Ch 21: The Evolution of Populations

Question 1

In a population with two alleles, B and b, the allele frequency of b is 0.4. What would be the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium?

Answer 1

0.48
( 2 x 0.4 (b) x 0.6 (B) )
(- heterozygotes = 2pq)

Question 2

Selection that acts over evolutionary time to preserve traits that increase an individual’s ability to mate is known as __________.

Answer 2

sexual selection

Question 3

Genetic analysis of a large population of mink inhabiting an island in Michigan revealed an unusual number of loci where one allele was fixed. Which of the following is the most probable explanation for this genetic homogeneity?

Answer 3

A very small number of mink may have colonized this island, and this founder effect and subsequent genetic drift could have fixed many alleles.

Question 4

An increase in the number of antibiotic-resistant individuals in a population of bacteria is an example of __________.

Answer 4

directional selection

Question 5

In the context of a population, how is evolution defined?

Answer 5

Evolution is a change in a population’s allele frequencies over generations.

Question 6

Assume a population is in Hardy-Weinberg equilibrium for a character with these genotypic frequencies: AA = 0.25, Aa = 0.50, and aa = 0.25. If you remove all the homozygous dominants and allow the remaining population to reproduce (again under Hardy-Weinberg conditions), what will be the frequency of homozygous dominants in the next generation?

Answer 6

0.11

(Since 2 x 0.25 = 0.50, one = 0.
AA (q) = 0, Aa (2pq) = 2/3 aa (p) = 1/3.
p^2 = 0.11)

Question 7

A population of squirrels is preyed on by small hawks at maturity. The smaller adult squirrels can escape into burrows. The larger adult squirrels can fight off the hawks. After several generations, the squirrels in the area tend to be very small or very large. This is an example of __________.

Answer 7

disruptive selection

Question 8

A population of 15 birds inhabits a small island. Ten of the birds are dark brown, and five of them are light brown. By chance, two of the dark brown birds and three of the light brown birds die before producing any offspring. All of the birds in the next generation are dark brown. This change in phenotypic frequency can be attributed to __________.

Answer 8

genetic drift

Question 9

The average heterozygosity of Drosophila melanogaster is estimated to be about 14%, which means that __________.

Answer 9

on average, 14% of a fruit fly’s gene loci are heterozygous

Question 10

In a particular central African population, 4% are born with sickle-cell disease (they are homozygous recessive at a hemoglobin loci). If this population is in Hardy-Weinberg equilibrium, what percentage of the population is heterozygous at the relevant allele and has the selective advantage of being more resistant to malaria?

Answer 10

32%

(4% = 1/population = 0.04
sqrt 0.04 = 0.2
1-0.2 = 0.8
2 x 0.2 x 0.8 = 0.32 = 32%)

Question 11

Which of the following mechanisms can alter allele frequencies?

Answer 11

All of the listed responses are correct.

Question 12

Birds with average-size wings survived a severe storm more successfully than other birds in the same population with longer or shorter wings. If severe storms occur regularly, then over time, one should expect these storms to bring about __________.

Answer 12

stabilizing selection

Question 13

Which of the following sets of conditions is required for Hardy-Weinberg equilibrium?

Answer 13

Random mating, no natural selection, and a large population

Question 14

An earthquake hits a small island. All but a few individuals of a lizard species are eliminated in this catastrophe, and an allele is lost from the population. This is an instance of __________.

Answer 14

the bottleneck effect

Question 15

Which of the following provides most of the genetic variation found among individuals in plant and animal populations?

Answer 15

Sexual reproduction

Question 16

A scientist observes that the height of a certain species of asters decreases as the altitude on a mountainside increases. She gathers seeds from samples at various altitudes, plants them in a uniform environment, and measures the height of the new plants. All of her experimental asters grow to approximately the same height. From this she concludes __________.

Answer 16

the height variation she initially observed was an example of environmental influence

Question 17

Which of the following is a true statement concerning sexual selection?

Answer 17

Sexual selection can result in sexual dimorphism.

Question 18

In a large population of randomly breeding organisms, the frequency of a recessive allele is initially 0.3. There is no migration and no selection. Humans enter this ecosystem and selectively hunt individuals showing the dominant trait. When the gene frequency is reexamined at the end of the year, __________.

Answer 18

the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up

Question 19

A plant population is found in an area that is becoming more arid. The average surface area of leaves has been decreasing over the generations. This trend is an example of __________.

Answer 19

directional selection

Question 20

Which of the following is the best example of gene flow?

Answer 20

Wind blows pollen from one population of plants to another and cross-fertilization occurs.

Question 21

Sexual recombination includes the shuffling of chromosomes in __________ and fertilization.

Answer 21

meiosis

Question 22

Approximately 1 out of every 2,500 Caucasians in the United States is born with the recessive disease cystic fibrosis. According to the Hardy-Weinberg equilibrium equation, approximately what percentage of people are carriers?

Answer 22

4%

(1/2500)

Question 23

A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both A1 and A2?

Answer 23

0.42

(70% in allele which means p or q.
If p = 0.7, q = 0.3, 2pq (2 x 0.7 x 0.3) = 0.42)

Question 24

For several years, scientists have warned doctors of the danger of overprescribing antibiotics, such as penicillin. Scientists are concerned because __________.

Answer 24

natural selection can lead to strains of microorganisms resistant to these drugs

Question 25

In a population that is in Hardy-Weinberg equilibrium for two alleles, C and c, for a character determined by complete dominance, 16% of the population show a recessive trait. Assuming C is dominant to c, what percent of individuals show the dominant trait?

Answer 25

84%

(dominant = p, recessive = q,
1 - 0.16 = 0.84)

Question 26

Three main mechanisms that cause allele frequency change

Answer 26

– Natural selection
– Genetic drift
– Gene flow

Question 27

Sexual reproduction can shuffle existing alleles into new
combinations through three mechanisms

Answer 27

– Crossing over
– Independent assortment
– Random fertilization

Question 28

a group of individuals that live in the
same area and interbreed, producing fertile offspring

Answer 28

population

Question 29

consists of all the alleles for all loci in a
population

Answer 29

gene pool

Question 30

A population in which mating is random and none of the
mechanisms of evolution are acting is in

Answer 30

Hardy-Weinberg
equilibrium

Question 31

Hardy- Weinberg formula

Answer 31

p squared + 2pq + q squared = 1

Question 32

How to find q for Hardy- Weinberg

Answer 32

1/ number of population, then sq rt of that

Question 33

How to find p for Hardy- Weinberg

Answer 33

1-q

Question 34

Five Conditions for Hardy-Weinberg Equilibrium

Answer 34

1. no mutations
2. random mating
3. no natural selection
4. Extremely large population size
5. no gene flow

Question 35

occurs when a few individuals
become isolated from a larger population

Answer 35

founder effect

Question 36

can result from a drastic reduction
in population size due to a sudden environmental change

Answer 36

bottleneck effect

Question 37

consists of the movement of alleles among
populations

Answer 37

Gene flow

Question 38

There are three modes of natural selection

Answer 38

• directional selection
• disruptive selection
• stabilizing selection

Question 39

occurs when conditions favor
individuals at one end of the phenotypic range

Answer 39

Directional selection

Question 40

occurs when conditions favor
individuals at both extremes of the phenotypic range

Answer 40

Disruptive selection

Question 41

occurs when conditions favor
intermediate variants and act against extreme
phenotypes

Answer 41

Stabilizing selection

Question 42

occurs when natural selection
maintains stable frequencies of two or more phenotypic
forms in a population

Answer 42

Balancing selection

Question 43

Balancing selection includes

Answer 43

– Heterozygote advantage
– Frequency-dependent selection

Question 44

occurs when heterozygotes
have a higher fitness than do both homozygotes

Answer 44

Heterozygote advantage

Question 45

the fitness of a
phenotype depends on how common it is in the
population

Answer 45

frequency-dependent selection,