Learning Objectives

1. To examine the conditions that cause evolutionary change.

2. To examine how mate choice influences the evolutionary process.

3. To become familiar with the role of mutation in the evolutionary process.

4. To examine the various modes of natural selection and become familiar with the concept of balanced polymorphism.

Key Concepts

1. Evolution occurs at the population leves as gene (allele) frequencies change. Algebra can be used to represent Hardy-Weinberg equilibrium, the unlikely situation in which evolution does not occur. Hardy-Weinberg equilibrium provides a background against which microevolution can be detected.

2. Nonrandom mating is one way for evolution to occur. Human behavior and migration make mating nonrandom. As a result, some alleles become more or less common in future generations.

3. Deleterious alleles in a population constitute the genetic load, and arise from mutation or are perpetuated in heterozygotes. Mutations influence evolution by introducing new genetic variants.

4. Natural Selection may favor one phenotype, two extreme phenotypes, or an intermediate phenotype. Balanced polymorphism maintains a deleterious allele when a heterozygote is unusually resistant to a specific, usually infectious, illness.

Chapter Concept 15.1: Why Is Evolution Much More Likely Than Not?

Evolution occurs at the population leves as gene (allele) frequencies change. Algebra can be used to represent Hardy-Weinberg equilibrium, the unlikely situation in which evolution does not occur. Hardy-Weinberg equilibrium provides a background against which microevolution can be detected.

Textbook Reading Assignment: Pages 280 - 282

The field of population genetics explains the mechanism behind microevolution—changes in the frequencies of alleles within the gene pool of a population. Microevolution is distinct from macroevolution—the process by which species and higher groups of taxa originate, change and go extinct. Population genetics explains variation at the population level and provides a basis for natural selection and other evolutionary forces.

The Hardy-Weinberg Principle suggests that sexual reproduction does not dilute variation within populations, but maintains the stability of genotype distribution from generation to generation. This principle is way to detailed for our purposes, so--skip it and focus on the conditions that cause evolutionary change:

1. mutation

2. migration

3. nonrandom mating

4. natural selection

5. genetic drift

This chapter will introduce you to the various ways in which gene frequencies change within populations (microevolution). Of all the causes of allelic frequency changes listed above, only natural selection consistently leads to adaptive changes in allele frequencies within populations over time. The other causes generally lead to random and non-adaptive effects.

Chapter Concept 15.2: How Does Mate Choice Influence Evolution?

Nonrandom mating is one way for evolution to occur. Human behavior and migration make mating nonrandom. As a result, some alleles become more or less common in future generations.

Textbook Reading Assignment: Pages 282 - 285

Nonrandom Mating
Both mammals and birds usually choose their mates and, therefore, do not mate at random. Such mating behavior is termed assortative mating. For example, mammals and birds seem to prefer mates with a high degree of symmetry. Inbreeding is an extreme case of assortative mating where individuals mate among closely related individuals. In general, both assortative mating and inbreeding result in an increase in the number of homozygous individuals within the population over time. This increase facilitates allelic frequency changes by exposing the alleles to natural selection.

Examples of non-random mating exist in the human population: read about "Arnold" and albinism on page 282. Extreme inbreeding tends to retain deleterious alleles with populations. As an example, read about the plight of the purebred animals discussed on page 283 of the text.

Migration
Animals and plants tend to mix their alleles with other populations. For example, animals tend to migrate and plants transfer pollen to new populations. In both cases, the resulting gene flow changes allele frequencies within populations over time. In general, gene flow tends to increase variation within local populations, but reduces variation between adjacent populations.

Genetic Drift
Interestingly enough, in small populations, sampling errors can cause allele frequencies to change randomly from generation to generation. The effect of chance in small populations can lead to genetic drift--changes in gene frequency due to random events. The smaller the population, the greater the chance of having random deviations from the average. Such random events may lead to a change in allele frequencies within the population.

The founder effect and a population bottleneck are two examples of random drift that can have profound effects in small populations (Figures 15.2 and 15.3).

The founder effect occurs when an entire population descends from a small number of individuals. Such a situation produces a population whose gene pool is a tiny sample of the original and the frequency of any allele within this population is likely to be different from that of the original population.

Figure 15.2

In the case of a population bottleneck, all of the organisms within the population have descended from a few individuals.

Figure 15.3

Unfortunately, the population that results from such a bottleneck suffers from a lack of genetic variation (ex: cheetahs as a case in point).   The importance of genetic variation within populations is evident when one considers evolutionary change in a constantly changing environment. Remember that natural selection acts on traits that best enable their possessors to survive and reproduce within the environment. Over time, the frequency of such traits increase due to natural selection. If variation within populations were not maintained, as in the case of a population bottleneck, natural selection would have very little to operate on. For example, if a population of individuals were all genetically similar to one another, then they would all be susceptible to the same environmental variables. If a disease were introduced into the population that affected one individual, it would most likely affect all individuals within the population. Such is the case with the current cheetah population.

In short, it is important to remember that genetic variation within populations ensures the survival of certain individuals during periods of environmental change. Such a scenario ensures that at least some individuals within the population survive and pass their genetic information on to the next generation.

 

Chapter Concept 15.3: How Does Mutation Fuel Evolution?

Deleterious alleles in a population constitute the genetic load, and arise from mutation or are perpetuated in heterozygotes. Mutations influence evolution by introducing new genetic variants.

Textbook Reading Assignment: Page 285

Mutations are the result of random processes that constantly occur in all genes. Since the number of genes within a species is extremely high, mutation can generate enormous amounts of variation within populations. Since mutations occur randomly, any change in the DNA is likely to be disadvantageous. Think of your DNA as being the product of millions of years of evolution. If you were to randomly change it, what are the chances that you would actually change it for the better? This might be similar to what would happen if you struck your car with a baseball bat. What are the chances that you would improve the phenotype of your car?

Although most mutations are harmful, some are beneficial. Beneficial mutations increase the likelihood of surviving and reproducing within the environment and, therefore, are passed on to the next generation. Interestingly enough, only those mutations that affect the phenotype of the individual are acted upon by natural selection. Since the phenotype (expression of the genotype) is what interacts with the environment, it is subject to natural selection. As a result, those individuals with advantageous phenotypes tend to survive and pass on their genotype to the next generation, therefore perpetuating the phenotype over time.

Interestingly enough, genetic drift (subject of Chapter Concept 15.2) plays an important role in the spread of new mutations. In fact, in small populations, new mutations can spread quickly in the absence of selection and eventually affect allelic frequencies. This demonstrates the importance of new mutations in the evolution of populations. In general, mutations “restock” genetic variation, which is the key to evolutionary change.

Chapter Concept 15.4: How Does Natural Selection Mold Evolution?

Natural Selection may favor one phenotype, two extreme phenotypes, or an intermediate phenotype. Balanced polymorphism maintains a deleterious allele when a heterozygote is unusually resistant to a specific, usually infectious, illness.

Textbook Reading Assignment: Pages 285 - 289

Natural Selection
One way natural selection alters allele frequencies within populations is by reducing the frequency of deleterious alleles within the population over time. Individuals with deleterious phenotypes do not survive and reproduce as well at those who lack the deleterious allele. As a result, the frequency of the deleterious allele is reduced over time. For example, individuals homozygous for the Tay-Sachs allele (allele that causes Tay-Sach disease) never survive to reproductive age and, as a result, the allele survives only at low frequencies. If not for the fact that the recessive allele can “hide” in the heterozygote (it is recessive), it would eventually be eliminated from the population.

Just as selection serves to reduce the frequency of one allele within a population, it can also serve to increase frequencies of others. One classic example is the evolution of pesticide resistant plants and antibiotic resistant bacteria. Since individuals with these traits are at an advantage within the environment, they tend to survive to reproductive age and pass these traits on to the next generation.

As indicated above, natural selection can push populations in a variety of directions: 

Directional selection results in a shift in the frequency of one or more traits in a particular direction. For example, many insect species have gradually increased the amount of pigment found within their cells in order to blend in with the environment and escape predation. Change associated with directional selection is relatively common in changing environments.

Disruptive selection serves to increase the frequency of extreme phenotypes and decrease the frequency of average phenotypes within populations. For example, marine snails tend to be either white or tan which allow them to blend in with their environment. Intermediate phenotypes are rare since these phenotypes are highly susceptible to predation.

Stabilizing selection acts against extreme phenotypes and selects for the average phenotype in the population. For example, most human babies have a birth weight between 3.1 and 3.5 kilograms—these babies have the greatest chance of survival. More importantly, the death rate is higher for infants outside of this range. Stabilizing selection is characteristic of a stable, unchanging environment. In fact, such selection explains the existence of horseshoe crabs, coelacanths and ginko trees which have remained unchanged for millions of years.

Figure 15.4

dividing line

Discussion Question 1
Average human height has increased over the past 200 years. What type of selection would characterize this trend?

dividing line

Most genetic variability comes from gene mutations which occur when single genes mutate into different forms called alleles. A population that has two or more alleles of a particular gene is said to be polymorphic. In many cases, natural selection can actually serve to promote genetic variation within polymorphic populations.

Populations, in some cases, maintain very high frequencies of deleterious alleles that may be lethal when homozygous. For example, sickle-cell anemia is caused by an autosomal recessive allele that is usually lethal in the homozygous condition. If evolution serves to decrease the frequency of deleterious alleles within populations over time, why is the frequency of the sickle-cell allele so high? The answer lies in the fact that the heterozygotes (whose blood contains both normal and abnormal hemoglobin) are resistant to malaria. Since heterozygotes have the advantage of malarial resistance, natural selection "selects for" this trait and keeps the frequency of the sickle-cell allele high. This fact is evidenced by the observation that in regions of the world where malaria is found, the frequency of the sickle-cell allele is quite high (refer to Figure 15.5).

dividing line

Discussion Questions 2 - 5
Below you will find Figure 15.6 from the text that demonstrates six different ways in which allele frequencies can be altered. View the figure and then answer the discussion questions that follow.

Figure 15.6a

Figure 15.6b

Figure 15.6c

Figure 15.6d

Figure 15.6e

Figure 15.6f

2. Which of the above conditions generates variation within populations?

3. Which of the above conditions alters gene frequencies within populations?

4. Which of the above is due to chance events?

5. Which of the above would be an example of a population bottleneck?

dividing line

Self-Test Questions

1. _____________ refers to changes in the frequencies of alleles within a population.

a) Microevolution

b) Macroevolution

c) The modern synthesis

d) August Weismann

2. A(n) __________ refers to all of the genes of all the individuals within a population.

a) evolutionary change

b) gene pool

c) allele frequency

d) microevolutionary change

3. The ______________ occurs when an entire population descends from a small number of individuals.

a) founder effect

b) genetic bottleneck

c) sexual selection

d) assortative mating

4. ____________, mating among close relatives, greatly increases the proportion of homozygotes within the population.

a) sexual selection

b) genetic drift

c) gene flow

d) inbreeding

5. ___________ selection selects for one extreme phenotype.

a) Directional

b) Disruptive

c) Stabilizing

d) Assortative

6. ______________ selection selects against both extreme phenotypes at the same time.

a) Directional

b) Sexual

c) Stabilizing

d) Disruptive

7. If a population has two or more alleles of a given gene, it is said to be _____________.

a) polygenic

b) polymorphic

c) digenic

d) heterozygous

8. Which of the following is the only "adaptive" way in which gene frequencies change?

a. mutation

b. migration

c. genetic drift

d. natural selection

9. Which of the following generate new alleles and increase variation within populations?

a. founder effect

b. natural selection

c. mutation

d. migration

10. Which of the following refers to the process by which species and higher groups of taxa originate, change and go extinct?

a. macroevolution

b. microevolution

c. polymorphism

d. None of the above

Answers to Self-Test Questions

1. a

2. b

3. a

4. d

5. a

6. c

7. b

8. d

9. c

10. a

Answers/Insight to Discussion Questions

1. directional selection

2. e

3. b, c, d, e, f

4. d

5. d