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					   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.

				
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