caused the cheetah bottleneck is not known. It is speculated that perhaps cheetahs were slaughtered by nineteenth-century cattle farmers protecting their herds, or were captured by Egyptians as pets 4,000 years ago, or were decimated by a mass extinction tens of thousands of years ago. Today, cheetahs suffer from relative infertility because of the intense inbreeding that occurred after the bottleneck.

Founder EffectThe founder effect is an example of genetic drift in which rare alleles, or combinations of alleles, occur at a higher frequency in a population isolated from the general population. After all, founding individuals contain only a fraction of the total genetic diversity of the original gene pool. Which particular alleles are carried by the founders is dictated by chance alone. The Amish of Lancaster County, Pennsylvania, are an isolated group that was founded by German settlers. Today, as many as one in 14 individuals carries a recessive allele that causes an unusual form of dwarfism (affecting only the lower arms and legs) and polydactylism (extra fingers) (Fig. 15.9). In most populations, only one in 1,000 individuals has this allele.

_{15.2 Natural Selection}

Natural selection is the process that results in adaptation of a population to the biotic (living) and abiotic (nonliving) environments. In the biotic environment, organisms acquire resources through competition, predation, and parasitism. The abiotic environment includes weather conditions, dependent chiefly upon temperatures and precipitation. Charles Darwin, the father of modern evolutionary theory, became convinced that species evolve with time and suggested natural selection as the mechanism for adaptation to the environment (see Chapter 14). In Table 15.1, Darwin’s hypothesis of natural selection is stated in a way that is consistent with modern genetics.

As a result of natural selection, the most fit individuals become more prevalent in a population, and this way a population changes over time. The most fit individuals are those that reproduce more than others because they are adapted to the environment.

Types of Selection

Most of the traits on which natural selection acts are polygenic and controlled by more than one pair of alleles located at different gene loci. Such traits have a range of phenotypes, the frequency distribution of which usually resembles a bell-shaped curve, as shown in Figures 15.10–15.12.

Three types of natural selection are possible for any particular trait: directional selection, stabilizing selection, and disruptive selection.

Directional Selection

Directional selection occurs when an extreme phenotype is favored and the distribution curve shifts in that direction. Such a shift can occur when a population is adapting to a changing environment.

Industrial melanism, discussed earlier and depicted in Figure 15.5, is an example of directional selection. Drug resistance is another. As you may know, widespread use of antibiotics and pesticides results in a wide distribution of bacteria and insects that are resistant to these chemicals. When an antibiotic is administered, some bacteria may survive because they are genetically resistant to the antibiotic. These are the bacteria that are likely to pass on their genes to the next generation. As a result, the number of resistant bacteria keeps increasing. Drug-resistant strains of bacteria that cause tuberculosis have now become a serious threat to the health of people worldwide.

Another example of directional selection is the human struggle against malaria, a disease caused by an infection of the liver and the red blood cells. The Anopheles mosquito transmits the disease-causing protozoan Plasmodium vivax from person to person. In the early 1960s, international health authorities thought malaria would soon be eradicated. A new drug, chloroquine, seemed effective against Plasmodium, and spraying of DDT (an insecticide) had reduced the mosquito population. But by the mid-1960s, Plasmodium was showing signs of chloroquine resistance, and worse yet, mosquitoes were becoming resistant to DDT. A few drug--resistant parasites and a few DDT-resistant mosquitoes had survived and multiplied, shifting the distribution curve toward the infective success of the parasite.

The gradual increase in the size of the modern horse, Equus, is an example of directional selection that can be correlated with a change in the environment—in this case, from forest conditions to grassland conditions (Fig. 15.10). Even so, as discussed previously, the evolution of the horse should not be viewed as a straight line of descent because we know of many side branches that became extinct.

Stabilizing Selection

Stabilizing selection occurs when an intermediate phenotype is favored. With stabilizing selection, extreme phenotypes are selected against, and individuals near the average are favored. This is the most common form of selection because the average individual is well-adapted to its environment.

As an example, consider that when Swiss starlings lay four to five eggs, more young survive than when the female lays more or less than this number (Fig. 15.11). Genes determining physiological characteristics, such as the production of yolk, and behavioral characteristics, such as how long the female will mate, are involved in determining clutch size. In humans, birth weight ranges from 0.89 to 4.9 kilograms. But most babies have an intermediate birth weight—the weight that favors survival.

Disruptive Selection

In disruptive selection, two or more extreme phenotypes are favored over any intermediate phenotype. For example, British land snails (Cepaea nemoralis) are found in low-vegetation areas (grass fields and hedgerows) and in forests. In low-vegetation areas, thrushes feed mainly on snails with dark shells that lack light bands, and in forest areas, they feed mainly on snails with light-banded shells. Therefore, these two distinctly different phenotypes are found in the population (Fig. 15.12).

Maintenance of Variations

A population always shows some genotypic variation. The maintenance of variation is beneficial because populations with limited variation may not be able to adapt to new conditions and may become extinct. How can variation be maintained in spite of selection constantly working to -reduce it?

First, we must remember that the forces that promote variation are still at work: Mutation still creates new alleles, and recombination still recombines these alleles during gametogenesis and fertilization. Second, gene flow might still occur. If the receiving population is small and is mostly -homozygous, gene flow can be a significant source of new alleles. Finally, natural selection favors certain phenotypes, but the other types may still remain in reduced frequency. And disruptive selection even promotes differences in form, called polymorphism, in a population. Diploidy and the heterozygote also help because they maintain recessive alleles in the gene pool.

Diploidy and the Heterozygote

Only alleles that are exposed (cause a phenotypic difference) are subject to natural selection. In diploid organisms, this fact makes the heterozygote a potential protector of recessive alleles that might otherwise be weeded out of the gene pool. Because the heterozygote remains in a population, so does the possibility of the recessive phenotype, which might have greater fitness in a changing environment. When natural selection favors the ratio of two or more phenotypes in generation after generation, it is called balanced polymorphism. Sickle cell disease offers an example of balanced polymorphism.

Sickle Cell DiseaseIndividuals with sickle cell disease have the genotype Hb^SHb^S and tend to die at an early age due to hemorrhaging and organ destruction. Those who are heterozygous and have sickle cell trait (Hb^AHb^S) are better off because their red blood cells usually become sickle-shaped only when the oxygen content of the environment is low. Ordinarily, those with a normal genotype (Hb^AHb^A) are the most fit.

Geneticists studying the distribution of sickle cell disease in Africa have found that the recessive allele (Hb^S) has a higher frequency (0.2 to as high as 0.4 in a few areas) in regions where the disease malaria is also prevalent (Fig. 15.13). What is the connection between higher frequency of the recessive allele and malaria? Malaria is caused by a parasite that lives in and destroys the red blood cells of the normal homozygote (Hb^AHb^A). However, the parasite is unable to live in the red blood cells of the heterozygote (Hb^AHb^S) because the infection causes the red blood cells to become sickle-shaped. Sickle-shaped red blood cells lose potassium, and this causes the parasite to die. In an environment where malaria is prevalent, the heterozygote is favored. Each of the homozygotes is selected against but is maintained because the heterozygote is favored in parts of Africa subject to malaria. Table 15.2 summarizes the effects of the three possible genotypes.

The Chapter in Review

Summary

15.1 Microevolution

Evolution in a Genetic Context

Microevolution involves several elements:

• All the various genes of a population make up its gene pool.

• Hardy-Weinberg equilibrium is present when gene pool allele frequencies remain the same from generation to generation. Certain conditions have to be met to achieve an equilibrium.

• The conditions are (1) no mutations, (2) no gene flow,
(3) random mating, (4) no genetic drift, and (5) no selection. Since these conditions are rarely met, a change in gene pool frequencies is likely.

• When gene pool frequencies change, microevolution has occurred. Deviations from a Hardy-Weinberg equilibrium allow us to determine when evolution has taken place.

Causes of Microevolution

Microevolution is caused by five conditions:

• Mutations are the raw material for evolutionary change. Certain genotypic variations may be of evolutionary significance, only if the environment changes. Recombinations help bring about adaptive genotypes.

• Gene flow occurs when a breeding individual (in animals) migrates to another population or when gametes and seeds (in plants) are carried into another population. Constant gene flow between two populations causes their gene pools to become similar.

• Nonrandom mating occurs when relatives mate (inbreeding) or when assortative mating takes place. Both of these cause an increase in homozygotes. Sexual selection occurs when a characteristic that increases the chances of mating is favored.

• Genetic drift occurs when allele frequencies are altered by chance—thatt is, by sampling error. Genetic drift is particularly evident after a bottleneck, when severe inbreeding occurs, or when founders start a new population.

• Natural selection (see Section 15.2).

15.2 Natural Selection

Today we believe that adaptation comes about because the more fit individuals, who reproduce more than others, are adapted to the environment.

Types of Selection

Most of the traits of evolutionary significance are polygenic, and a range of phenotypes in a population result in a bell-shaped curve. Three types of selection occur:

Directional SelectionThe curve shifts in one direction, as when dark-colored peppered moths become prevalent in polluted areas.

Stabilizing SelectionThe peak of the curve increases, as when most human babies have the intermediate birth weight. Babies that are very small or very large are less fit than those of intermediate weight.

Disruptive SelectionThe curve has two peaks, as when British land snails vary because a wide geographic range causes selection to vary.

Maintenance of Variations

Despite constant natural selection, variation is maintained because:

• Mutations and recombination still occur; gene flow among small populations can introduce new alleles; and natural selection still occurs.

• In sexually reproducing diploid organisms, the heterozygote acts as a repository for recessive alleles whose frequency is low. In sickle cell disease, the heterozygote is more fit in areas where malaria occurs, and therefore both homozygotes are maintained in the population.

Thinking Scientifically

Cystic fibrosis (CF) is the most common serious genetic disorder in Caucasians. People with CF have an average life span of 25 years. The disease is expressed in individuals who are homozygous recessive for the cystic fibrosis transmembrane regulator (CFTR) gene. They are not able to make a functional CFTR protein, which causes their cells to accumulate chloride ions. This results in the formation of a thick mucus in the lungs, leading to frequent lung infections. In addition, secretory ducts are blocked, causing nutritional problems. Since this is a serious and typically fatal disorder, why do you suppose natural selection has not eliminated (or at least dramatically reduced) the defective form of the gene from the human population?

Testing Yourself

Choose the best answer for each question.

1. A population consists of 48 AA, 54 Aa, and 22 aa individuals. What is the frequency of the A allele?

a. 0.60 d. 0.42

b. 0.40 e. 0.58

c. 0.62

2. Which of the following is the binomial equation?

a. 2p² + 2pq + 2q² c. 2p² + pq + 2q²

b. p² + pq + q² d. p² + 2 pq + q²

For questions 3 and 4, consider that about 70% of white North Americans can taste the chemical phenylthiocarbamide. The ability to taste is due to the dominant allele T. Nontasters are tt. Assume this population is in Hardy-Weinberg equilibrium.

3. What is the frequency of t?

a. 0.30 d. 0.09

b. 0.70 e. 0.60

c. 0.55

4. What is the frequency of heterozygous tasters?

a. 0.495 d. 0.2475

b. 0.21 e. 0.45

c. 0.42

5. Typically, mutations are immediately expressed and tested by the environment in

a. prokaryotes.

b. eukaryotes.

c. prokaryotes and eukaryotes.

d. neither prokaryotes nor eukaryotes.

6. The offspring of better-adapted individuals are expected to make up a larger proportion of the next generation. The most likely explanation is

a. mutations and nonrandom mating.

b. gene flow and genetic drift.

c. mutations and natural selection.

d. mutations and genetic drift.

7. The Northern elephant seal went through a severe population decline as a result of hunting in the late 1800s. The population has rebounded but is now homozygous for nearly every gene studied. This is an example of

a. negative assortative mating. d. a bottleneck.

b. migration. e. disruptive selection.

c. mutation.

8. Which of the following generally results in a net gain in genetic variability?

a. genetic drift d. bottleneck

b. mutation e. More than one of these are correct.

c. natural selection

For questions 9–15, indicate the effect of each of the conditions of the Hardy-Weinberg principle on genotype and allele frequencies. Each answer may be used more than once.

Key:

a. alters genotype and allele frequencies

b. alters genotype frequency only

c. alters allele frequency only

d. does not alter genotype or allele frequency

9. Mutation

10. Gene flow

11. Inbreeding

12. Assortative mating

13. Genetic drift

14. Bottleneck

15. Natural selection

16. A small, reproductively isolated religious sect called the Dunkers was established by 27 families that came to the United States from Germany 200 years ago. The frequencies for blood group alleles in this population differ significantly from those in the general U.S. population. This is an example of

a. negative assortative mating. d. bottleneck effect.

b. natural selection. e. gene flow.

c. founder effect.

17. Assuming a Hardy-Weinberg equilibrium, 21% of a population is homozygous dominant, 50% is heterozygous, and 29% is homozygous recessive. What percentage of the next generation is predicted to be homozygous recessive?

a. 21% d. 42%

b. 50% e. 58%

c. 29%

18. When a population is small, there is a greater chance of

a. gene flow.

b. genetic drift.

c. natural selection.

d. mutations occurring.

e. sexual selection.

19. Which of the following is not expected to help maintain genetic variability?

a. gene flow d. disruptive selection

b. mutation e. genetic drift

c. recombination

20. The sickle cell allele is maintained in regions where malaria is prevalent because

a. the allele confers resistance to the parasite.

b. gene flow is high in those regions.

c. disruptive selection is occurring.

d. genetic drift randomly selects for the allele.

21. Complete the following by drawing a curve in the other two graphs to show the effect of directional selection.

22. Complete the following by drawing a curve in the other two graphs to show the effect of stabilizing selection.

23. Complete the following by drawing curves in the other two graphs to show the effect of disruptive selection.

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical Issue

The highly regarded population geneticist Sir Ronald Aylmer Fisher published a book in 1930 entitled The Genetical Theory of Natural Selection. In the book, he claims that civilizations fail because the people with the highest level of fitness (those at the top of the societal ladder) do not reproduce as often as those in the less affluent classes. He suggests that high-income families be paid to have children in order to improve the population. In other words, Fischer is suggesting that we carry out selection within the human population.

Can you envision any scenario in which our society should encourage reproduction by the most-fit individuals and discourage it by the least-fit? If so, how should it be done? What characteristics would be appropriate to select for or against?

Understanding the Terms

assortative mating236

bottleneck effect237

directional selection239

disruptive selection240

founder effect238

gene flow236

gene pool233

genetic drift237

industrial melanism234

microevolution232

mutation235

natural selection238

nonrandom mating236

polymorphism241

population232

sexual selection236

stabilizing selection240

Match the terms to these definitions:

a. _______________ Frequencies of all the alleles of all the genes in all individuals in a population.

b. _______________ Mating between phenotypically similar males and females.

c. _______________ Causes nonrandom mating by favoring characteristics that increase the likelihood of obtaining a mate.

d. _______________ Sharp decline in the number of individuals in a population due to severe selection pressure or a natural calamity.

Do male competition and female choice also influence human evolution?

Male competition for mates can influence evolution.

Female choice of showy mates can influence evolution.

Check Your Progress

1.
Explain how gene flow alters allele frequencies.

2. Explain why assortative mating is a type of nonrandom mating.

3. Explain why genetic drift is more likely to happen in a small population.

Answers:1. Individuals add and remove alleles when they move into and out of populations.2. When individuals choose a phenotype like their own, mating is not random.3. The chance that a rare genotype might not participate in passing alleles to the next generation is greater.

Figure 15.1Population of lupine flowers.

Among the members of a flowering plant population, petal color can be a continuous variation, so that there is a range of phenotypes. Few individuals have the extreme phenotypes, white petals and yellow petals, and most individuals have the intermediate phenotype, pink petals. A graph of the phenotypes results in the bell-shaped curve shown above a photo of the population.

Check Your Progress

How does a population geneticist know that evolution has occurred?

Answer:Evolution has occurred if allele frequencies within a population have changed from one generation to the next.

Figure 15.3Calculating gene pool frequencies using the Hardy-Weinberg equation.

Figure 15.5Industrial melanism and microevolution.

Microevolution has occurred when there is a change in gene pool frequencies—in this case, due to natural selection. a. When birds cannot see light-colored moths on light tree trunks, the light-colored phenotype is more frequent in the population. b. When birds cannot see dark-colored moths on dark tree trunks, the dark-colored phenotype is more frequent in the population. Therefore, when trees become sooty due to pollution, the percentage of the dark-colored phenotype increases.

Check Your Progress

1.
List the five conditions that must be met if a population is to remain in Hardy-Weinberg equilibrium.

2. Explain why the Hardy-Weinberg principle is important even though the conditions for equilibrium are rarely met in real life.

3. Describe the significance of mutations in terms of evolution.

Answers:1. No mutations, no gene flow, random mating, no genetic drift, and no selection.2. The Hardy-Weinberg principle tells us how to calculate allele frequencies and how we can recognize that evolution has occurred.
3. Mutations are responsible for all the genetic variability upon which evolution depends.

Figure 15.6Gene flow.

Each rat snake represents a separate population of snakes. Because the populations are adjacent to one another, interbreeding occurs, and so does gene flow between the populations. This keeps their gene pools somewhat similar, and each of these populations is considered a subspecies of the species Elaphe obsoleta. Therefore, each has a three-part name.

Figure 15.8Bottleneck effect.

A bottleneck effect may occur when a catastrophic reduction in a population occurs, such as after a major epidemic or a hurricane. For example, a parent population contains roughly equal numbers of genotypes represented by blue, yellow, purple, green, and red marbles. The chance survivors of the catastrophe have genotypes represented by blue and yellow, resulting in a new population with altered gene pool frequencies.

Figure 15.7Genetic drift.

Genetic drift occurs when by chance only certain members of a population (in this case, green frogs) reproduce and pass on their genes to the next generation. The allele frequencies of the next generation’s gene pool may be markedly different from those of the previous generation.

Figure 15.9Founder effect.

A member of the founding population of Amish in Pennsylvania had a recessive allele for a rare kind of dwarfism linked with polydactylism. The percentage of the Amish population now carrying this allele is much higher compared to that of the general population.

Check Your Progress

Describe the evolutionary consequences of natural selection.

Answer:Natural selection results in the adaptation of a population to biotic and abiotic environments.

Figure 15.10Directional selection.

a. Natural selection favors one extreme phenotype, and the distribution curve shifts. b. Equus, the modern-day horse, evolved from Hyracotherium, which was about the size of a dog. This small animal could have hidden among trees, and it had low-crowned teeth for browsing. When grasslands began to replace forests, the ancestors of Equus may have been subject to selection pressure for the development of strength, intelligence, speed, and durable grinding teeth. Larger animals were stronger and more successful in combat, those with larger skulls and brains had better sensory processing, those with longer legs and better-developed hooves could escape enemies, and animals with durable teeth could feed more efficiently on grass. Animals with these characteristics tended to produce more offspring.

Figure 15.11Stabilizing selection.

Stabilizing selection occurs when natural selection favors the intermediate phenotype over the extremes. For example, Swiss starling birds that lay four to five eggs (usual clutch size) have more young survive than those that lay less than four eggs or more than five eggs.

Figure 15.12Disruptive selection.

a. Disruptive selection favors two extreme phenotypes. b. Today, British land snails mainly comprise two different phenotypes, each adapted to a different habitat.

Check Your Progress

1. Contrast directional selection with stabilizing selection.

2.
Describe the effect of disruptive selection.

Answers:1. Directional selection occurs when one extreme phenotype is favored; stabilizing selection occurs when the intermediate phenotype is favored.2. Since the intermediate phenotype is selected against, two distinctly different populations eventually develop at the phenotypic extremes.

Figure 15.13Sickle cell disease.

a. Hash marks show the areas where malaria was prevalent in Africa, the Middle East, southern Europe, and southern Asia in 1920, before eradication programs began; shown in orange are the areas where sickle cell disease most often occurred. The overlap of these two distributions suggested a causal connection. b. Micrograph of a sickled red blood cell.

Check Your Progress

1. List the forces that help maintain genetic variability in a population.

2.
Explain the connection between sickle cell disease and the incidence of malaria.

Answers:1. Mutation, recombination, gene flow, disruptive selection, and diploidy.2. The homozygote subject to sickle cell disease is maintained in regions with a high incidence of malaria because the heterozygote is favored.

Table 15.2

Sickle Cell Disease

Genotype	Phenotype	Result
Hb^AHb^A
Hb^AHb^S
Hb^SHb^S

Normal	Dies due to malarial infection
Sickle cell trait	Lives due to protection from both
Sickle cell disease	Dies due to sickle cell disease

Table 15.1

Natural Selection

Evolution by natural selection requires:

1.Variation. The members of a population differ from one another.

2.Inheritance. Many of these differences are heritable genetic differences.

3.Degrees of adaptiveness. Some of these genetic differences affect how well an organism is adapted to its environment.

4.Degrees of successful reproduction. Individuals that are better adapted to their environment are more likely to reproduce, and their fertile offspring will make up a greater proportion of the next generation.

Figure 15.2Freckles.

A dominant allele causes freckles—so why doesn’t everyone have freckles? The Hardy-Weinberg principle, which states that sexual reproduction in and of itself doesn’t change allele frequencies, explains why dominant alleles don’t become more prevalent with each generation.

Figure 15.4Random mating.

These data suggest that heterozygosity is so infrequent in cheetahs, dogs, and humans that the chance of random mating resulting in evolution is slim. Cheetahs went through a bottleneck (see page 237), and this reduced their heterozygosity to almost zero. Dogs have been so inbred by humans that each breed is practically a separate species. Humans also have limited heterozygosity; on the average only 65 pairs of alleles out of 1,000 are heterozygous, and all the rest are homozygous.