CHAPTER 15 Test Your Knowledge
Driving Question 1
What is a gene pool (and can you swim in it)?
By answering the questions below and studying Infographics 15.1 and 15.2, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Genetic diversity is measured in terms of allele frequencies (the relative proportions of specific alleles in a gene pool). A population of 3,200 mice has 4,200 dominant G alleles and 2,200 recessive g alleles. What is the frequency of g alleles in the population?
3,200 mice have 6,400 alleles, of which 2200 are g. 3,200/6,400 gives an allele frequency of 0.34.
Of the three populations described below, each of which has 1,000 members, which population has the highest genetic diversity? Note that only one gene is presented, and that this gene has three possible alleles: A1, A2, and a.
Population A: 70% have an A1/A1 genotype, 25% have an A1/A2 genotype, and 5% have an A1/a genotype.
Population B: 50% have an A1/A1 genotype, 20% have an A2/A2 genotype, 10% have an A1/A2 genotype, 10% have an A2/a genotype and 10% have an a/a genotype.
Population C: 80% have an A1/A1 genotype, and 20% have an A1/a genotype.
Population B appears to have the highest level of genetic diversity, based on allele frequencies and genotype frequencies in the population.
USE IT
A small population of 26 individuals has five alleles, A through E, for a particular gene. The E allele is represented only in one, homozygous individual:
Five individuals are D/A heterozygotes.
Five individuals are A/A homozygotes.
Five individuals are A/B heterozygotes.
Five individuals are C/D heterozygotes.
Five individuals are C/C homozygotes.
One individual is an E/E homozygote.
If five A/E heterozygotes migrate into the population, what will be the impact on the allele frequencies of each of the five alleles?
In the original population, there are 26 individuals and 52 alleles of the gene in question. The allele frequencies are: 10/52 = 0.19 D; 20/52 = 0.38 A; 5/52 =0.20 B; 15/52 = 0.29 C; 2/52 = 0.04 E. After the migration, the allele frequencies are: 10/62 = 0.16 D; 25/62 = 0.4 A; 5/62 = 0.08 B; 15/62 = 0.24 C; 7/62= 0.12 E. The frequency of E increases substantially.
Some populations, for example cheetahs, have gene pools with very few different alleles. What approach(es) could be taken to try and introduce new alleles into these kinds of population?
A common approach is to find distinct species or populations that will successfully interbreed with the cheetahs. If interbreeding with distantly related species or populations is successful, then new alleles will be introduced into the population.
From their gene pool and population size, which of the four populations in the accompanying table would you be most concerned about from a conservation perspective? Why would you be concerned?
Populations 1 and 4 are of most concern. Population 1 is not only small, but has only one allele for one of the genes examined. Population 4 is larger than population 1, but has limited allelic diversity (it has only one allele of two genes, and only two alleles of the third gene).
The global human population continues to grow, and more people than ever are living in crowded cities. Given this situation, what selective pressures might the human population be currently facing or be expected to face in the near future?
There are many possibilities. Some of these include the pressure of infectious diseases (which can spread easily in crowded populations), and the pressure of stress (associated with living in crowded cities). If city dwellers do not spend much time outside, there may be pressures associated with low exposure to sunlight. There may also be pressures exerted by air pollution.
Driving Question 2
How do different evolutionary mechanisms influence the composition of a gene pool?
By answering the questions below and studying Infographics 15.3, 15.4, and 15.5 and Table 5.1, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Which of the following are examples of genetic drift?
a. founder effect
b. bottleneck effect
c. inbreeding
d. a and b
e. a, b, and c
d
A bottleneck is best described as
a. an expansion of a population from a small group of founders.
b. a small number of individuals leaving a population.
c. a reduction in the size of an original population followed by an expansion in size as the surviving members reproduce.
d. the mixing and mingling of alleles by mating between members of different populations.
e. an example of natural selection.
c
A population of ants on a median strip has 12 different alleles, A through L, of a particular gene. A drunk driver plows across the median strip, destroying most of the median strip and 90% of the ants. The surviving ants are all homozygous for allele H.
a. What is the impact of this event on the frequency of alleles A through L?
b. What type of event is this?
a: The allele frequencies for every allele except H is now 0. The allele frequency for H is 1 (100% of the alleles are H). b: As a random event affecting allele frequencies, this is an example of genetic drift—specifically, it has created a bottleneck.
USE IT
Question 2 looked at the allele frequencies of three populations, A, B and C. From your answer to that question, which population would you predict to have the greatest chance of surviving an environmental change? Explain your answer.
From its overall diversity, population B appears to have the most resilience with which to face environmental change.
In humans, founder effects may occur when a small group of founders immigrates to a new country, for example to establish a religious community. In this situation, why might the allele frequencies in succeeding generations remain similar to those of the founding population rather than gradually becoming more similar to the allele frequencies of the population of the country to which they immigrated?
The founders bring a specific set of alleles to the new population in the new country. If this population remains isolated from other populations in the new country (e.g., because of religious or cultural practices), there may not be much mixing with other populations in the new country. Members of the founding group may choose to marry within their group. This will maintain alleles already present and will not introduce new alleles.
Why is genetic drift considered to be a form of evolution? How does it differ from evolution by natural selection?
Genetic drift changes allele frequencies in a population: this is the definition of evolution.
INTERPRETING DATA
The figure below shows a structure bar plot of moles from different parks in New York City. As in Infographic 15.2, each vertical bar represents genotypes from 18 genomic locations in one animal. The bars are color coded, with similar genotypes represented by the same color. From the data presented in the figure:
a. Are these three populations genetically isolated from one another? Explain your answer. What factors could explain their isolation, or lack thereof?
b. Is one of the populations potentially experiencing gene flow with another population? If so, which one, and how do you know?
a: Populations B and C share the blue genotype, suggesting that they are potentially sharing alleles. Population A has different alleles present in some mice (purple and green). These alleles are apparently not being shared with populations B and C. These alleles may be introduced into population A by sharing with other populations. When populations are separated geographically, they are less likely to share alleles than are populations with individuals that readily move between populations. b: Population A has alleles not found in populations B and C. This suggests that population A is sharing alleles with a population that is not sharing alleles with populations B and C.
Driving Question 3
How does the gene pool of an evolving population compare to the gene pool of a nonevolving population?
By answering the questions below and studying Infographic 15.5 and Up Close: Calculating Hardy-Weinberg Equilibrium, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Which of the following statements is/are true about a nonevolving population?
a. Allele frequencies do not change over generations.
b. Genotype frequencies do not change over time.
c. Individuals choose mates with whom they share many alleles.
d. all of the above
e. a and b
e
A starting population of bacteria has two alleles of the TUB gene, T and t. The frequency of T is 0.8 and the frequency of t is 0.2. The local environment undergoes an elevated temperature for many generations of bacterial reproduction. After 50 generations of reproduction at the elevated temperature, the frequency of T is 0.4 and the frequency of t is 0.6. Has evolution occurred? Explain your answer.
Evolution has occurred. The allele frequencies of the TUB gene have changed over many generations (in this case in response to a change in the environmental conditions).
Why is inbreeding detrimental to a population?
Inbreeding can result in matings between relatives that produce offspring with two (detrimental) recessive alleles. Over time, inbreeding reduces the frequency of heterozygotes, and produces homozygotes that have two deleterious alleles.
USE IT
Phenylketonuria (PKU) is a rare, recessive genetic condition that affects approximately 1 in 15,000 babies born in the United States. (You may have noticed on products that contain aspartame the statement “Phenylketonurics: contains phenylalanine,” a warning for people with PKU that they should avoid consuming that product.) Calculate the expected frequency of carriers (that is, of heterozygotes) in the U.S. population, based on the information provided about rates of PKU among U.S. births, assuming that the population is in Hardy-Weinberg equilibrium for this gene.
The frequency of homozygous recessives (q2) is 1/15,000. Therefore the frequency of the recessive allele (q) is the square root of 1/15,000 = 0.008. Therefore the frequency of the dominant allele (p) = 1 – q = 1 – 0.008 = 0.992. The frequency of carriers (heterozygotes) will be 2pq = 0.016 (1.6% of the population).
Assume a population of 100 individuals. Five are homozygous dominant (AA), 80 are heterozygous (Aa), and 15 are homozygous recessive (aa) for the A gene. Determine p and q for this gene for this population. Now use those values for p and q and plug them into the Hardy-Weinberg equation. Is this population in Hardy-Weinberg equilibrium? Why or why not?
p = 0.45 and q = 0.55. Therefore the predicted frequency of AA homozygotes is p2= 0.2025 or 20.25%. Only 5% of the population is actually AA homozygotes; therefore this population is not in Hardy-Weinberg equilibrium at this gene.
Driving Question 4
How do new species arise, and how can we recognize them?
By answering the questions below and studying Infographics 15.7 and 15.8, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
The biological species concept defines a species
a. on the basis of similar physical appearance.
b. on the basis of close genetic relationships.
c. on the basis of similar levels of genetic diversity.
d. on the basis of the ability to mate and produce fertile offspring.
e. on the basis of recognizing one another’s mating behaviors.
d
How does geographic isolation contribute to speciation?
When populations are geographically isolated, they do not exchange alleles. This means that if a mutation arises in one population and not the other, the mutation will be present in only one of the two populations. As the number of different mutations accumulates in each isolated population and the resulting phenotypes are acted on by natural selection, the two populations could diverge enough so that they cannot successfully interbreed if they come into contact with each other.
USE IT
Two populations of rodents have been physically separated by a large lake for many generations. The shore on one side of the lake is drier and has very different vegetation from that on the other side. The lake is drained by humans to irrigate crops, and now the rodent populations are reunited. How could you assess if they are still members of the same species?
According to the biological species concept, if members of different populations can successfully reproduce (and produce fertile offspring), then they are members of the same species. If they cannot mate or produce fertile offspring after their isolation, then they have (by definition) become different species.
If geographically dispersed groups of a given species all converge at a common location during breeding season, then return to their home sites to bear and rear their young, what might happen to the gene pools of the different groups over time?
While each population may live in a separate region, the fact they converge during breeding season means that alleles will be exchanged and shared throughout all the populations. They are not geographically isolated (at least at a genetic level).
MINI CASE
More than 50% of the global human population now lives in urban areas, and it is predicted that 70% will live in urban areas by 2050. Researchers have hypothesized that the emotional health of urbanites is influenced positively by interaction with nature. Given this information, and what you have read in this chapter, write a compelling paragraph on the need to conserve urban species and approaches to such conservation based on population genetics.
If natural settings are critical for human emotional health, then it is critical that humans live in settings that allow interactions with nature. Given the predicted shift of humans to urban settings, it is important that humans have the opportunity to interact with nature in urban settings. Such settings could include parks, ponds, lakes and nature trails. Many wildlife species face challenges in urban settings, including population isolation, which can lead to inbreeding depression. In order to maintain these populations in a healthy state, it is important to allow for distinct populations to exchange alleles of genes. These exchanges are facilitated by green connecting spaces which allow organisms to move between populations. Additionally, smaller green spaces such as container- or rooftop gardens can provide opportunities for pollinators. If enough people have such spaces, pollinators can transfer pollen between individual populations.
BRING IT HOME
The School of Ants is a citizen-scientist project to document the distribution and diversity of ants across the United States. Similarly, the Audubon Christmas Bird Count is a conservation-related project carried out by volunteer citizen scientists. The Audubon Society uses the data collected by these volunteers to evaluate the health of bird populations and make informed decisions about conservation. There are numerous other citizen-scientist projects (e.g., Project Squirrel and the Gravestone Project). Carry out an Internet search to find a citizen-scientist project that you find interesting. Now take the next step: enroll, collect some data, and contribute to science!
Answers will vary.