150 ❯ STEP 4. Review the Knowledge You Need to Score High
- Why do we assume that oxygen was not present
in the original atmosphere?
A. The presence of O 2 would have resulted in
the evolution of too many species too fast.
B. Oxygen would have slowed down the rate of
evolution.
C. We know the ozone layer, which is formed
by oxygen, has not been around that long.
D. Inorganic molecules could not have formed
in the presence of oxygen.
E. All the oxygen was held in the volcanoes.
10.All these are examples of random evolutionary
processesexcept:
A. An earthquake divides a single elk species
into two populations, forcing them to no
longer interbreed.
B. A mutation in a flower plant results in a new
variety.
C. An especially long winter causes a group of
migrating birds to shift their home range.
D. A mutation results in a population of trees
that spread their seeds more widely than
their peers, causing their population to
grow.
E. A spider species declines in an area because
individuals are consistently moving out of
an old range and into a new range.- B—Natural selection is the selective increase in
certain alleles because they confer an advantage to
their host organism. All other factors are random
with respect to the alleles (a “bottleneck” is a type
of genetic drift where a population is drastically
reduced in size). - A—All fruit flies need to fly not only to find
mates but also to survive. All the other characters
listed are sexually selected, meaning that they
have evolved because they confer specific advan-
tages in mating (and not survival). - D—Adaptations are defined as traits that affect
fitness if they are altered. Although adaptations
may have evolved to increase mating success
(answer C), they are not always intended for that
function (e.g., they may have remained because
they increase survival). - C—Natural selection can occur in asexually repro-
ducing organisms, as long as the other three neces-
sary conditions are met. “Survival of the fittest”
(answer E) is another way of saying that certain
organisms have higher reproductive success than
others. - C—Genetic drift is change in allele frequencies as
a result of random factors (e.g., natural disasters
or environmental change). In small populations,
genetic drift is a much more powerful force
because each individual represents a greater per-
centage of the population’s total genes than that
person would in a much larger population. Think
of it this way—if you have a population of
10 cheetahs, and 3 die, you have lost 30 percent
of the genes in that pool. If you have a population
of 100 cheetahs, and 3 die, you have lost only
3 percent. Since Hardy-Weinberg equilibrium
depends on no genetic drift, it is much more
likely to occur in very large populations.- C—Remember that pandqmust add up to 1 for
a population to be in Hardy-Weinberg equilib-
rium (this eliminates answers D and E ). Calculate
qfirst by taking the square root of 0.09, which is
0.30. Then simply subtract 0.30 from 1 to get
p=0.70. - B—Frequency-dependent selection is one process
by which multiple alleles are preserved in a popu-
lation. For traits that are selected for or against on
the basis of frequency, an allele becomes more
advantageous when it is rare, and therefore
increases. In this way, it is impossible for the allele
to become extinct (because as soon as it gets that
low, it increases again). When it gets too high, the
other allele is low, and that one then increases.
Frequency-dependent selection often exhibits
itself in this kind of seesaw effect.