13. Evolution and Natural Selection


13.10. A Summary of the Causes of Evolutionary Change


At the beginning of this chapter, evolution was described as a change in allele frequency over time. It is clear that several mechanisms operate to bring about this change. Mutations can either change one allele into another or introduce an entirely new piece of genetic information into the population. Immigration can introduce new genetic information if the entering organisms have genetic information that was not in the population previously. Emigration and death remove genes from the gene pool. Natural selection systematically filters some genes from the population, allowing other genes to remain and become more common. The primary mechanisms involved in natural selection are differences in death rates, reproductive rates, and the rate at which individuals are selected as mates (figure 13.14). In addition, gene frequencies are more easily changed in small populations, because events such as death, immigration, emigration, and mutation can have a greater impact on a small population than on a large population.



FIGURE 13.14. Processes That Influence Evolution

Several processes cause gene frequencies to change. New genetic information enters populations through immigration and mutation. Genetic information leaves populations through emigration and death. Natural selection operates within populations through death, mate selection, and rates of reproduction. Genetic drift can also result in evolutionary change but is not shown in this diagram.



20. Why is each of the following important for an understanding of evolution: mutation, migration, sexual reproduction, selective agents, and population size?



At one time, people thought that all organisms were unchangeable. Lamarck suggested that change does occur and thought that acquired characteristics could be passed from generation to generation. Darwin and Wallace proposed the theory of natural selection as the mechanism that drives evolution. All populations of sexually reproducing organisms naturally exhibit genetic diversity among individuals as a result of mutation and the genetic recombination resulting from meiosis and fertilization. The genetic differences are reflected in phenotypic differences among individuals. These genetic differences are important in changing environments, because natural selection must have genetic diversity to select from. Natural selection by the environment results in better-suited organisms having greater numbers of offspring than those that are less well off genetically. Not all genes are equally expressed. Some express themselves only during specific periods in the life of an organism, and some are recessive alleles that show themselves only when in the homozygous state. Characteristics that are acquired during the life of an individual and are not determined by genes cannot be raw material for natural selection. Sexual selection occurs when specific individuals are chosen as mates to the exclusion of others. In addition to natural selection and sexual selection, genetic drift in small populations can lead to evolutionary change.

Selecting agents change the gene frequencies of the population if the conditions of the Hardy-Weinberg concept are violated. The conditions required for the Hardy-Weinberg equilibrium are random mating, no mutations, no migration, large population size, and no selective advantage for any genes. These conditions are met only rarely, however, so that, after generations of time, the genes of the more favored individuals will make up a greater proportion of the gene pool. The process of natural selection allows the maintenance of a species in its environment, even as the environment changes.


Basic Review

1. Which of the following is not true?

a. All organisms produce more offspring than can survive.

b. All organisms of the same species are exactly alike.

c. Among organisms, there is a constant struggle for survival.

d. Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.

2. _____ characteristics are traits gained during an organism’s life and not determined genetically.

a. Genetic            

b. Acquired          

c. Sexual

d. Dominant

3. Who proposed the theory of natural selection?

a. Darwin and Wallace

b. Buffon             

c. Lamarck

d. Hardy and Weinberg

4. _____ is the success of an organism in passing on its gene to the next generation, compared with other members of its population.

5. _____ occurs within animal populations when some individuals are chosen as mates more frequently than others.

a. Stabilizing selection

b. Disruptive selection

c. Sexual selection

d. Normative         selection

6. _____ is how often an allele expresses itself when present.

a. Expressivity     

b. Dominance

c. Fitness             

d. Penetrance

7. Specific environmental factors that favor certain characteristics are called

a. selecting agents.   

b. hurricanes.      

c. mutations.

d. disruptive factors.

8. _____ _____ is a significant change in the frequency of an allele that is not the result of natural selection.

9. The conditions necessary for gene frequencies to remain constant include all the following except

a. mating must be completely random.

b. mutations must not occur.

c. the migration of individual organisms into and out of the population must not occur.

d. the population must be very small.

10. _____ occurs when there are minor differences in allele frequency between populations of the same species, as when genetic differences between subspecies are examined.

11. Natural selection can take place when genetic differences exist among individuals in a population and these differences affect the overall health or _____.

12. _____-evolution has taken place when the percentage of dark peppered moths in a population increases as a result of allele-frequency changes.

13. A change in allele _____ can result from changes resulting from rapid periods of climate warming.

14. _____ diseases were thought to be controlled but have become common in recent years.

a. Reemerging infectious       

b. Genetic abnormalities

c. Recurring infectious  

d. Incidental

15. A high _____ _____ tends to offset the high _____ _____ and population size remains stable.



1. b 2. b 3. a 4. Fitness 5. c 6. d 7. a 8. Genetic drift 9. d 10. Microevolution 11. fitness 12. Micro- 13. frequency 14. a 15. death rate/reproductive rate


Thinking Critically

Microevolution and Sexually Transmitted Diseases

Penicillin was introduced as an antibiotic in the early 1940s. Since that time, it has been found to be effective against the bacteria that cause gonorrhea, a sexually transmitted disease. The drug acts on dividing bacterial cells by preventing the formation of a new protective cell wall. Without the wall, the bacteria can be killed by normal body defenses. As time passed, a new strain of this disease-causing bacterium developed. This bacterium produces an enzyme that metabolizes penicillin. How can gonorrhea be controlled now that this organism is resistant to penicillin? How did a resistant strain develop? Include the following in your consideration: DNA, enzymes, selecting agents, and gene-frequency changes.