Download WORKING WITH THE FIGURES 1. Examining Figure 20

20
Evolution of Genes and Traits
WORKING WITH THE FIGURES
1.
Examining Figure 20-4, explain why the rate of evolution at nonsynonymous
sites is lower. Do you expect this to be true only of globin genes or of most
genes?
Answer: Both types of mutations are expected to arise at the same rate.
However, synonymous substitutions do not change the amino acid sequence
of a protein and generally have no phenotypic effect. They are therefore not
subject to natural selection and are expected to accumulate freely. On the
other hand, nonsynonymous substitutions do change the amino acid sequence
and potentially have a wide range of affects on phenotype. These effects are
subject to natural selection. Most nonsynonymous substitutions will be at
least slightly harmful and selected against, reducing their frequency, while a
few will be nearly neutral or advantageous; so nonsynonymous mutations will
tend to accumulate much more slowly and be fixed at a lower rate than
synonymous ones. This general tendency would not change from gene to
gene, so the effect would be expected to occur for most genes.
2.
In Figure 20-7, the overall survival rates of three genotypes are plotted.
Explain the reasons for the differences between the three survival curves.
Answer: The decrease in survivorship common to all three genotypes could be
due to any of many factors affecting early childhood mortality. The
differences between the genotypes can be explained as differences in
resistance to infection by malaria parasites (P. falciparum), an important
source of early childhood mortality in areas where malaria is common. The
graph indicates that SS genotypes suffer the greatest mortality, likely due to
the consequences of red blood cell sickling in homozygotes. Untreated, this is
frequently a lethal condition. AA has the second highest mortality. AA
homozygotes have no sickling but are subject to high childhood mortality
from infection by malaria parasites. AS has the lowest mortality because
heterozygotes are partially resistant to infection by the malarial parasite and
do not exhibit harmful sickling of red blood cells. There are at least two
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possible explanations for a decrease in the difference between AS and AA as
the children age. First, malaria mortality is highest among young children,
who are more susceptible because their natural immunity has not yet
developed. Second, AA genotypes that survive may have acquired immunity
to infection and are less likely to be infected as adults. In areas where malaria
is common, mortality from the disease drops dramatically as children age and
is uncommon among adults.
3.
From Table 20-4, would you expect the noncoding mutation g4205a to be
fixed before or after the coding mutation G238S in a population of bacteria
evolving resistance to the antibiotic cefotaxime? Give at least two reasons for
your answer.
Answer: The noncoding mutation g4205a would be fixed after the coding
mutation G238S. First, the noncoding mutation had a positive effect on only
8 alleles, whereas the coding mutation had a positive effect on all 16. Also,
the coding mutation had a positive effect independently of the state of other
genes. This gives it a strong advantage. Second, the noncoding mutation had a
negative effect on 2 other alleles, increasing its disadvantage relative to the
coding gene. The strength of the advantage of the coding gene is seen in the
mean proportional increase column-the increase in the coding mutation is
almost 3 orders of magnitude greater than the noncoding gene.
4.
Examining Table 20-5, what do you think would be the order of mutations
fixed during selection in a third evolving virus line? Would the mutations
become fixed in the same order as the TX or ID virus?
Answer: The differences in the order of mutations fixed between the two
different lines indicates that the order is not predictable-even though several
of the fixed mutations were the same in the two lines, the mutations were not
fixed in the same order. It is likely that the mutations in a third virus line
would be fixed in a different order but involve at least some of the same
mutations, for example those that were the same between TX and ID. It also
seems likely that the mutations in a third line would involve the A and F
proteins, since the fixed mutations commonly occur in those two.
5.
Using Figure 20-17, explain how the mutation in the GATA sequence of the
Duffy gene imparts resistance to P. vivax infection.
Answer: The GAT A sequence activates Duffy expression that is specific to
red blood cells, which results in Duffy protein on the surface of red blood
cells. The malarial parasite, P. vivax, invades red blood cells by using Duffy
protein as a recognition site. The mutation GACA prevents binding of the
GAT A 1 protein to the enhancer and prevents expression of Duffy protein on
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the red blood cells, but not on other cells. The lack of Duffy protein means
that P. vivax are no longer able to recognize and infect the red blood cells, and
resistance to malaria is conferred.
6.
In Figure 20-18, what is the evidence that polyploid formation has been
important in plant evolution?
Answer: Polyploidy doubles the diploid chromosome number, which
automatically produces an even haploid number.
In the absence of
polyploidy, the haploid number would be expected to be randomly distributed
between even and odd. The high frequency of even haploid chromosome
numbers compared to odd numbers indicates that polyploidy is common and
likely important in plant evolution.
BASIC PROBLEMS
7.
Compare Darwin's description of natural selection as quoted on page 730 with
Wallace's description of the tendency of varieties to depart from the original
type quoted on page 731. What ideas do they have in common?
Answer: The ideas of Darwin and Wallace were remarkably similar. Both
authors recognized a struggle for existence and that survival and reproduction
were not assured. Both also recognized the existence of variations that could
confer a reproductive advantage. Finally, both realized that if beneficial
variations were passed to the offspring they would increase in frequency over
time.
8.
What are the three principles of the theory of evolution by natural selection?
Answer: The three principles are: (1) organisms within a species vary from
one another, (2) the variation is heritable, and (3) different types leave
different numbers of offspring in future generations.
9.
Why was the neutral theory of molecular evolution a revolutionary idea?
Answer: Before the neutral theory was developed, evolutionary biologists
considered all change to be due to natural selection. In the absence of
molecular data, mutations were thought of as beneficial or harmful, even if
only slightly so, and so subject to natural selection. It was difficult to conceive
that modification of a highly specific enzyme, for example, could be neutral.
With a more complete understanding of the genetic code and the molecular
basis for mutation and protein function came the surprising realization that
neutral or nearly neutral mutations could arise. Neutral theory established the
even more revolutionary idea that evolution of neutral genes by genetic drift
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was not only possible but common, and that most amino acid differences
between species had not arisen through natural selection of adaptive variation
but through genetic drift of neutral alleles.
10.
What would you predict to be the relative rate of synonymous and
nonsynonymous substitutions in a globin pseudogene?
Answer: A pseudogene is a nonfunctional duplication of an active gene.
Because it is nonfunctional, changes in the pseudogene are not subject to
natural selection-all amino acid substitutions are neutral. As a result, there
should be no difference in the rate of synonymous and nonsynonymous
substitutions in a globin pseudogene or any other pseudogene.
11.
Are AS heterozygotes completely resistant to malarial infection? Explain the
evidence for your answer.
Answer: AS heterozygotes are not completely resistant to malarial infection.
The primary evidence for this is that AS heterozygotes have lower rates of
infection than AA homozygotes, but still may be infected by the parasite. One
study showed a 27.9 percent rate of infection for AS and a 45.7 percent rate
for AA. Other indirect evidence indicates mortality among AS heterozygotes
associated with the infection. Malaria is a primary source of early child
mortality where it is present, so complete resistance would be expected to
greatly increase survivorship for AS heterozygotes. However, survivorship
curves between AA and AS indicate a relatively moderate difference.
Additionally, the selective advantage for the AS genotype calculated using the
Hardy-Weinberg equilibrium suggests a moderate advantage, inconsistent
with complete resistance to a disease that is often lethal.
CHALLENGING PROBLEMS
Unpacking the Problem
12.
If the mutation rate to a new allele is 1o-s, how large must isolated
populations be to prevent chance differentiation among them in the frequency
ofthis allele?
Answer: A population will not differentiate from other populations by local
inbreeding if
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