AP Biology
Problem Set 21: Microevolution (Chapters 22-23)
due Tuesday, April 7, 2009
Instructions: There are a total of 4 required problems on this problem set. For Chapter 23, there
are also some optional problems. You should understand the optional problems; you may write
up your answers for extra credit ONLY if you turn in your problem set on time.
Chapter 22 – Choose 1 (but understand both)
1. Explain the Darwinian theory of natural selection and give an example of how this process
works. Be sure to include all key ideas that are part of this process (note, the idea about
mutation mentioned on your natural selection chart was NOT one of Darwin’s contributions,
because he didn’t know about genes/DNA). If you need ideas for examples, see the
following videos (they are about 5 minutes each)
Hummingbirds: http://www.pbs.org/wgbh/evolution/library/11/2/e_s_4.html
TB and antibiotic resistance:
http://www.pbs.org/wgbh/evolution/library/10/4/l_104_09.html
2. Four of Darwin’s contributions to evolutionary biology are:
 The non-constancy (variation) of species
 Branching evolution, which implies the common descent of all species
 Occurrence of gradual change in species
 Natural selection as the mechanism for evolution
For each of these concepts, give an example of supporting evidence. You may use the same
example or evidence for more than one idea, or you may use a different example for each; either
way, you must clearly explain how the evidence supports each idea.
Chapter 23 – All 3 are Required.
3. Describe the concept of Hardy-Weinberg equilibrium.
a) Explain what H-W equilibrium means.
b) Explain the equations used to calculate allele and genotype frequencies in a population.
c) What conditions must be met for a population to be in HW equilibrium?
d) Do you think all of these conditions are ever met in natural populations? If not, why is
the H-W framework useful?
4. Cystic fibrosis is a recessive genetic disease that affects about 1 in 2,500 babies in the
Caucasian population in the United States.
a) Calculate the frequency of the recessive allele in the population.
b) Calculate the frequency of the dominant allele in the population.
c) Calculate the percentage of heterozygous individuals (carriers) in the population.
d) Would you expect the population to be at Hardy-Weinberg equilibrium for the CF
gene? Why or why not?
5. Define and give examples of:
a)
b)
c)
d)
e)
f)
g)
directional selection
disruptive selection
stabilizing selection
intrasexual selection
intersexual selection
genetic drift
founder effect and population bottleneck
Chapter 23 – Optional (3 points each, extra credit)
6. The present-day Amish population in eastern Pennsylvania grew from a group of about 200
German immigrants who came to Pennsylvania in the 18th century for religious freedom.
Traditionally, the Amish do not marry outside of their own communities. This community
today has an unexpectedly high frequency of a recessive allele for Ellis-van Creveld
syndrome, a rare form of dwarfism (the frequency of this allele is 0.07 in the Amish
population compared to 0.001 in most populations.) This allele has been traced back to one
couple among the original founders – either Samuel King or his wife (they immigrated in
1744) was a carrier for this condition.
a) What’s going on here? What evolutionary force(s) are at work? What forces are not
at work?
b) Can you think of any other examples from human history in which populations would
have undergone an extreme form of genetic drift, such as a founder effect or
population bottleneck?
c) Explain why genetic drift has a greater effect on small populations.
7. Darwin said that evolution by natural selection favors individuals (or alleles) with the highest
fitness. What does “fitness” mean in this sense? What are the key components of fitness?
Give an example.
8. Explain how sexual selection can favor traits that actually decrease an individual’s likelihood
of survival. Give examples.
One more Hardy-Weinberg problem for extra credit
9. A certain species of cat is either completely black (recessive phenotype) or is black with
yellow polka dots (dominant phenotype; assuming we are dealing with alleles of a single
gene). The frequency of all-black cats in each of two different populations (A and B) is 0.25.
a) If colony A is infected by a lethal virus that only kills polka dotted individuals, what
will be the frequency of the black allele in the next generation?
b) If colony B is infected by a virus that only kills plain black cats, what will be the
frequency of black cats in the next generation? (Assume that the virus is gone by the
next generation.)