Unit 5 Review
Answer Key
1. Define the following terms:
a. Ernst Mayr was one of the twentieth century's leading evolutionary biologists.
He was also a renowned taxonomist, tropical explorer, ornithologist, and historian
of science. His work contributed to the conceptual revolution that led to the
modern evolutionary synthesis of Mendelian genetics, systematics, and
Darwinian evolution, and to the development of the biological species concept.
b. Biological species concept is a group of similar individuals capable of
interbreeding.
c. Allopatric speciation occurs due to geographic isolation between populations.
d. Sympatric speciation occurs when populations are not separated by
geographic isolation and is thought to rarely occur in nature.
e. Gradualism is Darwin/Wallace’s explanation for evolution in which speciation
occurs in small incremental steps with a cumulative effect over long periods of
time.
f. Punctuated equilibrium is an evolutionary theory that explains speciation
through rapid changes with long periods of time in which little or no change
occurs.
g. Prezygotic isolating mechanism is reproductive isolation that occurs prior to
fertilization of an egg.
h. Postzygotic isolating mechanism is reproductive isolation that occurs after
fertilization of an egg.
Use the following information to answer questions 2 – 5: A virus wipes out all the
waterfowl in a New Zealand watershed and temporarily removes the opportunity for the
trematode Microphallus parasite living in ponds in this area to reproduce sexually and
complete its life cycle. In response, a few Microphallus genotypes produce a mutation
that allows them to mature in their snail host and grow asexually for repeated
generations. Many generations later, you discover this population and become
interested in comparing the asexual population and a nearby population of Microphallus
with alternating sexual and asexual generations. Consider the predictions you might
make about the relative fitness of the asexual population compared to the population
with alternating generations.
Saylor URL: www.saylor.org/bio102 Unit 5
The Saylor Foundation
Saylor.org
Page 1 of 3
2. Why might you expect the population with sexual reproduction to have higher fitness
in the long run? Sex introduces recombination and recombination increases genetic
variation and improves fitness through various processes: -Removal of deleterious
mutations from the population -Increased parasite and pathogen resistance-Increased
genetic variation increases probability of solutions to future problems.
3. What conditions might favor the asexual population? Small population size for both
the sexual and asexual populations. The benefit of sex would be reduced by increased
influence of stochastic effects in the sexual populations. In addition, the chances of
encountering the opposite sex might be reduced due to low population density. The
availability of the snail host as a limiting resource would also favor the faster
reproducing asexual form.
4. You bring the individuals from the two populations into the lab, collect gametes, and
succeed in producing hybrid zygotes. However, very few of the fertilized eggs develop
into viable adults. Describe a genetic mechanism potentially responsible for this result.
Accumulation of fixed differences in regions where recombination has been blocked by
extensive sequence divergence has resulted in reproductive isolation and incipient
speciation.
5 Name one other mechanism that might prevent formation of hybrids if the two
populations were to come back into contact in the wild. Various answers are
acceptable, but include:
-Mate recognition
-Physical changes to sex organs
6. For each of the fictional examples given below, name the type of isolation mechanism
that is operating (be specific—don’t just say “pre-” or “post-zygotic”).
a. Western deer mice are unable to interact or mate with eastern deer mice,
because the Mississippi River divides them. Habitat
b. Brain corals and fan corals both release gametes (sperm and eggs) into the
water at night, but the two do not come into contact, because brain corals release
gametes when the moon is full and fan corals when the moon is dark. Temporal
c. Lollipop shrimp males would try to mate with candy cane shrimp females, but
the lollipop males’ claws are not the right shape to hold the candy cane females
during courtship/mating. Mechanical
D. Bullfrog males will try to mate with just about anything, including rocks (this is
true). But when they fertilize the eggs of tree frog females, the developing
tadpoles die before they reach adulthood. Hybrid inviability (or zygotic mortality)
Saylor URL: www.saylor.org/bio102 Unit 5
The Saylor Foundation
Saylor.org
Page 2 of 3
7. Label each of the following examples as “Prezygotic Isolating Mechanism” or
“Postzygotic Isolating Mechanism.”
a. Zygote mortality: Postzygotic Isolating Mechanism
b. Temporal: Prezygotic Isolating Mechanism
c. Hybrid inviability: Postzygotic Isolating Mechanism
d. Hybrid breakdown: Postzygotic Isolating Mechanism
e. Gametic isolation: Prezygotic Isolating Mechanism
f. Habitat: Prezygotic Isolating Mechanism
g. Behavioral: Prezygotic Isolating Mechanism
h. Hybrid sterility: Postzygotic Isolating Mechanism
i. Mechanical: Prezygotic Isolating Mechanism
8. Match the reproductive isolating mechanism with its definition.
ISOLATING
MECHANISM
a. Habitat/Geographical
b. Temporal
c. Behavioral
d. Mechanical
e. Gametic
f. Zygote mortality
g. Hybrid inviability
h. Hybrid sterility
i. Hybrid breakdown
DEFINITION
Differences in sex organs
embryo forms but viability is reduced
Behavior does not elicit mating
fertilization occurs zygote does not develop
hybrid offspring are sterile
Mate at different times
Occupy different geographic areas
first generation infertile but subsequent
generations are sterile
failure of sperm to meet egg
ANSWER
d
g
c
f
h
b
a
i
e
Saylor URL: www.saylor.org/bio102 Unit 5
The Saylor Foundation
Saylor.org
Page 3 of 3