Chapter 13
Answers
1
QUICK-CHECK questions
List two types of variation that may be seen in populations of dogs.
Examples of variation in dog populations include:
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 black/yellow coat colour
 black/liver coat colour
 presence/absence of body hair
 presence/absence of white spotting
 long/short leg length.
Give an example of developmental variation in:
a
a plant species
One example of developmental variation in a plant species is the change from
green, unripe fruit to red, ripe fruit in blackberries (see figure 13.12).
b
a bird species.
One example of developmental variation in a bird species is the change in
plumage as seagulls develop from juveniles to adults.
3
Give an example of a population that:
a
is polymorphic for a trait
Any population that shows two or a few discrete inherited variations for a trait is
said to be polymorphic; an example is lupin populations that show purple and pink
flower colour.
(In contrast, in a monomorphic population, all members are uniform for a particular
trait; an example is the pink and grey plumage of galahs (see figure 13.13).)
b
shows discontinuous variations in a trait.
Discontinuous variation occurs in a population when a small number of discrete,
non-overlapping variants of a trait exist. For example, in a human population,
people fall into one of four distinct ABO blood types; they may be group A or group
B or group AB or group O.
(Note: This is just one of many example of discontinuous variation that can be
seen in plant and animal populations. Table 13.4 lists some other examples.)
4
At different times of the year, members of a population of water fleas (Daphnia
sp.) vary in the size and shape of their outer head covering.
a
Is this likely to be an inherited variation? Explain.
The change in head shape and size is most likely to be an environmental variation
since the change is seasonal, occurring at certain times of the year. However, the
ability to make this change is an inherited characteristic that has evolved in only
some Daphnia species.
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Chapter 13: QUICK-CHECK answers
b
Suggest a possible cause for the observed change.
Daphnia are invertebrates that live in aquatic habitats such as transient pools,
small ponds and lakes. Many suggestions for the possible cause of this variation
are acceptable including:
 external environmental factors, such as water temperature, water pH and water
salinity
 internal environmental factors associated with breeding cycles.
(Note: In fact, studies have shown that the change in head shape and size in
populations of some Daphnia species was a response to chemical signals
released by invertebrates predators of Daphnia. This predator-induced change in
head shape in Daphnia occurred when predators invaded the habitat and has
been shown to give to the Daphnia individuals some protection from predation.)
5
Give an example of an inherited variation in human populations.
Examples of inherited variation in human populations include dark/fair hair colour, ABO
blood types and Rhesus blood types.
6
Give an example of an environmental variation in plants.
One example is the variation in leaf shape in arrowleaf plants depending on whether or
not they are submerged. When grown under water, the leaf is long and thin, but when
grown exposed to the air, the leaves are shaped like arrowheads (see figure 13.16).
7
An inherited trait in a population shows two variations only. How many genes
are likely to be involved?
If an inherited variation shows just two variants, it is reasonable to predict that a single
gene with two alleles is involved.
8
A population of cattle varies in coat colour, being red, roan or white. Is this
variation continuous or discontinuous?
This is an example of discontinuous variation since members of this population fall into
three discrete non-overlapping classes.
9
An inherited trait in a population is under the control of four genes, each with a
plus allele and a minus allele. How many different variants might be expected to
exist in this population?
Assume that the + alleles of the four genes make an identical contribution to the
phenotype. In this case, there would be nine phenotypic classes or variants, and these
variants would have a total of + alleles ranging from 8 to 0.
The variant showing the greatest phenotypic expression of the trait controlled by the
four polygenes has the genotype ++ ++ ++ ++, while the variant with the least
phenotypic expression has the genotype – – – – – – – –.
10
Over many generations, populations of a flowering plant showed only red flower
colour. A plant with white flower colour suddenly appeared. Suggest a likely
cause of this new variation.
One likely cause of this new variation is a gene mutation.
11
Distinguish between the members of the following groups:
a
genotype and gene pool
A genotype is the genetic information at one or more loci in one individual.
A gene pool is the genetic information present in a population.
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Chapter 13: QUICK-CHECK answers
b
allele frequency and genotype frequency and phenotype frequency.
Allele frequency is the proportion of a particular allele of a gene in the gene pool of
a population.
Genotype frequency is the proportion of a particular genotype in the gene pool of
a population.
Phenotype frequency is the proportion of a particular phenotype in the gene pool
of a population.
12
A population consists of 50 fish. What is the total number of alleles for one
autosomal gene in this population?
The total number of alleles for one autosomal gene in this fish population is 100, that
is, twice the number of fish since each fish has two copies of the chromosome that
carries this gene locus.
13
In the gene pool of a population, the frequency of the R allele is 0.2. Assuming
two alleles for a gene, what is the frequency of the r allele?
The frequency of the r allele = 1 – 0.2 = 0.8.
14
List two conditions that apply to a H–W population.
Features of a population in Hardy–Weinberg equilibrium include:
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 large population (so that genetic drift has no effect)
 random mating
 closed population (no inward or outward migration)
 all matings producing an equal number of viable offspring.
True or false?
a
p+q=1
True: The symbols p and q denote the allele frequencies of a gene with two
alleles, and the total of these frequencies must be 1.
b
p2 = 2pq = q2 = 1.
False: The symbols p2, 2pq and q2 denote genotype frequencies in a H–W
population. These values are not equal, as shown in the answer to question 15a.
However, the sum of these three frequencies is 1.
16
A herd of wild mountain goats moves permanently to a higher region and over
many generations shows an increase in average body size. If this change is a
result of selection, is it natural or artificial? Explain.
This is an example of natural selection since it involves a wild population.
17
What is meant by ‘fitness’ in a genetic sense?
In a genetic sense, fitness is the reproductive success (rather than physical strength).
18
In a plant population, some phenotypes survive periods of drought better than
other phenotypes. What might be expected if this population were exposed to
drought conditions for several generations?
It is reasonable to predict that continuous exposure of this population to drought over
several generations would result in selection of those plants that are physiologically
better adapted to drought conditions. These plants would survive and reproduce,
whereas plants lacking the drought-resistant alleles would reproduce less well and in
many cases would not survive to reproduce. It may be predicted that over several
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Chapter 13: QUICK-CHECK answers
generations, if drought conditions persisted, the frequency of alleles conferring drought
resistance in this population would increase.
19
Would complete selection have a greater effect against a dominant trait in a
population or a recessive trait? Explain.
Complete selection would have a far greater effect against a dominant trait than
complete selection against a recessive trait. In the former case, all individuals showing
the dominant trait, both homozygous AA and heterozygous Aa, would be completely
selected against. As a result, the trait and the allele responsible for it would be
eliminated from the population in one generation. In the case of complete selection
against a recessive trait, however, all homozygous recessive bb individuals would be
eliminated from the population, but the allele would still exist in the population carried
by heterozygous Bb individuals, who do not show the trait and so are not subject to
selection. Matings between heterozygotes would produce offspring showing the trait in
the next generation.
20
Migration is sometimes identified as an agent that can rapidly change the gene
pool of a population. Explain this statement.
Migration of individuals into (or out of) a population can bring about an immediate
change in the gene pool of that population provided that the gene pool of the migrant
group has different allele frequencies from the initial population. (In contrast, changes
in allele frequencies in a gene pool under the effects of natural selection can occur
only between generations.)
21
Define in your own words the following terms:
a
gene flow
Gene flow is an alternative term for migration into or out of a population, and it is a
change agent that can bring about immediate changes in the allele frequencies of
a population.
b
genetic drift
Genetic drift refers to chance effects that result in unpredictable changes in the
gene pool of a population and that are most marked in small populations. Genetic
drift is the basis of the biological phenomena known as the founder effect and the
bottleneck effect.
c
evolution within a species
Evolution within a species is a result of changes in the gene pool of one
population of the species due to the action of selection and/or the occurrence of
mutations. These phenotypic changes do not prevent interbreeding between
members of the changed population and members of the original population
should they meet. Evolution within a species results in the formation of races or
sub-species.
d
sub-species.
A sub-species is an identifiable race or a distinctive population of a species that
exists in a region so that the sub-species is typically geographically separated
from other populations of that species.
22
Identify the following statements as true or false:
a
mtDNA undergoes recombination during meiosis.
False: mtDNA is passes from a female parent to all her offspring with no
recombination.
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Chapter 13: QUICK-CHECK answers
b
Haplogroup L1 is the most ancient of existing haplogroups.
True: Using the simplified version shown in figure 13.43, it is reasonable to
conclude that haplogroup L1 is the most ancient because it occurs only in
indigenous populations in sub-Saharan Africa where modern humans evolved.
(Note: The study of haplogroups in human populations is a field of active research
and the various haplogroups are constantly being refined. For example, the
L haplogroup now consists of L0, L5, L1, L2 and L3, with the most ancient being
identified
as
L0.
For
more
information,
see
the
website
http://mbe.oxfordjournals.org/cgi/content/full/24/3/757.)
c
Study of mtDNA haplogroups supports the conclusion that the Pacific Island
populations originated in South America.
False: Study of mtDNA haplogroups supports the conclusion that Pacific Island
populations originated in the region of Taiwan.
d
The standard against which mtDNA sequences are compared is the
Cambridge Reference Sequence.
True: The Cambridge Reference Sequence (CRS) is the reference standard
against which all mtDNA sequences are compared and their differences identified.
(Note: The CRS belongs to haplogroup H.)
e
mtDNA studies have shown that Neanderthals were a different species from
modern human beings.
True: When compared with the mtDNA of modern humans, the large number of
differences seen in Neanderthal mtDNA supports the conclusion that these two
kinds of humans were different species.
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