22. Evolution of Populations

Genetic Variation

The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow

 No. Stabilizing selection occurs in stable environments, which resist factors that promote phenotypic variation. 

 No. Antibiotic resistance acts opposite to disruptive selection in that it promotes the growth of only those bacteria with the resistant gene. 

 No. This situation represents rapid evolution rather than equilibrium. 

 No. The spread of antibiotic resistance in a population would be an example of microevolution. 

Consider how the distribution of the trait is being affected.

Antibiotic resistance in bacteria is an example of which of the following?

 How strong the organism is when pitted against others of its species 

 No. This is not a generic measurement of fitness because not all organisms compete in this way. 

 Its ability to withstand environmental extremes 

No. But that might increase its fitness because it may be able to leave more offspring than other individuals.

 How much food it is able to make or obtain 

Consider how a population changes.

Which of the following most accurately measures an organism's fitness?

 Only random mating takes place among all the people that reproduce in North America. 

 No. That would decrease the possibility of microevolution in that population. 

 The incidence of skin cancer in adults over age 40 rises significantly. 

 No. Skin cells are somatic cells and would not affect reproduction. 

 Hundreds of thousands of people are killed in a natural disaster. 

 No. That would not be a significant enough change in the human gene pool to cause microevolution to take place. 

 All of the listed responses are correct. 

 A colony of humans on the moon is isolated from Earth. 

Consider the Hardy-Weinberg assumptions.

Which of the following scenarios would most likely result in the microevolution of a population of humans?

 humans will become tolerant to the effects of drugs 

 No. Consider why antibiotics are prescribed. 

 the organisms that produce many of the antibiotics are overexploited and could become extinct 

 No. Consider the topics of this chapter. 

 the drugs will be metabolized more quickly by our bodies, decreasing their effectiveness 

 No. This is possible, but it is not the major cause for concern. 

 None of the listed responses is correct. 

 No. One of the other choices is correct. 

 strains of microorganisms that are resistant to these drugs will be selected for 

Consider the directional mode of natural selection.

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

Mechanisms of Natural Selection Part 1: Types of Sexual Selection

Inbreeding and Sexual Selection

Natural Selection - Crash Course Biology #14

Evaluate this statement: Gene flow increases the genetic divergence of populations.

BioFlix: Natural Selection

Genetic Drift | Founder Effect and Bottleneck Effect Explained

 individual mosquitoes built up an immunity to an insecticide after being exposed to it 

 No. The insecticides are toxic chemicals. The insects could not have developed immunity. 

 mosquitoes needed to develop insecticide resistance to survive after the insecticide was used 

 No. This is not how evolution works. Selection can only edit existing variations. 

 mosquitoes attempted to adapt to their environment 

 No. Mosquitoes are not capable of such an action. 

 a new allele developed in response to the insecticide that provided future generations the benefit of resistance 

 No. This is not how evolution works. New alleles do not arise on demand. 

 a few mosquitoes were probably resistant to the insecticide before it was ever used, and these individuals were more likely to survive and reproduce in the presence of insecticide 

Does insecticide kill all members of a mosquito population? .

A number of mosquito populations today are resistant to insecticides that were once quite effective. Biologists think that insecticide resistance evolved in mosquitoes because __________.

Heterozygous Advantage

In the 1700s and 1800s, royalty in Europe often married their close relatives; furthermore, recessive genetic diseases such as hemophilia showed up much more often among royals than in the general population. Explain the likely connection.

Genetic drift, bottleneck effect and founder effect | Biology | Khan Academy

Directional, Stabilizing, and Disruptive Seleciton

STABILIZING, DIRECTIONAL, DISRUPTIVE SELECTION

 Homozygosity in some prairie chicken populations 

 No. These populations have almost no genetic variation. 

 Polymorphism of the Gal&#225;pagos finches 

 No. These variations are adaptive and selective rather than neutral. 

 No. The founder effect reduces the amount of variation in a population, but there is no reason to assume the variations are neutral. 

 No. Colors usually have a purpose, such as mating or camouflage. 

Consider which of these does not affect fitness.

Which of the following would seem to be an example of neutral variation?

 No. Chromosomes are duplicated but not shuffled in mitosis. 

 No. This is the random change in a small gene pool. 

 No. Natural selection refers to an organism's reproductive success. 

 No. Mutation is the ultimate source of genetic variation, but it has nothing to do with sexual recombination. 

Consider when genetic recombination occurs.

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

Suppose you were studying several species of monkeys. In one, males never helped females raise offspring. In another, males provided just as much parental care as females except for actually carrying the fetus during pregnancy. How does the fundamental asymmetry of sex compare in the two species? How would you expect sexual dimorphism to compare between the two species?

No two people are genetically identical, except for identical twins. The main source of genetic variation among humans is
a. new mutations that occurred in the preceding generation.
b. genetic drift.
c. the reshuffling of alleles in sexual reproduction.
d. environmental effects.

 No. Stabilizing selection favors intermediate variants. 

 No. Directional selection acts against individuals at one of the phenotypic extremes. 

 No. Balancing selection maintains stable frequencies of two or more phenotypic forms in a population. 

 No. The type of selection referred to in the question would favor evolution rather than equilibrium. 

Consider the effect of the predation on the trait distribution.

A population of squirrels is preyed on by small hawks. The smaller squirrels can escape into burrows. The larger squirrels can fight off the hawks. After several generations, the squirrels in the area tend to be very small or very large. What process is responsible for this outcome?

 DNA forensics relies on neutral variation among humans. 

 No. DNA forensics does use neutral variation to distinguish one human from another, but this use of neutral variation does not affect evolution. 

 Relative frequencies of neutral variations are controlled by natural selection. 

 No. Variations are considered neutral if they are neither advantageous nor deleterious. Natural selection does not act on variation that is neutral in the current environment. 

 Neutral variation is usually removed from the population by natural selection because it does not confer an advantage in the current environment. 

 No. Natural selection does not act on neutral variation. 

 Many mutations are silent and do not affect the amino acid that is coded for. 

 No. Although this statement is true, it does not explain the importance of this kind of variation to evolution. 

 Neutral variation increases genetic variation, allowing a population to carry more alleles that may help it respond to environmental change. 

Is neutral variation always neutral?

What is the importance of neutral variation in evolution?

The largest unit within which gene flow can readily occur is a
a. population.
b. species.
c. genus.
d. hybrid.

Natural selection changes allele frequencies because some _________ survive and reproduce better than others.
a. alleles
b. loci
c. species
d. individuals

Balancing Selection and Gene Flow

Animation: Causes of Evolutionary Change

Human activities are causing the fragmentation of the Brazilian Atlantic rain forest. One result is that toucans have become extinct or nearly extinct in some of the forest fragments. Does the absence of toucans affect the forest?

Toucans can eat fruits with large seeds because their large bills can open wide. Most other birds in the same forest can only eat small seeds. Ecologist Mauro Galetti and his colleagues measured the seed sizes of palms in forest fragments with and without toucans. The graphs show two of the forest populations they studied. What is the take-home message of the data? <IMAGE>

Why is genetic drift aptly named? Select True or False for each statement.

T/F It causes allele frequencies to drift up or down randomly.
T/F It occurs when alleles from one population drift into another.
T/F It occurs when mutations drift into a genome.

Ivory from elephant tusks is a valuable commodity on the world market. As a result, male African elephants with large tusks have been heavily hunted for the past few centuries. Today, male elephants have significantly shorter tusks at full adulthood than male elephants in the early 1900s. This is an example of                   .
a. diversifying selection;
b. stabilizing selection;
c. directional selection;
d. chance;
e. more than one of the above is correct

Describe three ways that evolution can occur via genetic drift.

 No. The founder effect, another example of genetic drift, would occur if a small group of lizards migrated to the island. 

 No. Gene flow would occur if lizards migrated in and out of the population. 

 No. There are no mutations described here. 

 No. There is no indication that the lizards that survived did so because of any beneficial trait. 

What are some of the mechanisms that change allele frequencies?

An earthquake hits a small island. All but a small group of closely related lizards are eliminated, and the survivors spread out over the island. This is an instance of __________.

Genetic Drift

The risks faced by small populations include                   .
a. erosion of genetic variability through genetic drift;
b. decreased fitness of individuals as a result of inbreeding;
c. increased risk of experiencing natural disasters;
d. A and B are correct;
e. A, B, and C are correct

 No. This refers to genetic variations that have little or no impact on reproductive success. 

 No. This answer is too specific. Heterozygote advantage is just one mechanism of balancing selection. 

 No. This mode of selection actually reduces variation and maintains the status quo for a particular phenotypic character. 

 No. Directional selection is selection for a single form; selection for two or more forms is balancing selection. 

Review the types of selection. .

Which type of selection maintains stable frequencies of two or more phenotypic forms in a population?

True or false? Gene flow can either increase or decrease the average fitness of a population. Explain.

Genetic variation, gene flow, and new species

Antibiotic resistance is becoming common among organisms that cause a variety of human diseases. All of the following strategies help reduce the risk of antibiotic resistance evolving in a susceptible bacterial population except                   .
a. using antibiotics only when appropriate, for bacterial infections that are not clearing up naturally;
b. using the drugs as directed, taking all the antibiotic over the course of days prescribed;
c. using more than one antibiotic at a time for difficult-to-treat organisms;
d. preventing natural selection by reducing the amount of evolution the organisms can perform;
e. reducing the use of antibiotics in non–health-care settings, such as agriculture

 Intrasexual selection leads to the death of most unfit males in combat. 

 No. More often than not, the victor of these struggles wins through psychological methods rather than physical ones. 

 Showy secondary sexual characteristics cannot be explained because they break all of the rules of natural selection. 

 No. On the surface, this does appear to be true. For example, the colorful plumage of some birds makes them much more visible to predators. However, there are some theories to explain how these characteristics can increase an individual's fitness. 

 In most vertebrates, females court the males. 

 No. Males are usually the suitors in vertebrate populations. 

 There is no evidence that intrasexual selection takes place between females. 

 No. There is evidence in some populations of females establishing hierarchies of dominance. 

 Sexual selection can result in sexual dimorphism&mdash;marked differences between the sexes in secondary sexual characteristics that are not associated directly with reproduction. 

Remember that organisms are successful if they survive and reproduce.

Which statement below is true about sexual selection?

Types of Natural Selection

 No. This would slightly decrease the number of genes in the gene pool. 

 No. Because the genetic code is redundant, even point mutations in genes that code for proteins may have little effect if they do not alter the amino acid composition. The chances are slight that such a mutation would have a profound effect on an organism. 

 No. Such a mutation would most likely be harmful, and it does not generate new genetic material. 

No. A change in nucleotide sequence describes the result of any type of mutation but is not in and of itself a type of mutation.

Think about the end result of different types of mutations.

Which type of mutation plays the most important role in increasing the number of genes in the gene pool?

Draw a small concept map showing how selection, genetic drift, gene flow, and mutation relate to genetic variation.

 No. Natural selection leads to adaptation, but it is not the origin of genetic variation. 

 No. This just leads to a change in the frequency of alleles in a gene pool. 

 No. Sexual recombination does not generate new alleles; it merely recombines available alleles. 

 No. The environment elicits adaptation but not genetic variation. 

Only one of these choices actually introduces new alleles into a population.

Which of the following mechanisms can form entirely new alleles?

Heterozygote Advantage

Natural Selection animation (stabilizing, disruptive and directional selection)

 Genetic drift; natural selection; mutation 

 No. Genetic drift and natural selection cause changes in the frequency of alleles in the gene pool. They do not result in genetic variation. Mutations do not usually result in adaptation to the environment. 

 Overproduction of offspring; mutation; sexual recombination 

 No. Although mutation results in genetic variation, overproduction does not. 

 Natural selection; mutation; sexual recombination 

 No. Although mutation results in genetic variation, natural selection does not. 

 Sexual recombination; natural selection; overproduction 

 No. Although sexual recombination results in genetic variation, natural selection does not. 

 Mutation; sexual recombination; natural selection 

Evolution is a blend of chance and sorting.

__________ and __________ generate variation, whereas __________ results in an adaptation to the environment.

 No. Directional selection shifts the frequency curve for some phenotypic character in one direction or the other, by favoring individuals that deviate from the average. 

 No. Disruptive selection occurs when environmental conditions favor individuals on both extremes of a phenotypic range over intermediate phenotypes. 

 No. Stabilizing selection acts against extreme phenotypes and favors intermediate variants. 

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

 No. Stabilizing selection has nothing to do with mutation rates. 

 occurs when some individuals migrate to an area with different environmental conditions 

 No. That would likely result in directional selection. 

 usually results in two distinct phenotypes 

 No. Stabilizing selection has a homogenizing effect; disruptive selection, on the other hand, often results in two different phenotypes. 

 favors intermediate variants in a population 

Consider how stabilizing selection affects the distribution of a trait.

Stabilizing selection __________.

Sexual Selection Explained: Evolution 101

Why isn’t inbreeding considered an evolutionary process?
a. It does not change genotype frequencies.
b. It does not change allele frequencies.
c. It does not occur often enough to be important in evolution.
d. It does not violate the assumptions of the Hardy–Weinberg principle.

Natural Selection & Sexual Selection: An Illustrated Introduction

Selection

 No. There is no evidence that the birds' color affected their fitness. 

 No. This refers to the genetic exchange between populations. In this example, there is only one population. 

 No. There is no evidence that the birds' color affected their attractiveness to potential mates or subsequent fitness. 

 No. A new mutation affecting color would be unlikely to occur in a single generation in such a small population. A single mutation would not cause such a large change in phenotypic frequency. 

Which causes of evolution involve chance?

A population of 15 birds inhabits a fairly new 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 __________.

 A small population of humans colonizes a newly formed island. 

 No. Gene flow is the loss or gain of alleles from the gene pool by migration. This is an example of the founder effect. 

 Genes are shuffled by the crossing over of chromosomes during meiosis. 

 No. Gene flow is the loss or gain of alleles from the gene pool. 

 An earthquake results in the formation of a canyon, splitting a population of toads apart. 

No. Gene flow is the loss or gain of alleles from the gene pool. Though this example might result in a cessation of gene flow, it is not an example of gene flow occurring.

 A fire drastically reduces the size of a white-tailed deer population. The remaining individuals spread out throughout the remaining forest. 

 No. This is an example of a bottleneck, not gene flow. 

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

Consider that gene flow refers to a genetic exchange between populations that results from the movement of fertile individuals or gametes.

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

Human activities are causing the fragmentation of the Brazilian Atlantic rain forest. One result is that toucans have become extinct or nearly extinct in some of the forest fragments. Does the absence of toucans affect the forest?

Toucans disperse seeds of key forest species such as juçara palms by eating the fruit and defecating the seeds in new locations, sometimes more than a kilometer away. If there are no toucans, is the genetic diversity of palms likely to increase or decrease within forest fragments? Why?
a. increase (due to increased genetic drift)

b. decrease (due to decreased gene flow)

c. decrease (due to decreased mutation rate)

d. decrease (due to decreased natural selection)

Balancing Selection

 Because homozygous recessive individuals die, the recessive allele will eventually be lost from the population. 

 No. The recessive allele has been maintained in the population because heterozygotes survive and may pass the recessive allele on to offspring. 

 Only homozygous dominant individuals will be able to survive and reproduce. 

 No. In order to be affected by the disease, an individual must be homozygous recessive. Heterozygotes survive and may pass the recessive allele on to offspring. 

 In the heterozygous condition, the dominant allele will overcome the recessive allele and only the dominant allele will be passed on to offspring. 

 No. All alleles within an organism have an equal chance of being passed on to offspring. 

 Homozygous dominant individuals will be more likely to reproduce than heterozygous individuals. 

 No. The presence of the Tay-Sachs allele in a heterozygous individual would not affect his or her ability to reproduce. 

 Heterozygous individuals will survive and may pass the recessive allele on to their offspring. 

Consider the conditions under which the Tay-Sachs allele is not expressed.

The lethal genetic disorder Tay-Sachs disease occurs in individuals with two copies of a recessive allele of the responsible gene. Which of the following statements is true?