The Evolution of Populations

In evolutionary terms, an organism's fitness is measured by its ________. stability in the face of environmental change contribution to the gene pool of the next generation genetic variability mutation rate health Answer: B

In evolutionary terms, an organism's fitness is measured by its ________. stability in the face of environmental change contribution to the gene pool of the next generation genetic variability mutation rate health 3

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 4

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. 5

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Some moths on a tree are easier to see due to their lighter color and therefore are eaten by predators more often. large population size no mutation no gene flow random mating occurring no selection Answer: E The variation in color is allowing some moths to survive more easily than others; therefore, selection is occurring. 6

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Some moths on a tree are easier to see due to their lighter color and therefore are eaten by predators more often. large population size no mutation no gene flow random mating occurring no selection 7

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Male elephant seals show aggression toward other males, resulting in dominant males mating with several females and other males mating with few or no females. large population size no mutation no gene flow random mating occurring no selection Answer: D There is a pattern to the mating that is occurring, making this nonrandom mating. 8

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Male elephant seals show aggression toward other males, resulting in dominant males mating with several females and other males mating with few or no females. large population size no mutation no gene flow random mating occurring no selection 9

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Due to global warming, a river has dried up, allowing two different rabbit populations to mate with one another, whereas they were isolated before. large population size no mutation no gene flow random mating occurring no selection Answer: C When the river was there and the species were isolated, no gene flow was occurring. Now that gene flow can happen, this can reduce differences between the populations. 10

Indicate which of the Hardy-Weinberg conditions is being violated in this example: Due to global warming, a river has dried up, allowing two different rabbit populations to mate with one another, whereas they were isolated before. large population size no mutation no gene flow random mating occurring no selection 11

The textbook discusses how the conversion of quality habitat into farmland has greatly depleted the greater prairie chicken populations and, as a result, their genetic diversity. Which of the following occurred in this example? founder effects mutation natural selection gene flow bottlenecking Answer: E The drastic drop in population size and genetic diversity is causing the greater prairie chicken to undergo a genetic bottleneck. 12

The textbook discusses how the conversion of quality habitat into farmland has greatly depleted the greater prairie chicken populations and, as a result, their genetic diversity. Which of the following occurred in this example? founder effects mutation natural selection gene flow bottlenecking 13

Indicate what type of selection is occurring in the example here: Due to less snowfall in an area, white mice are predated on more than intermediate- or dark-colored mice. directional selection disruptive selection stabilizing selection Answer: A The population is shifting toward the darker colors, moving the bell curve over to the right. 14

Indicate what type of selection is occurring in the example here: Due to less snowfall in an area, white mice are predated on more than intermediate- or dark-colored mice. directional selection disruptive selection stabilizing selection 15

Indicate what type of selection is occurring in the example here: During a drought, it was discovered that finches with large beaks and those with small beaks were more successful due to the food sources available during the drought. directional selection disruptive selection stabilizing selection Answer: B The extremes are being favored in this example, and the intermediates are not being favored. 16

Indicate what type of selection is occurring in the example here: During a drought, it was discovered that finches with large beaks and those with small beaks were more successful due to the food sources available during the drought. directional selection disruptive selection stabilizing selection 17

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, or about 0.04% 1/50, or about 2% 1/25, or about 4% 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 = 1/2,500. P is close to 1. You may want to discuss why option D does not apply. 18

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, or about 0.04% 1/50, or about 2% 1/25, or about 4% The frequency cannot be calculated because selection violates Hardy-Weinberg assumptions. 19

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: D 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. 20

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% 21

Which of the following evolutionary mechanisms increases the amount of genetic variation in a population? genetic drift mutation sexual selection directional natural selection stabilizing natural selection Answer: B, the creation of new alleles (via mutation) and genes (via gene duplication) are the mechanisms by which the genetic variation of a population can be increased. Genetic drift, all forms of natural and sexual selection tend to decrease the genetic diversity. 22

Which of the following evolutionary mechanisms increases the amount of genetic variation in a population? genetic drift mutation sexual selection directional natural selection stabilizing natural selection 23

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 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. 24

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 25

Scientific Skills Questions One way to test whether evolution is occurring in a population is to compare the observed genotype frequencies at a locus with those expected for a nonevolving population based on the Hardy-Weinberg equation. In this exercise, you’ll test whether a soybean population is evolving at a locus with two alleles, CG and CY , that affect chlorophyll production and hence leaf color. Students planted soybean seeds and then counted the number of seedlings of each genotype at day 7 and again at day 21. Seedlings of each genotype could be distinguished visually because the CG and CY alleles show incomplete dominance: CGCG seedlings have green leaves, CGCY seedlings have green-yellow leaves, and CYCY seedlings have yellow leaves.

Using the day 7 data, what is the frequency of the CG allele (p)? Answer: A

Using the day 7 data, what is the frequency of the CG allele (p)? Answer: A

Using the day 7 data, what is the frequency of the CY allele (q)? 0.234 0.484 1.516 1.484 Answer: C

Using the day 7 data, what is the frequency of the CY allele (q)? 0.234 0.484 1.516 1.484

What is the expected genotype frequency for the CGCG genotype? Answer: A

What is the expected genotype frequency for the CGCG genotype?

Calculate the observed genotype frequency for the genotype CGCY at day 7. 49  216  0.227 111  216  0.514 56  216  0.259 Answer: A

Calculate the observed genotype frequency for the genotype CGCY at day 7. 49  216  0.227 111  216  0.514 56  216  0.259

The population is in Hardy-Weinberg equilibrium. Compare the observed genotype frequencies you just calculated with the genotype frequencies expected if the population is in Hardy-Weinberg equilibrium. At day 7, is the seedling population in Hardy-Weinberg equilibrium, or is evolution occurring? The population is evolving, and there appears to be selection against genotype CGCY. The population is evolving, and there appears to be selection for genotype CGCG. The population is in Hardy-Weinberg equilibrium. The population is evolving, and there appears to be selection for genotype CYCY. Answer: C

Compare the observed genotype frequencies you just calculated with the genotype frequencies expected if the population is in Hardy-Weinberg equilibrium. At day 7, is the seedling population in Hardy-Weinberg equilibrium, or is evolution occurring? The population is evolving, and there appears to be selection against genotype CGCY. The population is evolving, and there appears to be selection for genotype CGCG. The population is in Hardy-Weinberg equilibrium. The population is evolving, and there appears to be selection for genotype CYCY.

The population is in Hardy-Weinberg equilibrium. Compare the genotype frequencies at day 21 to the expected frequencies and the observed frequencies at day 7. Is the seedling population at day 21 in Hardy-Weinberg equilibrium, or is evolution occurring? The population is evolving, and there appears to be selection against genotype CYCY. The population is evolving, and there appears to be selection against genotype CGCG. The population is evolving, and there appears to be selection against genotype CGCY. The population is in Hardy-Weinberg equilibrium. Answer: A

Compare the genotype frequencies at day 21 to the expected frequencies and the observed frequencies at day 7. Is the seedling population at day 21 in Hardy-Weinberg equilibrium, or is evolution occurring? The population is evolving, and there appears to be selection against genotype CYCY. The population is evolving, and there appears to be selection against genotype CGCG. The population is evolving, and there appears to be selection against genotype CGCY. The population is in Hardy-Weinberg equilibrium.

Homozygous CYCY individuals cannot produce chlorophyll Homozygous CYCY individuals cannot produce chlorophyll. The ability to photosynthesize becomes more critical as seedlings age and begin to exhaust the supply of food that was stored in the seed from which they emerged. What hypothesis could explain the data for days 7 and 21? At day 7, CYCY individuals were not being selected against because they were using chlorophyll inherited from the mother plant; by day 21, they had run out of chlorophyll and many plants were not surviving. Some external factor caused a disproportionate number of deaths among the CYCY seedlings. If the experiment were repeated, the researchers would obtain a different result. At day 7, CYCY individuals were not being selected against because they had plenty of water; by day 21, they had run out of water and many plants were not surviving. At day 7, CYCY individuals were not being selected against because they had not used up the supply of food that was stored in the seed; by day 21, they had run out of stored food and many plants were not surviving. Answer: D

Homozygous CYCY individuals cannot produce chlorophyll Homozygous CYCY individuals cannot produce chlorophyll. The ability to photosynthesize becomes more critical as seedlings age and begin to exhaust the supply of food that was stored in the seed from which they emerged. What hypothesis could explain the data for days 7 and 21? At day 7, CYCY individuals were not being selected against because they were using chlorophyll inherited from the mother plant; by day 21, they had run out of chlorophyll and many plants were not surviving. Some external factor caused a disproportionate number of deaths among the CYCY seedlings. If the experiment were repeated, the researchers would obtain a different result. At day 7, CYCY individuals were not being selected against because they had plenty of water; by day 21, they had run out of water and many plants were not surviving. At day 7, CYCY individuals were not being selected against because they had not used up the supply of food that was stored in the seed; by day 21, they had run out of stored food and many plants were not surviving.

How do you expect the frequencies of the CG and CY alleles to change beyond day 21? The frequency of the CY allele will decrease, and the frequency of the CG allele will stay the same. The frequency of the CY allele will increase, and the frequency of the CG allele will decrease. The frequency of the CY allele will decrease, and the frequency of the CG allele will increase. The frequencies of the CY and CG alleles will stay the same. Answer: C

How do you expect the frequencies of the CG and CY alleles to change beyond day 21? The frequency of the CY allele will decrease, and the frequency of the CG allele will stay the same. The frequency of the CY allele will increase, and the frequency of the CG allele will decrease. The frequency of the CY allele will decrease, and the frequency of the CG allele will increase. The frequencies of the CY and CG alleles will stay the same.