Answers to Mastering Concepts Questions
12.1
1. What are two ways to define evolution?
Evolution can be defined in reference to macroevolution or microevolution.
Macroevolutionary events are large-scale evolutionary changes that occur over long
periods of time, while small scale microevolutionary events occur over shorter time
periods. Microevolutionary events lead to macroevolutionary events.
2. Why can evolution act only on populations and not on individuals?
Evolution can only act on populations because it is defined as a change in allele
frequencies. Alleles cannot be changed in an individual, but alleles in a population can
change.
12.2
1. How did people think that species arose and diversified before Darwin published his
theory of evolution by natural selection?
People believed that organisms arrived by planned, purposeful events.
2. What did Darwin observe that led him to develop his ideas about the origin of species?
Darwin noticed that the finches on the Galapagos Islands must have had a common
ancestor with the finches on the mainland. He noticed that there was variation in
individuals within a population. He also noticed that not all individuals were able to
reproduce and that not all offspring would survive.
3. How is artificial selection different from natural selection?
Artificial selection occurs when humans select for specific traits in organisms that are
beneficial to humans, and then breed the organisms accordingly. In natural selection,
natural forces drive the selection for traits that benefit the survival and reproduction of
the organism.
4. What is the modern evolutionary synthesis?
The modern evolutionary synthesis represents the fact that mutations in the genetic
structure of organisms bring about heritable variation which is what the forces of natural
selection can act upon.
12.3
1. What is an adaptation?
An adaptation is a feature that provides an advantage that improves the ability of an
individual to survive and reproduce.
2. What is the role of genetic variation in natural selection?
Genetic variation provides the phenotypic variation upon which natural selection can act.
3. How can natural selection favor different phenotypes at different times?
Environmental conditions change; favorable traits in one condition might not be
favorable in another.
4. What is “fitness” in the context of evolution?
Fitness is an organism’s contribution to the gene pool in the next generation. The more
offspring an individual produces, the more fit the individual is.
12.4
1. What five conditions are required for Hardy-Weinberg equilibrium?
Hardy-Weinberg equilibrium requires a large population, random mating, no natural
selection, no mutation, and no migration.
2. Explain the components and meaning of the equation p + q = 1 and p2 + 2pq + q2 =
1?
The first equation represents the allele frequencies in the population assuming that there
are two alleles for a given trait. The second equation represents the genotypes and
frequencies for a typical monohybrid cross (where p2 is the frequency of homozygous
dominant genotypes, 2pq represents heterozygous genotypes, and q2 represents
homozygous recessive genotypes).
3. Why doesn’t Hardy-Weinberg equilibrium occur in real populations?
In real populations, at least one of the requirements for the equilibrium model is violated.
Populations do not always exhibit random mating, for example.
12.5
1. Distinguish between directional, disruptive, and stabilizing selection?
Directional selection occurs when individuals at one extreme are selected for. Disruptive
selection occurs when both extremes are selected for. Stabilizing selection occurs when
individuals in the middle are selected for.
2. How can natural selection maintain harmful alleles in a population?
Harmful alleles can be maintained if there is a survival or reproductive advantage for
individuals that are heterozygous for the trait.
12.6
1. How does sexual selection promote traits that would seem to decrease fitness?
If the trait is preferred by the mate, the trait is advantageous for reproduction. Those
individuals that display the trait will be more likely to mate and produce offspring.
2. Describe two ways that competition for access to mates can lead to sexual selection?
Members of one sex may compete for access to members of the other sex (e.g., male
rams fight for access to females) or members of one sex may be choosy about mates (e.g.,
females choose the male peacock with the biggest, brightest tail feathers).
12.7
1. How do mutations affect an organism’s phenotype?
Mutations change alleles. Alleles are alternate forms of a gene. Genes produce proteins.
A change in the protein will change the phenotype.
2. Under what conditions does a mutation in one organism pass to subsequent
generations?
If a mutation occurs in eggs or sperm then the offspring will have the mutation.
3. How does sampling error lead to genetic drift?
If the sample does not match the allele frequency of the larger group from which it is
taken, it will lead to drift.
4. What is the difference between the founder effect and a population bottleneck?
The founder effect occurs when a small population breaks off from a larger population.
A bottleneck occurs when a population’s size is reduced dramatically over a short period
of time.
5. How do nonrandom mating and migration result in evolutionary change?
Nonrandom mating leads to evolutionary change because individuals with particular traits
are selected for in reproduction which leads to those traits being prevalent in the
population over time. Migration moves alleles among populations.
12.8
1. What hypothesis did Conover and Munch test?
The researchers where testing whether fishing regulations play a role in the increase in
small, slow-growing fish.
2. How is a population reared in a tank different from a population in the “real world”?
The population in the tank is controlled for many environmental factors (no predators, no
competitors, etc.) and the tank fish would be fed instead of looking for food.
3. The heritability of a trait is the extent to which it is genetically determined. Heritability
ranges from 0 (entirely under environmental control) to 1.0 (100% controlled by genes).
In Atlantic silversides, the heritability of body size is about 0.2. How would the results of
this experiment differ if heritability of body size were higher? What if it approached
zero?
If heritability were higher, the results would be seen much quicker because if all large
fish were removed, there would be no ‘large’ alleles left in the population. If the
heritability were zero, then the removal of large individuals would not be related to
alleles, and would not change the size structure of the population.
Answers to Write It Out Questions
1. List and describe five mechanisms of evolution.
Genetic drift: random loss of alleles in a population.
Selection: differential survival and reproduction of individuals within a population.
Mutation: provides the new alleles in a population.
Sexual selection: individuals with preferred traits are selected for mating and leave more
offspring.
Migration: moves alleles from one population to another.
2.How did the work of other scientists influence Charles Darwin’s thinking?
Lyell’s work led Darwin to realize that events that he was witnessing would have been
the same in the geological past. Malthus’s essay challenged Darwin to realize that not all
individuals would survive, but that there would be more produced than the habitat could
support, leading to a ‘survival of the fittest’. Wallace had a similar theory which
prompted Darwin to realize his idea was worthy of publication.
3. Your boss is a plant breeder who asks you to develop a sweeter variety of apple. How
would you use artificial selection to achieve this goal? How might natural selection
produce the same result? Which would occur faster, artificial selection or natural
selection?
You would use the seeds from the sweetest apples to plant trees. When those trees
mature they should be pollinated with pollen from other trees that produce sweet apples.
This would occur naturally because animals would prefer to eat the sweeter fruit which
would disperse the seeds. If sweet fruit trees also had sweeter nectar, then there would be
a preference for sweeter trees for insects pollinating the trees. The process would occur
more quickly through artificial selection.
4. How does variation arise in an asexually reproducing population? A sexually
reproducing population?
Variation arises by mutation in both types of reproduction. A population that reproduces
sexually also has genetic recombination during meiosis to provide variation.
5. What happens to a population if conditions change and no individuals have the allele
combinations required to survive and produce offspring?
The population would die off or go extinct.
6. Many articles about the rise of antibiotic-resistant bacteria claim that overuse of
antibiotics creates resistant strains. How is this incorrect?
The use of the soap doesn’t ‘create’ resistant bacteria. There were already bacteria that
had the alleles for resistance through mutation. By killing all of the individuals that aren’t
resistant to the antibiotic, the remaining population has a higher frequency of individuals
which are resistant.
7. Explain how harmful recessive alleles can persist in populations, even though they
prevent homozygous individuals from reproducing?
If a harmful recessive allele actually makes a heterozygote more able to survive and
reproduce, then the allele will be maintained in the population.
8. Fraggles are mythical, mouselike creatures that live underground beneath a large
vegetable garden. Of the 100 Fraggles in this population, 84 have green fur, and 16 have
gray fur. A dominant allele F confers green fur, and a recessive allele f confers gray fur.
Assuming Hardy–Weinberg equilibrium is operating, answer the following questions. (a)
What is the frequency of the gray allele f ? (b) What is the frequency of the green allele
F? (c) How many Fraggles are heterozygotes (Ff)? (d) How many Fraggles are
homozygous recessive (ff)? (e) How many Fraggles are homozygous dominant (FF)?
a)
b)
c)
d)
e)
0.4
0.6
48
16
36
9. One spring, a dust storm blankets the usually green garden of the Fraggles in gray. The
green Fraggles therefore become visible to the Gorgs, who tend the gardens and try to kill
the Fraggles to protect their crops. The gray Fraggles, however, blend easily into the
dusty background. How might this event affect microevolution in the Fraggles? What
mode of natural selection does this represent? .
The gray Fraggles will become more common, so the gray allele will become more
common in the population. This would represent directional selection.
10. Describe examples of directional, disruptive, and stabilizing selection other than those
mentioned in this chapter?
Answers will vary but should represent what is graphically seen in figure 12.13.
11. A male cardinal is bright red; his mate is drab brown. How does sexual selection
explain this difference?
Females select the mates and choose males that have a bright red coloration which would
also be more attractive to predators.
12. Many American farmers routinely use an herbicide called glyphosate to keep their
fields weed-free. They are now reporting the emergence and spread of glyphosateresistant species of “superweeds.” Use natural selection to explain this situation. What
might farmers do to minimize the problem?
The herbicide kills the individuals that do not have the trait that provides resistance.
Killing the individuals without resistance means that only the resistant individuals
survive to reproduce. The more resistant individuals, the more resistant individuals there
are for reproduction. The allele frequency of the population has shifted to the resistant
form. Farmers could reduce the frequency and amount of herbicide used, or they could
alternate types of herbicide.
13. The giraffe in the chapter opening photo has a long neck. Use natural selection to
explain how a long-necked species could evolve from an ancestral population with short
necks. How does your explanation compare to how Lamarck might have explained it?
The population had a few individuals with necks that were longer than other individuals.
The individuals with the long necks were more able to reach leaves on the trees and
therefore, survive and produce offspring. Long necked individuals mate with long
necked individuals so there are more long necked individuals in the population. Lamarck
would have said that an individual stretched its neck to reach the leaves in the trees.
Since its neck is now long from stretching, when it had offspring, their necks would also
be long.
Answers to Pull It Together Questions
1. What is the biological definition of evolution?
Evolution is the change in allele frequencies in a population over time.
2. Add the terms genotype, phenotype, allele frequencies, founder effect, bottleneck
effect, and sexual selection to this concept map.
EVOLUTION acts on “Phenotype” which is determined by “Genotype”. A “Founder
effect”, “Bottleneck”, and “Sexual selection” change “Allele frequencies”.
3. How does each mechanism of evolution change allele frequencies in a population?
A mutation will change the actual allele. Genetic drift will randomly change which alleles
are more common in the population. Sexual selection changes alleles in the population
which are related to characteristics that are preferred by mates for reproduction.
Migration brings new alleles to the population, or eliminates alleles from the population,
if the only individuals with those alleles leave.
4. Describe the modes of natural selection.
Selection can be directional, disruptive, or stabilizing. In directional selection,
individuals of an extreme phenotype become more common. In disruptive selection,
individuals of both extreme phenotypes become more common. In stabilizing selection,
individuals with the average phenotype are the most common.