Mastering Concepts
11.1
1. What is evolution?
Evolution is genetic change in a population over time.
2. How did people think that species arose and diversified before Charles Darwin
published his theory of evolution by natural selection?
The major belief was in a special creation in which God formed all species and placed
them on the earth. Jean Baptiste de Lamarck proposed that acquired characteristics were
heritable; differential use and disuse of body parts accounted for changes between
generations and species diversification.
3. What did Darwin observe that led him to develop his ideas about the origin of species?
As Darwin journeyed in the HMS Beagle, he observed the uniformity of geological
processes such as volcanism, earthquakes, and erosion. He collected fossils and observed
that each continent had animals that were characteristic and different from those on other
continents. On the Galapagos, he observed and collected finches from different islands
and noted differences in giant tortoises on different Galapagos Islands. These
observations led Darwin to the idea of “descent with modification.”
4. How is artificial selection different from natural selection?
In artificial selection, humans select for the traits they desire in plants or animals. In
natural selection, environmental factors favor some traits over others.
5. How did Darwin’s ideas challenge prevailing beliefs about life’s diversity and the
status of humans?
Darwin’s ideas challenged prevailing beliefs about life’s diversity by using natural
processes to explain how species arise. Darwin saw humans as just another species and
rejected the idea that humans were specially created. He thereby demoted humans from
an exalted position, putting them on an equal footing with other species competing for
resources.
6. What is the modern evolutionary synthesis?
The modern evolutionary synthesis combines the work of Darwin with a modern
understanding of genes, chromosomes, and the origins of variation.
11.2
1. What is an adaptation, and how do adaptations become more common within a
population?
An adaptation is a characteristic that helps an individual survive and reproduce in its
environment. Adaptations become more common within a population when they are
heritable and when they increase the odds of survival and reproduction.
2. What is the role of genetic variation in natural selection?
Genetic variation is the raw material that natural selection acts on since genetic variation
creates variations in phenotypes, some of which are better suited to the current
environment.
3. How can natural selection favor different phenotypes at different times?
Natural selection can favor different phenotypes at different times because environments
are always shifting in dramatic or subtle ways.
4. Why doesn’t natural selection produce perfectly adapted organisms?
Natural selection does not produce perfectly adapted organisms because each
population’s evolution is constrained by its existing gene pool, which limits the possible
allele combinations and may not contain every allele necessary to confront every
challenge. Chance events may also wipe out adaptive allele combinations. Genetic
illnesses that produce symptoms only after reproductive age also can maintain harmful
alleles in a population.
5. What is evolutionary fitness?
Evolutionary fitness is measured by reproductive success.
11.3
1. What are the five conditions required for Hardy–Weinberg equilibrium?
The five conditions required for Hardy-Weinberg equilibrium are:
- no mutations;
- random mating;
- no emigration or immigration;
- the population is large enough to eliminate random changes in allele frequencies
(genetic drift);
- natural selection does not occur.
2. Why is the concept of Hardy–Weinberg equilibrium important?
The concept of Hardy-Weinberg equilibrium is important because it shows that evolution
has the potential to occur at all times; in fact, evolution is probably occurring at all times
in all populations.
3. Explain the components and meaning of the equation p2 + 2pq + q2 = 1.
In the Hardy-Weinberg equation, p= the frequency of the dominant allele in a population,
q= the frequency of the recessive allele, and pq= the product of p and q. The equation p2
+ 2pq + q2 = 1 says that all individuals in a population are homozygous dominant,
heterozygous, or homozygous recessive (for a gene with only two alleles). By applying
the Hardy-Weinberg equation at different times, it is possible to observe changes in allele
frequency and determine whether evolution is occurring.
4. Why doesn’t Hardy–Weinberg equilibrium occur in real populations?
Hardy-Weinberg equilibrium does not occur in real populations because the conditions
for it are never met in real populations.
11.4
1. Distinguish among directional, disruptive, and stabilizing selection.
Directional selection selects against phenotypes at one end of the variation spectrum in a
population; the variation in the population shifts in the opposite direction. In disruptive
selection, extreme phenotypes are more fit than the intermediate phenotype; the
frequency of the intermediate phenotype decreases in the population. In stabilizing
selection, both extreme variations of phenotype are selected against; the most common
phenotype in the population remains around the median.
2. How can natural selection maintain harmful alleles in a population?
Natural selection can only select against phenotypes. If the dominant phenotype is
selected for, heterozygous individuals will retain the recessive allele in the population.
Also there are times when the heterozygous condition gives advantages to the individual
(e.g., cystic fibrosis carriers are less susceptible to cholera). In these instances a balanced
polymorphism is seen in the population.
11.5
1. How does sexual selection promote traits that would seem to decrease fitness?
Sexual selection selects for extreme phenotypes in one sex, and those extreme variations
often place the individual at a disadvantage for survival (e.g., a peacock’s longer tail will
make him more visible to predators and slower in escaping). However, since these traits
increase fitness by increasing mating success, sexual selection remains an active force in
the population.
2. What is the difference between intrasexual selection and intersexual selection?
Intrasexual selection occurs when members of the same sex compete for mating
opportunities with the opposite sex. Intersexual selection occurs between males and
females of the same species: one sex chooses a mate of the other sex based on the
presence of a characteristic.
11.6
1. What are some ways that mutations affect an organism’s phenotype?
Mutations introduce new alleles, which may or may not alter the phenotype of an
organism.
2. Under what conditions does a mutation in one organism pass to subsequent
generations?
A mutation in one organism passes to subsequent generations if it is a mutation that can
be inherited. In sexually reproducing organisms, the mutation must appear in a gameteproducing cell. Mutations in somatic cells will not be passed to the next generation. By
contrast, a mutation in an organism that produces asexually will be transmitted to all of
its offspring.
3. How does sampling error cause genetic drift?
Sampling error occurs when the selected sample (e.g. the alleles making it to the next
generation) does not match the larger original pool. Such chance sampling errors result
in a change in the allele frequencies of the population, not as a result of adaptive fitness,
but as a result of pure chance.
4. What is the difference between the founder effect and a population bottleneck?
In the founder effect, a new population is started by a small group of individuals that
colonize a new area. In a population bottleneck, a large and genetically variable
population experiences a massive die-off, and only a few individuals survive to continue
the population. Both effects result in populations with restricted variety in their gene
pools and are therefore prone to genetic drift.
5. How do nonrandom mating and gene flow disrupt Hardy–Weinberg equilibrium?
Nonrandom mating means that not all individuals have an equal chance of reproducing,
and this will shift the equilibrium equation. In gene flow, alleles enter or leave the
population; this by its very nature disrupts the allele balance in the population.
11.7
1. What hypothesis did Conover and Munch test?
The researchers tested the hypothesis that fishing results in population shifts towards fish
that are smaller at maturity or that mature more slowly.
2. How is a population reared in a tank different from a population in the “real world”?
In the real world the population would be subject to gene flow as fish entered or left. The
artificial population might also be subject to genetic drift.
3. The heritability of a trait 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 0?
If heritability of body size were higher then the results of the experiment would be even
more pronounced with the yellow and blue lines even farther from the random control
line. If heritability approached zero then all three lines on the graphs should remain
about the same.
Write It Out
1. How did James Hutton, Georges Cuvier, Georges-Louis Buffon, Jean Baptiste de
Lamarck, Charles Lyell, and Thomas Malthus influence Charles Darwin’s thinking?
James Hutton influenced Darwin by suggesting that changes in nature were gradual and
uniform (a concept called uniformitarianism). Georges Cuvier suggested that catastrophic
events may cause extinctions and that nature can act abruptly. Georges Buffon said that
species change as they spread from their original location. Jean Baptiste de Lamarck
proposed the inheritance of acquired characteristics. Charles Lyell renewed the argument
of uniformitarianism and applied gradual geologic change to the history of the Earth and
life; Lyell also suggested that the Earth was much older than 6000 years, allowing plenty
of time for evolution by means of natural selection to occur. Thomas Malthus illustrated
that populations produce many offspring and compete for resources. Darwin extended
Malthus’ theory, suggesting that those individuals that are able to obtain resources best
would be most likely to survive and reproduce.
2. 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 process would occur faster?
Have a group of volunteers taste the apples and categorize them based on sweetness.
Take the seeds from the sweetest apples, plant them, and repeat the experiment. Natural
selection could mimic this if some animal relied on the apple as a food source, and the
animal were more attracted to sweeter apples. The seeds of the sweeter apples might get
carried farther away from their parent plants than the less sweet apples, increasing the
chance of the seeds of the sweeter apple maturing. Artificial selection would be much
faster since scientists could ensure that all seeds from the sweetest trees successfully
germinate.
3. How does variation arise in an asexually reproducing population? A sexually
reproducing population?
Variation arises in all species through mutations. In sexually reproducing species,
variation spreads rapidly as a result of genetic recombination during meiosis.
4. What happens to a population if conditions change and no individuals have the allele
combinations required to survive and produce offspring?
The population goes extinct.
5. Many articles about the rise of antibiotic-resistant bacteria claim that overuse of
antibiotics creates resistant strains. How is this incorrect?
Within populations of bacteria, random mutations are constantly occurring. Exposure to
antibiotics does not cause such mutations, but it does cause the death of the bacteria that
do not inherit alleles that confer resistance.
6. Explain how harmful recessive alleles can persist in populations, even though they
prevent homozygous individuals from reproducing.
Harmful recessive alleles can “hide” behind dominant alleles in the heterozygous
condition of balanced polymorphisms, especially if the heterozygote individual carries
some advantage (e.g., in sickle cell anemia where individuals resist malaria).
7. 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) The frequency of the gray allele is 0.4. (b) The frequency of the green allele is 0.6.
(c) The number of heterozygotes is 48. (d) The number of homozygous recessive
individuals is 16. (e) The number of homozygous dominant individuals is 36.
8. 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 homozygous recessive condition (gray) has a better chance of survival and will
therefore have greater reproductive success. Their numbers will increase in future
generations. This represents directional selection.
9. Describe examples of directional, disruptive, and stabilizing selection other than those
mentioned in the chapter.
Many examples possible. Below are a few:
Directional: increasing pesticide resistance, increasing body size in horse evolution
Disruptive: banding or no banding patterns in Lake Erie water snakes, different body
shapes in cichlid fish from the same cohort in African lakes
Stabilizing: clutch size in birds, anatomical structures in alligators and horseshoe crabs
10. How does sexual selection maintain sexual dimorphism?
If a trait seems harmful but somehow increases an individual’s reproductive success, it
will become more common in the population.
11. One of the most striking examples of convergent evolution occurs among animals that
permanently live in caves. Among other shared traits, many of these species are eyeless.
Develop a hypothesis to explain how an eyeless species could evolve from an ancestral
population with eyes. How does your hypothesis compare to how Lamarck might have
explained it?
The allele for eyelessness is recessive but is carried in the population by a few
heterozygous individuals. In every generation there would be a chance that some
offspring receive the recessive allele from both parents (homozygous recessive). In
normal environments, these offspring would be at a selective disadvantage and most
would not breed. In a cave, they would not be at a disadvantage and could live to mate. If
two homozygous recessive individuals mate, then all of their offspring are homozygous
recessive (eyeless). The allele frequency of the population slowly shifts toward recessive;
a larger percentage of each successive generation would be eyeless. Lamarck might have
suggested that in the cavefish did not use their eyes and so they lost them. This loss was
passed on to their offspring.
Pull It Together
1. What is the biological definition of evolution?
The biological definition of evolution is the change of 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” leads with “alters” to “Allele frequency”. “Nonrandom mating” leads with
“is not seen in nature due to” to “Sexual selection”. “Founder effect” and “Bottleneck
effect” lead with “are specific types of” to “Genetic drift”. “Mutation” leads with “alters
an organism’s” to “Genotype”, which leads with “partly determines” to “Phenotype”.
3. How does each mechanism of evolution change allele frequencies in a population?
(See Figure 11.20) Natural selection changes allele frequency by selecting against alleles
that reduce the chance of survival and selecting for alleles that enhance the chance of
survival. Mutation changes allele frequency by introducing new alleles. Genetic drift
changes allele frequency by chance. Nonrandom mating changes allele frequency by
selecting for traits that other mates find attractive. Gene flow changes allele frequency
by moving alleles from one population to another.
4. Describe the three modes of natural selection.
Phenotypes with a range of expressions in a population, from common to extremes, are
subject to selective pressures. Directional selection chooses for one extreme or against
one extreme of a phenotype. Disruptive selection chooses against the average phenotype,
leaving different extremes of a phenotype in the population. Stabilizing selection
chooses against the extreme phenotypes in the population, leaving the average phenotype.