"Ch OLC sterility"
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.