Download 1. Prezygotic isolating mechanisms include all of the following except

1. Prezygotic isolating mechanisms include all of the following except—
a. hybrid sterility
b. courtship rituals
c. habitat separation
d. seasonal reproduction
The correct answer is a—hybrid sterility.
A. Answer a is correct. Hybrid sterility is what results when individuals from two
different species are able to mate but their offspring is infertile, meaning that their
offspring are not able to reproduce. Because this occurs after the formation of the
zygote, this is a postzygotic isolating mechanism, not a prezygotic mechanism.
The correct answer is a—
B. Answer b is incorrect. Courtship rituals are mechanisms of prezygotic isolation
called behavioral isolation. Individuals of different species will often display
different courtship mating rituals that are identified by individuals of their same
species but are not recognized by individuals of other species. By establishing
different rituals, species are able to recognize individuals of their own species and
not expend energy mating with individuals of other species.
The correct answer is a—
C. Answer c is incorrect. Habitat separation, also called ecological isolation, is a
mechanism of prezygotic isolation. If individuals of two different species are
located in different areas, or occupy different niches, they will rarely come into
contact with each other and so will not have the opportunities to mate, thereby
preserving their species identity.
The correct answer is a—
D. Answer d is incorrect. Seasonal reproduction, also called temporal isolation, is a
mechanism of prezygotic isolation. If individuals of two different species mate at
different times of the year (that is, one species mates in spring and the other in
summer), they will rarely mate because they are not ready to mate at the same
time.
Hint: How do species keep their separate identities? Reproductive isolating
mechanisms fall into two categories: prezygotic isolating mechanisms, which
prevent the formation of zygotes; and postzygotic isolating mechanisms, which
prevent the proper functioning of zygotes after they form, including fertility of
offspring.
2. Reproductive isolation—
a. is a result of individuals not mating with each other
b. is a specific type of postzygotic isolating mechanism
c. is required by the biological species concept
d. none of the above
The correct answer is c—
A. Answer a is incorrect. Behavior differences are just one type of reproductive
isolation.
The correct answer is c—
B. Answer b is incorrect. Postzygotic isolating mechanisms are specific kinds of
reproductive isolation, not the other way around.
The correct answer is c—is required by the biological species concept
C. Answer c is correct. Without reproductive isolation (brought about by any
effective mechanism) populations would exchange genes and not be identified as
individual species.
The correct answer is c—
D. Answer d is incorrect. One of the answers is correct, answer c.
3. Leopard frogs from different geographic populations of the Rana pipiens
complex—
a. are members of a single species because they look very similar to on another
b. are different species shown to have pre- and postzygotic isolating mechanisms
c. frequently interbreed to produce viable hybrids
d. are genetically identical due to effective reproductive isolation
The correct answer is b—
A. Answer a is incorrect. Species are not defined merely by external characteristics.
The correct answer is b—are different species shown to have pre- and postzygotic
isolating mechanisms
B. Answer b is correct. The isolating mechanisms prevent genetic exchange and
continuity that is an important characteristic of species.
The correct answer is b—
C. Answer c is incorrect. Isolating mechanism prevent the production of viable
hybrids in the wild and in the lab.
The correct answer is b—
D. Answer d is incorrect. Reproductive isolation enhances genetic differences, not
similarity.
4. __________________ isolating mechanisms include improper development of
hybrids and failure of hybrids to become established in nature.
a. Prezygotic
b. Postzygotic
c. Temporal
d. Mechanical
The correct answer is b—
A. Answer a is incorrect. Prezygotic isolating mechanisms are mechanisms that
occur prior to the formation of the zygote. They include mechanisms such as
ecological isolation, behavioral isolation, temporal isolation, mechanical isolation,
and prevention of gamete fusion.
The correct answer is b—Postzygotic
B. Answer b is correct. Postzygotic isolating mechanisms are mechanisms that occur
after the formation of the zygote but prevent viable embryos or, if hybrids are
born, prevent the viability or the reproductive abilities of the hybrid.
The correct answer is b—
C. Answer c is incorrect. Temporal isolating mechanisms are prezygotic mechanisms
that restrict mating between different species. Temporal isolation occurs when
species have different breeding seasons and so they don't produce hybrids because
they are not mating at the same time.
The correct answer is b—
D. Answer d is incorrect. Mechanical isolation mechanisms are prezygotic
mechanisms that restrict mating between different species. Mechanical isolation
occurs when species have different copulatory organs that are not compatible.
Therefore, they are not able to mate and produce offspring.
Hint: Prezygotic isolation mechanisms prevent hybridization. If hybrid matings
do occur and zygotes are produced, many factors known as postzygotic isolating
mechanisms may still prevent those zygotes from developing into normally
functioning, fertile individuals.
5. Problems with the biological species concept include the fact that—
a. many species reproduce asexually
b. postzygotic isolating mechanisms decrease hybrid viability
c. prezygotic isolating mechanisms are extremely rare
d. all of these
The correct answer is a—many species reproduce asexually
A. Answer a is correct. Many species do not reproduce sexually so a strict
application of the test of interbreeding does not apply.
The correct answer is a—
B. Answer b is incorrect. Although it is true that postzygotic mechanisms decrease
hybrid viability, such an effect reduces the incidence of hybridization and adds to
the usefulness of the biological species concept.
The correct answer is a—
C. Answer c is incorrect. Prezygotic mechanisms are not rare, in fact they are very
common, and such mechanisms help substantiate the biological species concept.
The correct answer is a—
D. Answer d is incorrect. Only answer a is the correct answer.
6. Cladogenesis—
a. is a type of prezygotic isolating mechanism
b. is a type of postzygotic isolating mechanism
c. only occurs in plants
d. none of the above
The correct answer is d—
A. Answer a is incorrect. Cladogenesis refers to the process of new species
formation, which is enhanced by reproductive isolating mechanisms but is not a
type of isolating mechanism.
The correct answer is d—
B. Answer b is incorrect. Cladogenesis refers to the process of new species
formation, which is enhanced by reproductive isolating mechanisms but is not a
type of isolating mechanism.
The correct answer is d—
C. Answer c is incorrect. New species form in all kinds of organisms, not just plants.
The correct answer is d—none of the above
D. Answer d is correct. None of the answers provided is a correct statement about
cladogenesis.
7. If reinforcement is weak and hybrids are not completely infertile—
a. genetic divergence between populations may be overcome by gene flow
b. speciation will occur 100% of the time
c. gene flow between populations will be impossible
d. the speciation will be more likely than if hybrids were completely infertile
The correct answer is a—genetic divergence between populations may be overcome by
gene flow
A. Answer a is correct. When hybridization occurs and hybrids are fertile, the hybrid
offspring will breed with the parental populations, leading to gene flow between
the populations, which will genetically homogenize the two species and diminish
whatever reproductive isolation has evolved.
The correct answer is a—
B. Answer b is incorrect. Under the circumstances described, the probability of
speciation is expected to be low, at least it wouldn’t be a certainty.
The correct answer is a—
C. Answer c is incorrect. If hybrids have nonzero fertility, then reproductive isolation
would not be complete, leading to nonzero gene flow.
The correct answer is a—
D. Answer c is incorrect. The probability of speciation should increase as the
probability of hybrid fertility decreases.
8. Natural selection—
a. can enhance the probability of speciation
b. can enhance reproductive isolation
c. can act against hybrid survival and reproduction
d. all of these
The correct answer is d—
A. Answer a is incorrect. Natural selection can lead to phenotypic changes that
promote divergence in different habitats, as the example with physiological
tolerance to drought by fruit flies demonstrates. This is not the only correct
answer.
The correct answer is d—
B. Answer b is incorrect. Natural selection can directly or indirectly favor individuals
that have mating preferences that reduce gene flow among populations. Dewlap
coloration in some Anoles lizards is a good example of this phenomenon. This is
not the only correct answer.
The correct answer is d—
C. Answer c is incorrect. This answer is true, but this is not the only correct answer.
The correct answer is d—all of these
D. Answer d is correct. Natural selection can be a powerful force enhancing
speciation. The extent to which it is always an important component of speciation
is a matter of much current study.
9. Allopatric speciation—
a. is less common than sympatric speciation
b. involves geographic isolation of some kind
c. is the only kind of speciation that occurs in plants
d. requires polyploidy
The correct answer is b—
A. Answer a is incorrect. Sympatric speciation, due to the difficulty of achieving
genetic divergence, is thought to be much less common.
The correct answer is b—involves geographic isolation of some kind
B. Answer b is correct. The isolation is usually thought of on a very large scale, but
even small barriers can separate populations of some kinds of organisms.
The correct answer is b—
C. Answer c is incorrect. Allopathic speciation is not restricted to any particular kind
of organism. Moreover, it is currently estimated that many plant species have an
episode of polyploidy in their history, increasing the possibility that sympatric
speciation may be an important mechanism in plants
The correct answer is b—
D. Answer d is incorrect. Allopatric speciation can occur even without polyploidy
occurring.
10. Cichlid diversity can be attributed to—
a. adaptive radiation
b. new habitats and geographic isolation
c. a second set of jaws in the throat of the fish
d. all of the above
The correct answer is d—
A. Answer a is incorrect. Although adaptive radiation aided in the explosion of
cichlid diversity after the introduction of the first group of cichlids about 200,000
years ago, this is not the only correct answer.
The correct answer is d—
B. Answer b is incorrect. Although the rising and falling of the water level in the lake
caused the formation of new habitats and geographic isolation of some
populations, which contributed to cichlid diversity, this is not the only correct
answer.
The correct answer is d—
C. Answer c is incorrect. Although the evolution of a second set of jaws in the throat,
that aided in eating, allowed for a large amount of variation of the mouthparts that
led to the evolution of many species of cichlids, this is not the only correct
answer.
The correct answer is d—all of the above
D. Answer d is correct. The first cichlids occurred in Lake Victoria some 200,000
years ago (not long ago in terms of evolutionary change) and have undergone a
great amount of evolutionary change in that time. Adaptive radiation occurs
when a large amount of variation evolves from an ancestral population, which
occurred with the cichlids. The adaptation to new habitats and the occasional
geographic isolation that occurred helped speciation along, as well as the
evolution of a second set of jaws that allowed the other mouthparts to adapt in
other ways.
Hint: Lake Victoria is an immense shallow freshwater sea in East Africa that
until recently was home to an incredibly diverse collection of over 300 species of
cichlid fishes. The cluster of species appears to have evolved recently and quite
rapidly. Scientists estimate that the first cichlids occurred in the lake about
200,000 years ago. Dramatic changes in water level encouraged species
formation. Cichlid fish have a remarkable trait that may have been instrumental
in this evolutionary radiation: a second set of functioning jaws in the throats of the
fish which freed the oral jaws to evolve for other purposes, creating great
diversity.
11. The hypothesis that evolution occurs in spurts, with great amounts of evolutionary
change followed by periods of stasis, is—
a. punctuated equilibrium
b. allopatric speciation
c. gradualism
d. the Hardy–Weinberg equilibrium
The correct answer is a—punctuated equilibrium
A. Answer a is correct. Niles Eldredge and Stephen Jay Gould proposed the
hypothesis of punctuated equilibrium, which challenged the then widely accepted
evolutionary model, gradualism. According to the model of gradualism,
evolutionary changes occur very slowly, over long periods of time. Punctuated
equilibrium states instead that evolutionary changes occur in short time intervals
separated by long periods of little or no evolutionary change.
The correct answer is a—
B. Answer b is incorrect. Allopatric speciation is the evolution of species in
populations that are isolated from one another. Although evolutionary change in
these populations could occur in spurts, they could also occur gradually.
Allopatric speciation is not the hypothesis referred to in this question.
The correct answer is a—
C. Answer c is incorrect. Gradualism was the widely accepted model of evolution
before this hypothesis was presented. This hypothesis challenged the gradualism
model, which stated that evolutionary change occurred slowly, and gradually,
over long periods of time.
The correct answer is a—
D. Answer d is incorrect. The Hardy–Weinberg equilibrium describes populations
that are static, undergoing no evolutionary change. This is not the hypothesis
referred to in this question.
Hint: Gradualism was challenged in 1972 by paleontologists Niles Eldredge and
Stephen Jay Gould who argued that species experience long periods of little or no
evolutionary change (termed stasis), punctuated by bursts of evolutionary change
occurring over geologically short time intervals. Moreover, they argued that these
periods of rapid change occurred only during the speciation process.
12. Gradualism and punctuated equilibrium are—
a. two ends of a continuum of the rate of evolutionary change over time
b. mutually exclusive views about how all evolutionary change takes place
c. mechanisms of reproductive isolation
d. none of the above
The correct answer is a—two ends of a continuum of the rate of evolutionary change over
time
A. Answer a is correct. We see evidence of taxa and periods characterized by
gradualism and punctuated equilibrium. Which mechanism predominates is still
the subject of active research and debate.
The correct answer is a—
B. Answer b is incorrect. There is enough variability in the evolutionary history of
organisms that numerous examples supporting both views can be found. It is
unlikely that one or the other view is correct, but rather that there is a continuum
of possibilities whose ends are defined by the two extremes.
The correct answer is a—
C. Answer c is incorrect. Neither is an example of reproductive isolation, but rather a
description of the pace of evolution.
The correct answer is a—
D. Answer d is incorrect. One of the answers is correct, answer a.
13. During the history of life on Earth—
a. there have been major extinction events
b. species diversity has steadily increased
c. species diversity has stayed relatively constant
d. extinction rates have been completely offset by speciation rates
The correct answer is a—there have been major extinction events
A. Answer a is correct. There is good evidence from the fossil record that on at least
five occasions in the last 600 million years, large numbers of species went extinct
in a relatively short time. These events have been related both to climate change
and/or impact events with extraterrestrial objects (asteroids).
The correct answer is a—
B. Answer b is incorrect. The numbers of species has increased and decreased over
time, even without including the major extinction events.
The correct answer is a—
C. Answer c is incorrect. There is good evidence from the fossil record of variation
in species diversity over time.
The correct answer is a—
D. Answer d is incorrect. For this reason, c is also incorrect. If speciation rates offset
extinction rates, that would lead to no change in species diversity over time, and
the fossil record tells a different story.
14. Character displacement—
a. arises through competition and natural selection, favoring divergence in
resource use
b. arises through competition and natural selection, favoring convergence in
resource use
c. does not promote speciation
d. reduced speciation rates in Galápagos finches
The correct answer is a—arises through competition and natural selection, favoring
divergence in resource use
A. Answer a is correct. When similar individuals compete for a shared limited
resource, the supply may not meet the total demand of either. Under those
circumstances, individuals that utilize slightly different sets of resources will be
favored by natural selection. Over time, selection could favor a divergence in
resource use, leading to “character displacement” because it enhances the fitness
of individuals by reducing competition.
The correct answer is a—
B. Answer b is incorrect. Convergence in resource use would only exacerbate
competition. If resources are limited, that would be an unsustainable situation.
The correct answer is a—
C. Answer c is incorrect. Character displacement occurs between two species; a
process similar to character displacement occurs in some types of sympatric
speciation, in which the starting condition is a population of a single species, not
of two different species.
The correct answer is a—
D. Answer d is incorrect. Although character displacement has occurred in
Galápagos finches, there is no reason to believe that it has slowed down the rate
of speciation.
15. Hybridization between incompletely isolated populations—
a. always leads to reinforcement due to the inferiority of hybrids
b. can serve as a mechanism for preserving gene flow between populations
c. only occurs in plants
d. never affects rates of speciation
The correct answer is b—
A. Answer a is incorrect. Reinforcement depends on many variables, not just the
viability of hybrids. Even if hybrids are inferior, they may still be vigorous
enough to provide a small amount of gene flow between populations. As we saw
in chapter 20, small amounts of gene flow can keep populations from diverging
genetically.
The correct answer is b—can serve as a mechanism for preserving gene flow between
populations
B. Answer b is correct. As mentioned above, if hybrids are not sterile, and if they are
capable of reproducing, then they can provide a pathway of genetic exchange
between individuals of the two different populations. That might especially be
true if hybrids are intermediate in their phenotype between individuals from the
two populations.
The correct answer is b—
C. Answer c is incorrect. Plants and animals hybridize, although it appears to be
more common in plants.
The correct answer is b—
D. Answer d is incorrect. This is the case especially where hybrids have very low
viability or are sterile. Thus, any individuals that mate with members of the other
population would be severely selected against, reducing the probability of gene
flow.
Challenge Questions
1. Natural selection can lead to the evolution of prezygotic isolating mechanisms,
but not postzygotic isolating mechanisms? Explain.
Answer—If hybrids between two species have reduced viability or fertility, then natural
selection will favor any trait that prevents hybrid matings. The reason is that individuals
that don’t waste time, energy, or resources on such matings will have greater fitness if
they instead spend the time, energy, and resources on mating with members of their own
species. For this reason, natural selection will favor any trait that decreases the
probability of hybridization. By contrast, once hybridization has occurred, the time,
energy, and resources have already been expended. Thus, there is no reason that less fit
hybrids would be favored over more fit ones. The only exception is for species that
invest considerable time and energy in incubating eggs and rearing the young; for those
species, selection may favor reduced viability of hybrids because parents of such
individuals will not waste further time and energy on them.
2. If there is no universally accepted definition of a species, what good is the term?
Will the idea of and need for a “species concept” be eliminated in the future?
Answer—The biological species concept, despite its limitations, reveals the continuum
of biological processes and the complexity and dynamics of organic evolution. At the
very least, the biological species concept provides a mechanism for biologists to
communicate about taxa and know that they are talking about the same thing! Perhaps
even more significantly, discussion and debate about the meaning of “species” fuels a
deeper understanding about biology and evolution in general. It is unlikely that we will
ever have a single unifying concept of species given the vast diversity of life, both extinct
and extant.
3. Refer to Figure 22.6. In Europe, pied and collared flycatchers are dissimilar in
sympatry, but very similar in allopatry, consistent with character divergence in
coloration. In this case, there is no competition for ecological resources as in
other cases of character divergence discussed. How might this example work?
Answer—The principle is the same as in character displacement. In sympatry,
individuals of the two species that look alike may mate with each other. If the species are
not completely interfertile, then individuals hybridizing will be at a selective
disadvantage. If a trait appears in one species that allows that species to more easily
recognize members of its own species and thus avoid hybridization, then individuals
bearing that trait will have higher fitness and that trait will spread through the population.
4. Refer to Figure 22.14. Geospiza fuliginosa and Geospiza fortis are found in
sympatry on at least one island in the Galápagos and in allopatry on several
islands in the same archipelago. Compare your expectations about degree of
morphological similarity of the two species in these two contexts, given the
hypothesis that competition for food played a large role in the adaptive radiation
of this group. Would your expectations be the same for a pair of finch species
that are not as closely related? Explain.
Answer—I would expect the two species to have more similar morphology when they
are found alone (allopatry) than when they are found together (sympatry), assuming that
food resources were the same from one island to the next. This would be the result of
character displacement expected under a hypothesis of competition for food when the two
species occur in sympatry. A species pair that is more distantly related might not be
expected to show the pattern of character displacement since they show greater
differences in morphology (and presumably in ecology and behavior as well), which
should reduce the potential for competition to drive character divergence.