1. The Evolution of Populations
Chapter 23
The Evolution of Populations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

2. Allele Frequencies Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies?
red = 0.36, white = 0.16
red = 0.6, white = 0.4
red = 0.5, white = 0.5
Allele frequencies cannot be determined unless the population is in equilibrium.
Answer: b
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

3. Hardy-Weinberg Equilibrium Determination Which of these populations are in Hardy-Weinberg equilibrium? A B
both A and B
neither A nor B
Answer: a
This question shows that two populations can have the same allele frequencies but may differ regarding whether they are in equilibrium. Also, many students have the misconception that if pp + 2pq + qq = 1, then the population is in equilibrium. Ask students with that particular misconception to calculate the outcome of random mating in each population. Population A will have the same proportions of phenotypes in the next generation, whereas population B will change to the same proportions as in population A.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

4. Cystic Fibrosis 1 The frequency of cystic fibrosis, a recessive genetic disease, is 1 per 2,500 births among Northern Europeans. Assuming random mating, what is the frequency of carriers?
1/2,500
1/50
1/25
The frequency cannot be calculated because selection violates Hardy-Weinberg assumptions.
Answer: c
The frequency of carriers is 2pq. The allele frequency, q, is 1/50 since qq = 1/2500. P is close to 1. You may want to discuss why option d does not apply.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

5. Cystic Fibrosis 2 Until the 1950s, infants born with cystic fibrosis did not survive longer than a few months. If the frequency of carriers was 4% in the year 1900, what proportion of CF alleles was eliminated in one generation?
100%
50% 4% 2%
<0.1%
Answer: c
The frequency of CF alleles lost from the death of infants with CF is 2/2,500. The frequency of CF alleles present in heterozygotes is 1/25 or 100/2,500. Thus approximately 2/100, or 2%, of CF alleles are eliminated from the population because the homozygous infants do not survive to have progeny.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

6. Cystic Fibrosis 3 The frequency of cystic fibrosis is extremely low among Asians and Africans. What may explain the relatively high frequency of cystic fibrosis among Northern Europeans?
bottleneck effect
founder effect
genetic drift
directional selection
balancing selection
Answer: e
Balancing selection (heterozygote advantage) is the most likely answer, analogous to the high frequency of the sickle-cell disease allele in equatorial African populations.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

7. Figure 22.14 Evolution of Drug Resistance in HIV, with Time after Initiation of Treatment Before treatment with 3TC, did 3TC-resistant viruses exist in these patients?
yes no
no way to tell
Answer: a
This question follows up the example of HIV as an evolutionary system introduced in Chapter 22 and explores the origin of hereditary variation and the dynamics of evolutionary change in populations as a result of selection.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

8. Mutation What will happen to the proportion of the drug-resistant virus after drug treatment stops?
It will decline exponentially.
It will decline logarithmically.
It will stay the same.
It will decline linearly.
It will drift randomly.
Answer: b
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

9. A high proportion of the cats on Key West have extra toes (polydactyly). What is the most likely explanation?
high rate of mutation
founder effect
bottleneck effect
directional selection for extra toes
Extra toes are sexually appealing to female cats.
Answer: b
This question asks students to apply an understanding of the different mechanisms of evolution to formulate a hypothesis that explains an actual situation. In this case, history tells us that the so-called Hemingway cats descended from a six-toed cat brought to the island by a ship captain in the 1800s.
Image from Wikipedia under the Creative Commons Attribution 3.0 license.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.