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Answers to Study Questions
Chapter 1: Animals and Environments: Function on the Ecological Stage
1. There is a chance that a calcium atom or carbon atom that was once part of Caesar’s or
Cleopatra’s body is now part of your body. Part of the reason is that most calcium and
carbon atoms that were parts of these rulers’ bodies did not go to their graves with them.
Explain both statements. (If you enjoy quantifying processes, also see Question 11.)
Answer: Animals exchange atoms with their environments throughout their lives. Thus,
over the decades of Caesar’s life, numerous atoms that were part of his body had returned
to the environment. Because of this process, the number of atoms in the environment that
were once part of Caesar’s body far exceed the number in his body at his death, raising
the likelihood that some atoms that were once his are still among us.
2. Animals do not keep all their detoxification enzymes in a constant state of readiness.
Thus they depend on phenotypic plasticity to adapt to changing hazards. An example is
provided by the enzyme alcohol dehydrogenase, which breaks down ethyl alcohol.
People who do not drink alcoholic beverages have little alcohol dehydrogenase.
Expression of the enzyme increases when people drink alcohol, but full expression
requires many days, meaning that people are incompletely defended against alcohol’s
effects when they first start drinking after a period of not drinking. Consider, also, that
muscles atrophy when not used, rather than being maintained always in a fully developed
state. Propose reasons why animals depend on phenotypic plasticity, instead of
maintaining all their systems in a maximum state of readiness at all times.
Answer: Maintaining proteins costs energy; thus, unused capacities are not cost-free.
Protein molecules occupy space in cells, and dissolved proteins add to the total solute
concentrations of intracellular fluids. In terms of hypotheses we can offer to explain the
phenomena described, failure to maintain proteins at all times may be a means of (i)
avoiding unnecessary energy costs or (ii) keeping cell contents or concentrations below
acceptable maxima.
3. Whereas the larvae of a particular species of marine crab are bright orange, the adults
of the species are white. An expert on the crabs was asked, “Why are the two different
life stages different in color?” She replied, “The larvae accumulate orange-colored
carotenoid pigments, but the adults do not.” Did she recognize all the significant
meanings in the question asked? Explain.
Answer: No. While recognizing the question of mechanism, she did not recognize the
question of origins: Why did larvae evolve orange coloration? Do larvae gain an
advantage by being orange? Do adults benefit by being white?
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4. Referring to Figure 1.11, do plains zebras, warthogs, and greater kudus have normal or
exceptional gestation lengths? Justify your position in each case.
Answer: The line in Figure 1.9 shows the statistically expected gestation length at each
body weight. Of course, to compare any particular species with what is expected for
animals of its weight, statistical tests would need to be carried out. Based on visual
inspection, however, the zebras have exceptionally long gestation lengths, because they
plot far above the line of expected gestation length. Compared to the length that is
statistically expected for their body weight, warthogs have a far shorter gestation. Greater
kudus plot on the line of expectation and therefore have a normal, or expected, gestation
length.
5. At least three hemoglobin alleles in human populations alter hemoglobin structure in
such a way as to impair the transport of O2 by the blood but enhance resistance of red
blood cells to parasitization by malaria parasites. Explain how such alleles exemplify
pleiotropy, and discuss whether such alleles could lead to nonadaptive evolution of blood
O2 transport in certain situations.
Answer: Each allele of this sort causes an individual to exhibit two distinct, seemingly
unrelated traits: impaired O2 transport and improved malaria resistance. In an
environment where people are likely to get malaria, these alleles could be favored by
natural selection during evolution because of their benefits for malaria resistance. High
frequencies of the alleles would lead to high frequencies of impaired O2 transport.
6. What are some of the microclimates that a mouse might find in your professor’s home?
Answer: The microclimate in the space inside one of my professor’s shoes will likely be
more humid than elsewhere in the house. If my professor opens windows, the surfaces
just inside the windows will experience more wind than most parts of the house. If a
secluded part of the attic is occupied by several families of mice, the CO2 concentration
in the air there might be especially high.
7. Figure 1.16 seems at first to be simply a description of the physical and chemical
properties of a lake. Outline how living organisms participate in determining the physical
and chemical (i.e., temperature and O2) patterns. Consider organisms living both in the
lake and on the land surrounding the lake. Consider also a research report that shows that
dense populations of algae sometimes change the temperature structure of lakes by
raising the thermocline and thereby increasing the thickness of the deep, cold layer; how
could algal populations do this, and what could be the consequences for deep-water
animals?
Answer: Living organisms often affect the pattern of O2 concentration in a lake. For
example, if there are lots of fish in a lake, the fish use O2 at a high rate and may lower O2
concentration at all depths. If organisms living around the lake, such as trees along the
shores, add lots of organic matter to the lake (e.g., falling leaves), bacteria using the
organic matter as food may tend to deplete O2. In the case of high algal growth in the
lake, the high concentration of algal cells may block penetration of sunlight, meaning the
sun does not warm the water at as great depths as usual, raising the thermocline.
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8. Do you agree with François Jacob that evolution is more like tinkering than
engineering? Explain.
Answer: Yes. In air-breathing fish there is no rhyme or reason to the location where the
air-breathing organ evolved. Sometimes the gill cavity is the site of air-breathing,
sometimes it’s the intestines, etc. The fish seem to have improvised like a tinkerer would.
Also, why would there be 30 different molecules that act as luciferins during
bioluminescence if evolution occurred as though an engineer were in charge?
9. Explain how the comparative method, knockout animals, and geographical patterns of
gene frequencies might be used to assess whether a trait is adaptive. As much as possible,
mention pros and cons of each approach.
Answer: In the comparative method, you look – for example – to see if all animals
evolved in deserts have similar characteristics beneficial to desert life. A problem with
this method is that there are no absolute rules regarding which animal species should or
should not be included in a comparison. In using knockout animals, you engineer animals
to lack functional copies of a gene and look at how they are inferior by comparison to
normal animals. A problem with this method is that physiological systems other than
those controlled by the gene may cover up for deficiencies. In using geographical
patterns, if you find, for example, that the frequency of an allele varies systemically from
low to high latitude, you will have good reason to believe that the allele helps with some
challenge that varies with latitude. Determining which challenge, however, may be
difficult.
10. Certain species of animals tolerate body temperatures of 50°C, but the vast majority
do not. Some species can go through their life cycles at very high altitudes, but most
cannot. What are the potential reasons that certain exceptional species have evolved to
live in environments that are so physically or chemically extreme as to be lethal for most
animals? How could you test some of the ideas you propose?
Answer: If a species can evolve to live where most organisms cannot, it may avoid
predation or disease, because predators or pathogens cannot live there. Another
hypothesis is that if a species can evolve to live where most organisms cannot live, it may
be able to get most of the food there. To test this last idea, if a herbivore species lives
over a range of habitats that are non-extreme and extreme, you could carry out
measurements to see if the percentage of vegetable matter it eats is greater in the extreme
parts of its range that in the non-extreme parts where competitors are better able to live.
11. Using the set of data that follows, calculate how many of the molecules of O2 that
were used in aerobic catabolism by Julius Caesar are in each liter of atmospheric air
today. All values given are expressed at Standard Conditions of Temperature and
Pressure (see Appendix C) and therefore can be legitimately compared. Average rate of
O2 consumption of a human male during ordinary daily activities: 25 L/h. Number of
years after his birth when Caesar was mortally stabbed near the Roman Forum: 56 years.
Number of liters of O2 per mole: 22.4 L/mol. Number of moles of O2 in Earth’s
atmosphere: 3.7  1019 mol. Number of molecules per mole: 6  1023 molecules/mol.
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Amount of O2 per liter of air at sea level (20°C): 195 mL/L. Be prepared to be surprised!
Of course, criticize the calculations if you feel they deserve criticism.
Answer: First estimate the total liters of O2 Caesar consumed in his lifetime:
25 L O2/h  24 h/day  365 day/year  56 years = 12,264,000 L O2
Then calculate the moles of O2 he used in his lifetime:
12,264,000 L  mol/22.4 L = 547,500 mol
Then calculate the fraction of atmospheric O2 that Caesar processed:
547,500 mol/3.7  1019 mol = 1.5  10‾14
If a liter of air contains 0.195 L O2, then it contains 0.195/22.4 mol O2 = 0.00871 mol O2.
Accordingly, it contains 0.00871  (6  1023) = 5.2  1021 molecules of O2.
If each liter of air contains 5.2  1021 molecules of O2, and if Caesar processed 1.5  10‾14
of those, then the liter contains 7.8  107 molecules that were in Caesar’s cells and
participated in his metabolism.
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