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Answers to Thinking Critically Questions
Mader: Inquiry into Life, Twelfth Edition
Chapter 27
1. Viruses such as HIV are rapidly replicated and have very high mutation rates;
thus, evolution of the virus can be observed in a single infected person. Using
what you have learned in this chapter, explain why HIV is so hard to treat, even
though multiple drugs to treat HIV have been developed.
Explanation/Answer: Using drugs against HIV “artificially selects” for resistant
strains. Because the mutation rate is so high, resistant strains come about, and
then increase in the viral population within a host in the presence of drugs. That
is, strains that are resistant survive and reproduce more than those strains that are
susceptible. The frequency of resistant virus strains increases, and eventually, the
population of HIV virus molecules within a host are resistant to the drugs that are
being used.
2. If the conditions for the Hardy-Weinberg principle are rarely, if ever, met in
nature, why is it such an important idea?
Explanation/Answer: Although it is unrealistic to assume that populations do not
experience any mutations, gene flow, genetic drift, or selection, and that they
mate randomly, the Hardy-Weinberg principle provides a baseline for detecting
evolution in populations. That is, deviations from expected allele frequencies
under Hardy-Weinberg equilibrium suggest that evolution has occurred in a
population.
3. You observe a wasting disease in cattle that you know is genetically caused and
thus heritable. The disease is fatal in young cattle. The allele frequency for the
gene that causes the disease is 0.05 in the United States, but it is 0.35 in South
America. Explain why such a difference in allele frequencies might exist?
Explanation/Answer: This disease is likely caused by a homozygous recessive
allele. Since it kills young cattle, they do not have the ability to reproduce and
thus the allele should decrease in frequency in a population. However, in this
case, heterozygotes probably have a less severe form of the disease or are
unaffected; otherwise the allele would be eliminated completely from a
population. To explain the difference in allele frequency between the US and
South America, it is likely that the heterozygote form may provide some
advantage in South America, such as resistance to a disease not found in the US.
Similar to sickle cell disease and malaria, perhaps the advantage to a heterozygote
in South America is sufficient enough to maintain the harmful allele at relatively
high frequency in the population.
4. Why are homologous structures, as opposed to analogous ones, used to
determine the evolutionary relationships of species and to reconstruct
phylogenies?
Explanation/Answer: Homologous structures are those that arose due to
common descent. Because phylogenies reconstruct the relationships among
species based on ancestry, homologous structures are informative. Homologous
structures tend to be similar in structure (e.g., the bones of mammal limbs)
because they arose in a common ancestor, but different in function because they
diverged in evolutionary time (e.g., human arm versus bat wing versus whale fin).
Analogous structures are those that look similar, but did not arise from a common
ancestor. A similar environment selects for similar structures. For example,
wings of birds and insects are shaped similarly not because those organisms arose
from a common ancestor, but rather because a certain wing shape is efficient for
flying. Thus, these similarities arose due to adaptation to a common environment,
not common ancestry. Therefore, they are not useful for reconstructing
evolutionary relationships among organisms.
5. Why do scientists continue to devise experimental systems that mimic the
conditions of the early Earth, despite the fact that it is difficult to do and there is
no way to know for sure what the conditions of the Earth were billions of years
ago? Are you aware of any such experiments and their results?
Explanation/Answer: Scientists are interested in re-creating the Earth’s
environmental conditions billions of years ago to understand the origin of life. At
some point, life came from non-life, and scientists are trying to establish how this
occurred billions of years ago. The Miller-Urey experiment recreated the early
Earth’s environment (about 4.5 billion years ago) and showed that basic building
blocks of proteins (amino acids) and small organic molecules could be formed in
a closed system over a week. This was a major breakthrough in determining how
the origin of life may have occurred. Sidney Fox later showed that amino acids
could polymerize to form proteins when exposed to dry heat.