Download Answers

Lecture I. Practice Question Answers.
1. (4 pts) Questions on energy.
a. Why is it necessary for organisms to obtain energy?
Energy necessary for biological activity:
growth, maintenance, reproduction, movement, etc.; more generally to locally reverse
2nd Law of Thermodynamics.
b. How do green plants obtain energy?
From the sun via photosynthesis.
c. How do animals obtain energy?
By eating plants or other animals (living or
dead).
d. What is the ultimate source of energy for all organisms?
Conventional answer is “the sun”, i.e., photosynthesis at the base of the food chain. However, as we will later discuss, many microorganisms extract energy from inorganic molecules. The atoms (save hydrogen) of which
these (and all other molecules) are composed
1
were produced via nucleosynthesis in ancient
stars that later exploded.
2. (2 pts) The figure below shows some of the levels
in the biological hierarchy discussed in introductory texts. List four additional levels that fit between genes and populations.
Organelles, cells, tissues, organs, individuals,
family groups. The lowest level, “genes,” would
be better labeled “macromolecules.”
2
3. (1½ pts) Disorder in the Department of Entropy.
List three things you could do to reduce entropy
in the office. What would each reduction require?
Repair the floor; re-hang the door, put the trash
back in the wastepaper basket, etc. All require
expenditure of energy.
3
4. (8 pts) Consider gas molecules in a box divided into
two equal sized compartments by a permeable membrane. Let 𝑝1 be the fraction
of molecules in compartment
1, and 𝑝2 , the fraction in
compartment 2. The entropy, 𝐸, of the system is
𝐸=
𝑝1 log 𝑝1 +𝑝2 log 𝑝2
−
log 2
(1)
where log 𝑝1 is the common (base 10) logarithm of 𝑝1,
etc. Note that by definition,
𝑝1 + 𝑝2 = 1.
(2)
a. What are the values of p1 and p2 in the figure?
p1 = 5/15= .333… ; p2 = 10/15 = .666… .
4
b. Using Equation (1), compute E for p1 = .01,
.10, .25, .5, .75, .9, and .99.
p1
.01
.10
.25
.50
E
.0808
.4690
.8113
1.000
p1
.75
.90
.99
E
.8113
.4690
.0808
c. For what value(s) of p1 on [0, 1] is E minimal?
E(0) = E(1) = 0. This one is tricky. Calculus
students can use L’Hôpital’s rule to evaluate
𝐥𝐢𝐦 𝒑 𝒍𝒐𝒈 𝒑 and 𝐥𝐢𝐦(𝟏 − 𝒑)𝒍𝒐𝒈(𝟏 − 𝒑). Non𝒑→𝟎
𝒑→𝟏
calculus students can calculate E(p) for multiple p values successively closer to 0 and 1,
thereby
convincing
themselves
that
𝐥𝐢𝐦 𝒑 𝒍𝒐𝒈 𝒑 = 𝐥𝐢𝐦(𝟏 − 𝒑)𝒍𝒐𝒈(𝟏 − 𝒑) = 𝟎.
𝒑→𝟎
𝒑→𝟏
d. For what value(s) of p1 on [0, 1] is E maximal?
E(0.5) = 1.0
5
5. (4 pts) Hoyle argued that the spontaneous origin
of life, essentially because the time required to
create biological complexity by random mutation
vastly exceeds the time that was available. How
might an evolutionary biologist respond?
Evolution proceeds in small steps, each of which
is accepted or rejected by selection when it first
arises. Such a process proceeds much faster
than if selection is deferred until all steps have
been taken. For a whimsical analogy, go to
http://www.blc.arizona.edu/courses/schaffer/182/george.htm.
6. (4 pts) Crudely, one can think of cells as “bags”
of biological molecules that permit controlled molecular exchange with their environment. From
the perspective of the Second Law, why should
biological molecules be so packaged?
According to the 2nd Law, chemicals diffuse
down concentration gradients. Thus, a local
concentration of biological molecules would be
diluted (the biological molecules would diffuse
out; water would diffuse in) absent a “bag” to
separate it from the surrounding environment.
6
7. (4 pts) Why did Schrödinger imagine the information-bearing crystal to
be aperiodic, i.e., why
wouldn’t a “periodic” crystal such as NaCl (right)
have sufficed?
Table salt (NaCl) is
a “periodic” crystal.
A periodic crystal can only code for a limited amount of information;
likewise a periodic DNA molecule.
7
8. (4 pts) Regarding cells and viruses: a. If cells
from cells, whence cometh the first cell? Give two
possibilities. b. Viruses lack most of the structure
of even the simplest cells. Whence cometh viruses? Give two possibilities.
a. 1. Originated de novo on earth after a
period of chemical evolution.
2. Transported to earth by comets, meteorites, etc. (panspermia).
b. 1. Descended from pre-cellular (more primitive than a bacterium) life forms.
2. Degenerate descendants of simple cells or
their genetic material.
8
9. (4 pts) Regarding the evolution of multicellular
organisms: a. In unicellular organisms, the cell is
the organism; in multicellular organisms, cells are
building blocks. How might the first multicellular
organisms have evolved? b. If complete genomes
are replicated when cells divide, how can one account for different cell types in multi-cellular organisms?
a. Products of mitosis that failed to separate; alternatively compartmentalization
of large cells with many nuclei.
b. Different genes turned on or off during
development.
9
10. (8 pts) Darwin and Alfred
Wallace, the often neglected co-discoverer of
natural selection, both
imagined phyletic descent as a branching
tree. By this, it is meant
that any two living species have a unique common ancestor, and that,
by extension, there is a
universal common ancestor of all living species.
Famous
a. Does decent with
modification necessitate a tree-like pattern
of relatedness? b. Why
or why not?
'branching tree”
sketch from Notebook B
(1837) was Darwin’s first attempt at illustrating his theory. From Shu, 2005. Guidance to Reading 'On the
Origin of Species'. Peking
University Press.
a. By definition, Descent with Modification
necessitates change, the “modification
part. It does not, however, necessitate lineage splitting, i.e.., speciation.
10
b. Tree of Life assumes that that lineages
split but do not fuse. This is often the case.
But there are exceptions. For example, as
we will later discuss, new species can result from hybridization. Likewise, viruses
can move genes from one species to another, and at least some of the organelles
in eukaryotic cells are descended from
formerly free-living prokaryotes. Finally,
as we also will later discus, the entire domain Eukarya may be the result of symbiogenesis.
11. (8 pts) Redraw Figure 20
under the assumption
that the environment is
never limiting, i.e., that
reproduction and survival
rates do not change with
increasing
population
size. Do superior descendant varieties still
exterminate their ancestors?
If rates of reproduc11
tion and survival remain constant, both mutant
and wild type grow exponentially. Therefore the
mutant does not exterminate the wild type. On
the other hand, wild type frequency goes to zero, as 𝒕 → ∞.
12. (8 pts) Regarding altruistic behavior, i.e., acting
in a way that benefits another individual’s fitness
at cost to your own: a. Why is this a problem for
the theory of evolution by natural selection? b.
Give three reasons why altruistic behavior might
nevertheless evolve.
a. Everything else being equal, selection
will remove genes that reduce individual
fitness.
b. 1. Kin selection. Beneficiaries related to
altruists. By increasing the fitness of
kin, altruists promote the survival of
their own genes.
2. Group selection. Altruistic behavior
can enhance the probability of group
12
survival, i.e., groups with altruists may
be more likely to survive than groups
without, may compete less effectively In
such situations, however, selection
within the group will favor “cheaters”
who benefit from the altruism of others,
but do not themselves contribute.
3. Reciprocal altruism – I scratch your
back; you scratch mine. Works in social
species with individual recognition.
4. Societal rules of behavior, laws, etc. especially in man – see May (2009) for
discussion.
5. Altruism may be only apparent, i.e.,
apparently altruistic acts, for example,
helping parents at the nest instead of
making your own in scrub jays, may allow young individuals to acquire parental skills that will increase reproductive
success in future years.
13
13. (8 pts) As an example of unexpected relationships consequent to ecological complexity, Darwin (1859, pp. 73-74) argued that village cats
can promote the abundance of red clover as
shown below.
Can you think of another example of complex ecological interactions that produce unexpected and/or important effects?
14
Two examples:
a. “Keystone predators” promote prey species
diversity by preventing one or more from
out-competing the rest.
Intertidal community studied by Robert Paine. The starfish,
Pisaster, is the keystone predator. Without it, competition reduces prey species diversity i.e., the number of coexisting
species declines.
15
b. Sea otters promote fish availability.
Otter-urchin-kelp ecosystem. Protecting sea otters indirectly promotes the growth of kelp forests because
otters eat sea urchins that graze kelp. Kelp forests also promote fish larva survival, and therefore indirectly
make more fish available for species that eat fish. Unfortunately for fisherman, otters eat fish too.
16
14. (2 pts) What do you think Darwin and Wallace
might each have had to say about the “little yappers” (below) that infest retirement communities?
“Real” dogs (left) and “rodent” dogs (right), both shown in their
native habitats, share a common wolf-like ancestor.
Darwin would have emphasized the ability of
breeder’s to produce dogs (via AS) that deviated from their presumptive wolf-like ancestor. Wallace would have emphasized that
“rodent dogs” are unfit. They only survive
because humans provide food, shelter, etc.
17
15. (4 pts) Regarding William Paley. a. Who was he?
b. What was his iconic example? c. In what way
does the existence of vestigial structures such as
the human coccyx, undercut Paley’s argument?
a. Wm. Paley (1743-1805) was an English
minister and philosopher best known for
his use of apparent design in nature as
evidence for the existence of God.
b. His iconic example was a watch. In Natural Theology (1809), he wrote as follows:
“IN crossing a heath, suppose I pitched my foot
against a stone, and were asked how the stone came
to be there; I might possibly answer, that, for any
thing I knew to the contrary, it had lain there for ever: … But suppose I had found a watch upon the
ground, and it should be inquired how the watch
happened to be in that place; I should hardly think of
the answer which I had before given … . [And] For
this reason, … that, when we come to inspect the
watch, we perceive (what we could not discover in
the stone) that its several parts are framed and put
together for a purpose … .
18
… the inference, we think, is inevitable, that the
watch must have had a maker: that there must have
existed, at some time, and at some place or other, an
artificer or artificers who formed it for the purpose
which we find it actually to answer; who comprehended its construction, and designed its use.”
c. To the extent that vestigial structures
serve no purpose, the argument of Design fails.
16. Male bighorn sheep contest with each other for mating
privileges. Discuss the factors selecting for and against
large horn size in this species.
Favoring the evolution of large horn size is
the fact that dominant males sire a disproportionate fraction of offspring.
Opposing the evolution of large is the energetic cost of growing the horns and to the
extent that horn growth and aggression are
correlated, (both likely consequences of
high testosterone), increased risk of injury.
19