CH 23

A population of butterflies has two alleles for color: blue alleles and green alleles. The blue allele is dominant. Under what circumstances will the frequency of the blue allele increase?

Female butterflies prefer to mate with blue butterflies, so blue butterflies leave more offspring than green ones. A volcanic explosion kills many butterflies. By chance, the few survivors are all blue. Three green butterflies migrate out of this population.

Which of the following are basic components of the Hardy-Weinberg model? Hint 1: What were Hardy and Weinberg trying to determine about the consequences of matings among all the individuals in a population?

Frequencies of two alleles in a gene pool before and after many random matings. Hardy and Weinberg were trying to determine how and whether allele frequencies in a population change from one generation to the next.

Evolution in a population of sea turtles can be caused by the following agent(s).

Gene flow Genetic drift Natural selection All of the above can be agents.

How does inbreeding alter genotype and allele frequencies? See Section 23.2 (Page). Hint: What is changing through time with each generation?

Homozygotes increase in frequency in the population over generations. Inbreeding increases homozygosity.

How could genetic drift explain differences in the genotype frequencies among the Caucasian and Aborigine populations? See Section 23.4 (Page). Hint: Which populations are more isolated?

Isolation of certain populations results in an increased incidence of homozygosity. Genetic drift reduced the incidence of heterozygosity in the Aborigines.

How might gene flow be important in managing an endangered population? See Section 23.5 (Page). Hint: What is gene flow?

It increases genetic diversity by introducing alleles from one population into another. Gene flow is the movement of alleles between populations.

Does this model show that evolution has occurred? Select the best answer.

Yes; the allele frequencies have changed. Evolution has occurred. This model shows that the allele frequencies have changed over time.

This model shows which process?

natural selection, The model shows that only the red fish are dying at Time 2, which suggests that these fish have a lower fitness than the others. Natural selection is decreasing the frequency of the aa genotype, thus the a allele, over time.

\If a population is not in Hardy-Weinberg equilibrium for a specific gene, which statement is correct? See Section 23.1 (Page). Hint: What are the possible conditions that violate the Hardy-Weinberg principle?

At least one of the Hardy-Weinberg assumptions has been broken. We do not have evidence indicating which assumption has been broken, but at least one must be.

Swine are vulnerable to infection by bird flu virus and human flu virus, which can both be present in an individual pig at the same time. When this occurs, it is possible for genes from bird flu virus and human flu virus to be combined. If the human flu virus contributes a gene for Tamiflu resistance (Tamiflu is an antiviral drug) to the new virus, and if the new virus is introduced to an environment lacking Tamiflu, then what is most likely to occur?

If the Tamiflu-resistance gene involves a cost, it will experience directional selection leading to reduction in its frequency.

Use this Punnett square to calculate the predicted genotype frequencies of the offspring.

MM:p2=0.36, Mm:2pq=0.24+0.24=0.48, mm:q2=0.16, The genotype frequencies within each square are calculated by multiplying the frequencies of the two gametes that contribute to the genotype. For example, 0.6M×0.6M=0.36MM.

Considering the concept of the fundamental asymmetry of sex, why is it adaptive for red deer mice to fight one another? See Section 23.3 (Page). Hint: What limits male fitness?

Male fitness is limited by their access to females and the winner of these fights is able to mate with more females than the loser. Males with alleles that increase their likelihood of winning these competitions mate more often and leave more offspring.

Which of the following evolutionary forces consistently results in adaptive changes in allele frequencies Hint: Adaptive changes in allele frequencies often occur due to the varying fitness of individuals in a population.

Selection Selection is the only evolutionary force that consistently results in adaptation. Mutation without selection and genetic drift are random processes that may lead to adaptive, maladaptive, or neutral effects on populations.

Which statement about genetic drift is correct? See Section 23.4 (Page). Hint: Remember the underlying mechanism that causes genetic drift.

The direction of evolutionary change due to genetic drift is random. Genetic drift is not driven by a directional effect but occurs as a magnification of chance effects due to small population size.

What genotype frequencies are expected under Hardy-Weinberg equilibrium for a population with allele frequencies of p = 0.8 and q = 0.2 for a particular gene? Hint: Remember that all genotype frequencies of a particular gene in a population add up to one.

The expected genotype frequencies are 0.64, 0.32, and 0.04 for A1A1, A1A2 , and A2A2 , respectively. The expected frequency of the A1A1 genotype is p 2 = (0.8)(0.8) = 0.64; the expected frequency of the A1A2 genotype is 2pq = 2(0.8)(0.2) = 0.32; the expected frequency of the A2A2 genotype is q 2 = (0.2)(0.2) = 0.04. To verify your calculations, confirm that the three frequencies add up to one.

Genetic drift occurs in a population. Which of the following statements might be TRUE?

The population experiences a decrease in genetic variation. Genetic drift increased the population's fitness. The population was relatively small. Genetic drift decreased the population's fitness. Any of these statements can be true.

What is a gene pool?

a hypothetical collection of all the possible alleles in a population. While "allele pool" would be a more precise name, "gene pool" is the term that is traditionally used.

The Dunkers are a religious group that moved from Germany to Pennsylvania in the mid-1700s. They do not marry members outside their own immediate community. Today, the Dunkers are genetically unique and differ in gene frequencies, at many loci, from all other populations, including those in their original homeland. Which of the following likely explains the genetic uniqueness of this population?

founder effect, inbreeding, and genetic drift

Genetic variation ________.

must be present in a population before natural selection can act upon the population

Examine this Punnett square. What does the number 0.6 refer to?

the gamete allele frequency p The numbers outside the Punnett square represent gametes and the allele frequencies are represented by p+q=1.

Which of the following is the most predictable outcome of increased gene flow between two populations?

decreased genetic difference between the two populations

Show how selection, genetic drift, gene flow, and mutation relate to genetic variation.

1. Selection may increase, decrease or maintain genetic variation. 2. Mutation increases genetic variation. 3. Genetic drift decreases genetic variation. 4. Gene flow may increase or decrease genetic variation of a population.

Long necks make it easier for giraffes to reach leaves high on trees, while also making them better fighters in "neck wrestling" contests. In both cases, which kind of selection appears to have made giraffes the long-necked creatures they are today? Hint: Ancestral giraffes presumably had short necks, like a horse.

Directional selection Directional selection drives the average of the population in one direction, in this case, toward longer necks.

Black-bellied seedcrackers have either small beaks (better for eating soft seeds) or large beaks (better for hard seeds). There are no seeds of intermediate hardness; therefore, which kind of selection acts on beak size in seedcrackers? Hint: There are no birds with intermediate beak size, either.

Disruptive selection Disruptive selection causes both extreme phenotypes (large and small beaks) to be favored over the intermediate phenotypes.

For a gene suspected of causing hypertension in humans, you observe the following genotype frequencies: A1A1 0.574; A1A2 0.339; A2A2 0.087. Is this gene in Hardy-Weinberg equilibrium? Why or why not? (Assume that a difference of three percent or more in any of the observed versus expected frequencies is statistically significant.) See Section 23.1 (Page). Hint: What are the expected frequencies for Hardy-Weinberg equilibrium?

No. The expected genotype frequencies are A1A1 0.553; A1A2 0.381; A2A2 0.066. The expected genotype frequencies are significantly different from those expected from the Hardy-Weinberg Principle.

Interpret this model of a lake that was drained for irrigation. Assume that the death of certain fish was random with respect to their genotypes.

The allele frequencies at Time 2 are 0.25 A, 0.75 a. True The arrow in this model represents gene flow. False This model is not useful because it shows far fewer individuals than would actually occur in a population in nature. False This model is not useful because the fish do not look very realistic. False This model shows the bottleneck effect. True, This model shows genetic drift. True The color code in this model may or may not represent fish color. True

In a bell-shaped curve, the x-axis (horizontal direction) of the graph represents which of the following? Hint: This is what is measured in the population.

The value of a particular characteristic; characteristics of an organism can include such traits as size and color. The value of the characteristic increases from left to right.

Interpret this student model of palm tree populations on two islands. Assume that at Time 1 (not shown) all the circles on Island 2 were white. Now, at Time 2, some are green.

This model shows the founder effect. False To be accurate, the blue arrows would have to go both directions, not just from one island to the other. False The color code in this model represents genotypes. True The blue arrows in this model represent gene flow. True This model shows genetic drift. False None of the palm trees on the larger island have the f allele. False The allele frequencies on the smaller island are 0.2 F, 0.8 f. True The color code in this model represents phenotypes. False

In the beetles described in the animation, there were two alleles for color, brown and green. Suppose that you discover a very small population of these beetles, consisting of the individuals shown below. How can you calculate the frequency of each allele in this population?

1. To calculate the frequency of the brown allele, count the number of brown alleles and divide by the total number of alleles in this population. 2. In this beetle population, the number of brown alleles is 8. 3. In this beetle population, the total number of alleles is 20. 4. The frequency of the brown allele in this beetle population is 0.4. 5. The frequency of the green allele in this beetle population is 0.6. Allele frequencies measure the relative proportions of different alleles in a population. By looking at how allele frequencies change over time, you can see how populations are evolving.

Which type of selection tends to increase genetic variation? Hint: Think about what kinds of phenotypes are favored in these three selection types.

Disruptive selection Disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes, resulting in increased genetic variation in a population.

Three major mechanisms of evolution are natural selection, genetic drift, and gene flow.

1. The evolution of populations due to chance events is genetic drift. 2. The transfer of alleles into and out of populations due to the movement of fertile individuals or their gametes is gene flow. 3. Natural selection occurs when individuals with inherited traits better suited to their environment survive and produce more offspring than other individuals.

The three major mechanisms of evolution differ in how they work, and as a result often have different effects on a population. Review your understanding of natural selection, genetic drift, and gene flow by sorting the statements below into the correct bins.

Natural selection, genetic drift, and gene flow cause a population to evolve in different ways. Natural selection is the result of differential survival and reproduction. It is the only mechanism of evolution that consistently causes a population to become better adapted to its environment. Genetic drift describes evolution due to chance events and causes unpredictable fluctuations in allele frequencies. Genetic drift can have a particularly significant effect in a small population, such as during a bottleneck or founder event. Gene flow results when alleles are transferred into or out of a population due to the migration of fertile individuals or their gametes. Gene flow can bring new alleles (beneficial, harmful, or neutral) into a population

Use the following information to answer the question below. In 1983, a population of dark-eyed junco birds became established on the campus of the University of California, San Diego (UCSD), which is located many miles from the junco's normal habitat in the mixed-coniferous temperate forests in the mountains. Juncos have white outer tail feathers that the males display during aggressive interactions and during courtship displays. Males with more white in their tail are more likely to win aggressive interactions, and females prefer to mate with males with more white in their tails. Females have less white in their tails than do males and display it less often. (Pamela J. Yeh. 2004. Rapid evolution of a sexually selected trait following population establishment in a novel habitat. Evolution 58[1]:166—74.) The UCSD campus male junco population tails were, on average, 36 percent white, whereas the tails of males from nearby mountain populations averaged 40-45 percent white. If this observed trait difference were due to a difference in the original colonizing population, it would most likely be due to ________.

a founder effect

When imbalances occur in the sex ratio of sexual species that have two sexes (that is, other than a 50:50 ratio), the members of the minority sex often receive a greater proportion of care and resources from parents than do the offspring of the majority sex. This is most clearly an example of _____.

frequency-dependent selection

People with cystic fibrosis (CF) are homozygous recessive for mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. These mutations cause the CFTR protein to become dysfunctional, and it is unable to move chloride to the cell surface. Ultimately mucus can build up in organs, and in the lungs, the mucus clogs the airways and traps bacteria, leading to infections, inflammation, and respiratory failure. In a human population, if 9 in 10,000 newborn babies are born with CF, what are the expected frequencies of the dominant (A 1) and recessive (A 2) alleles according to the Hardy-Weinberg model in that population?

p = 0.9700, q = 0.0300

Populations evolve for many reasons. Suppose there is a population of plants that have either purple flowers or white flowers, and the allele for purple flowers is dominant. This means that plants with two purple alleles have purple flowers. Plants with one purple allele and one white allele also have purple flowers. Only plants with two white alleles have white flowers. For each event or condition described below, answer the following questions. Which mechanism of evolution is at work? How does this event affect the population's gene pool? Do the frequencies of the two alleles change, and if so, how?

Different events and conditions result in natural selection, genetic drift, and gene flow. In turn, these mechanisms cause allele frequencies to change--and populations to evolve--in different ways. Natural selection, which results from differential survival and reproduction, is the only mechanism of evolution that consistently causes a population to become better adapted to its environment. While genetic drift and gene flow may cause alleles to increase in frequency regardless of whether they are harmful, neutral, or beneficial, natural selection causes only beneficial alleles to become more common in a population. Think about the events and conditions described in this exercise. Can you see how only those associated with natural selection produce a better match between the plants and their environment?

True or false? Heterozygote advantage refers to the tendency for heterozygous individuals to have better fitness than homozygous individuals. This higher fitness results in less genetic variation in the population Hint: What is genetic variation?

False Heterozygote advantage results in more genetic variation in the population.

What is the frequency of the A1A2 genotype in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals? Hint 1: Remember that all genotype frequencies of a particular gene in a population add up to one.

0.4 The calculation to determine the frequency of the A1A2 genotype is: 80 A1A2 individuals / (20 + 80 + 100) total individuals = 0.4, the frequency of the A1A2 genotype.

Calculate the allele frequencies in this fish population.

0.46 A, 0.54 a There are 12 fish total, for a total of 24 alleles since they are diploid. The frequency of A = 2 × 3(AA) + 1 × 5(Aa) = 11/24 = 0.46. The frequency of a = 2 × 4(aa) + 1 × 5(Aa) = 13/24 = 0.54.

Calculate the allele frequencies in the iguana population following the mutation event.

0.95 G, 0.05 g, There are 10 iguanas total, for a total of 20 alleles since they are diploid. The frequency of G = 2 × 9(GG) + 1 × 1(Gg) = 19/20 = 0.95. The frequency of g = 1 × 1(Gg) = 1/20 = 0.05.

Suppose 64 percent of people living in a remote, isolated mountain village can taste phenylthiocarbamide (PTC) and must, therefore, have at least one copy of the dominant PTC taster allele. If this population conforms to Hardy-Weinberg expectations for this gene, what percentage of the population must be heterozygous for this trait?

48 percent

Which of the following evolutionary forces could create new genetic information in a population Hint: Think about how new genetic information can arise.

Mutation Mutations, which are changes in a cell's DNA, can introduce new genetic information in a population.

Which evolutionary mechanism results in adaptation? See Section 23.2 (Page). Hint: What causes adaptation?

Natural selection Adaptation is the ability to reproduce and produce viable offspring in a particular environment. Natural selection is the evolutionary mechanism that produces adaptation.

Women often have complications during labor while giving birth to very large babies, whereas very small babies tend to be underdeveloped. Which kind of selection is most likely at work regarding the birth weight of babies? Hint: Babies sometimes die if labor is complicated, especially if medical care is poor or unavailable. Similarly, underdeveloped babies have low survival rates.

Stabilizing selection Stabilizing selection causes no change in the average of the population; extreme phenotypes (in this case, large and small babies) become less common.

Small Aristelliger lizards have difficulty defending territories, but large lizards are more likely to be preyed upon by owls. Which kind of selection acts on the adult body size of these lizards? Hint: Consider what lizard size has the advantage.

Stabilizing selection Stabilizing selection causes no change in the average of the population; extreme phenotypes (large and small lizards) become less common.

Why don't the spontaneous deleterious mutations that occur in an individual E.coli cause downward drops in fitness in this population? See Section 23.6 (Page).

The cells that acquire these deleterious mutations are selected against by natural selection and these mutations are lost from the population over time.

Suppose that Gene B occurs in a sea turtle gene pool where the frequency of the B1 allele is 0.5 and the frequency of the B2 allele is 0.4. What is the frequency of the B3 allele, assuming that there are only three alleles in the population?

The frequency of B3=0.1 because 0.5+0.4+?=1. The frequencies of the three alleles must add up to 1.

What is the frequency of the A1 allele in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals? Hint: Remember that these individuals are diploid and that all allele frequencies of a particular gene in a population add up to one.

The frequency of the A1 allele is 0.3. The frequency of the A1 allele is p = (number of A1 alleles) / (total of all alleles) = [(2 ( 20) + 80] / [(2 × 20) + (2 × 80) + (2 × 100)] = 0.3.

Some of the steelhead trout (Oncorhynchus mykiss) in the Hood River of Oregon were raised in hatcheries and released to supplement dwindling wild populations. In one study, researchers captured and determined the parents of a group of trout and found some were offspring of wild x wild, others had one wild parent and one hatchery-raised parent, and a third group were offspring of captive x captive parents. The data in the accompanying figure show the relative fitness of these offspring. What statement best summarizes the results?

The gene flow between hatchery-reared and wild populations is leading to a decline in fitness of wild populations.

Which of the following statements is not a part of the Hardy-Weinberg principle?

The genotype frequencies in the offspring generation must add up to two. This statement is not true; the genotype frequencies in the offspring generation must add up to one.

True or false? The Hardy-Weinberg model makes the following assumptions: no selection at the gene in question; no genetic drift; no gene flow; no mutation; random mating.

True These are the five assumptions of the Hardy-Weinberg model.