| Chapter 24 Review |
To review the Activities and/or Investigations listed in Chapter 24 below, go to the Campbell BIOLOGY 7th edition Web site and select "Chapter 24" from the dropdown menu.
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| SUMMARY OF KEY CONCEPTS |
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 Concept 24.1
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The biological species concept emphasizes reproductive isolation
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 The Biological Species Concept |
A biological species is a group of populations whose individuals have the potential to interbreed and produce fertile offspring with each other but not with members of other species. The biological species concept emphasizes reproductive isolation through prezygotic and postzygotic barriers that can result in separating the gene pools of different populations.
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| Other Definitions of Species |
| Although particularly helpful in thinking about speciation processes, the biological species concept has some major limitations. For instance, it cannot be applied to organisms that are known only as fossils or to organisms that reproduce only asexually. Thus, scientists maintain alternative species concepts, such as the morphological species concept, that are useful in various contexts. |
Activity Overview of Macroevolution
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| Concept 24.2
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Speciation can take place with or without geographic separation
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| Allopatric (“Other Country”) Speciation |
Allopatric speciation may occur when two populations of one species become geographically separated from each other. One or both populations may undergo evolutionary change during the period of separation. Should they come into contact once more, they may be separated by the prezygotic and postzygotic isolating mechanisms that have accumulated.
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| Sympatric (“Same Country”) Speciation |
A new species can originate while remaining in a geographically overlapping area with the parent species. In particular, many plant species have evolved sympatrically through polyploidy (multiplications of the chromosome number). Autopolyploids are species derived this way from one ancestral species. Allopolyploids are species with multiple sets of chromosomes derived from different species. Sympatric speciation can also result from the appearance of new ecological niches and from nonrandom mating in polymorphic populations.
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| Allopatric and Sympatric Speciation: A Summary |
In allopatric speciation, a new species forms while geographically isolated from its parent population. In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation.
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| Adaptive Radiation |
Adaptive radiation can occur when a population encounters a multiplicity of new or newly available ecological niches. This may happen during colonization of a new environment, such as newly formed volcanic islands, or after an environmental change that has resulted in mass extinctions of other species in an area.
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| Studying the Genetics of Speciation |
| The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation. |
Investigation How Do New Species Arise by Genetic Isolation?
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| The Tempo of Speciation |
Eldredge and Gould’s punctuated equilibrium model draws on fossil evidence showing that species change most as they arise from an ancestral species, after which they undergo relatively little change for the rest of their existence. This model contrasts with a model of gradual change throughout a species’ existence.
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| Concept 24.3
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Macroevolutionary changes can accumulate through many speciation events
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| Evolutionary Novelties |
Most novel biological structures evolve in many stages from previously existing structures. Some complex structures, such as the eye, have had similar functions during all stages of their evolution. The most important functions of others, such as feathers, have changed.
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| Evolution of the Genes That Control Development |
| Many large evolutionary changes may have been associated with mutations in genes that regulate development. Such changes can affect the timing of developmental events (heterochrony) or the spatial organization of body parts. Some of these changes result from mutational changes in homeotic genes and in the genes that regulate them. |
Activity Allometric Growth
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| Evolution Is Not Goal Oriented |
Long–term evolutionary trends may arise because of adaptation to a changing environment. In addition, according to the species selection model, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics.
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| TESTING YOUR KNOWLEDGE |
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Self–Quiz
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1. The biological species concept is not useful for organisms known only from fossils because |
| a. fossils are rarely preserved well enough to distinguish species based on morphology. |
| b. it is not possible to test reproductive isolation of fossil forms. |
| c. it is not possible to infer the types of habitats occupied by the fossil forms before their extinction. |
| d. in examining fossil organisms, it is not possible to distinguish males from females. |
| e. the fossil record can only be used for studying anagenesis, but not cladogenesis. |
2. The largest unit within which gene flow can readily occur is a |
| a. population. |
| b. species. |
| c. genus. |
| d. hybrid. |
| e. phylum. |
3. Bird guides once listed the myrtle warbler and Audubon’s warbler as distinct species. Recently, these birds have been classified as eastern and western forms of a single species, the yellow–rumped warbler. Which of the following pieces of evidence, if true, would be cause for this reclassification? |
| a. The two forms are observed to interbreed successfully where their habitats overlap. |
| b. The two forms live in similar habitats. |
| c. The two forms have many genes in common. |
| d. The two forms have similar food requirements. |
| e. The two forms are very similar in coloration. |
4. Males of different species of the fruit fly Drosophila that live in the same parts of the Hawaiian islands have different elaborate courtship rituals that involve fighting other males and stylized movements that attract females. What type of reproductive isolation does this represent? |
| a. habitat isolation |
| b. temporal isolation |
| c. behavioral isolation |
| d. gametic isolation |
| e. postzygotic barriers |
5. Which of the following factors would not contribute to allopatric speciation? |
| a. A population becomes geographically isolated from the parent population. |
| b. The separated population is small, and genetic drift occurs. |
| c. The isolated population is exposed to different selection pressures than the ancestral population. |
| d. Different mutations begin to distinguish the gene pools of the separated populations. |
| e. Gene flow between the two populations is extensive. |
6. Plant species A has a diploid number of 12. Plant species B has a diploid number of 16. A new species, C, arises as an allopolyploid from A and B. The likely diploid number for species C would probably be |
| a. 12. |
| b. 14. |
| c. 16. |
| d. 28. |
| e. 56. |
7. The speciation episode described in question 6 is most likely a case of |
| a. allopatric speciation. |
| b. sympatric speciation. |
| c. speciation based on sexual selection. |
| d. adaptive radiation. |
| e. anagenesis. |
8.Mimulus lewisii and M. cardinalis are plants that do not hybridize in nature but can be readily crossed in the laboratory to produce fertile offspring. Which of the following is least likely to keep the gene pools of these two plants separate in nature? |
| a. gametic incompatibility |
| b. different attractiveness to pollinators |
| c. different ecological niches |
| d. different geographic ranges |
| e. seasonal differences in flowering |
9. According to the punctuated equilibrium model, |
| a. natural selection is unimportant as a mechanism of evolution. |
| b. given enough time, most existing species will branch gradually into new species. |
| c. most new species accumulate their unique features as they come into existence, then change little for the rest of their duration as a species. |
| d. most evolution is goal oriented. |
| e. speciation is usually due to a single mutation. |
10. A genetic change that caused a certain Hox gene to be expressed along the tip of a vertebrate limb bud instead of farther back made possible the evolution of the tetrapod limb. This type of change is illustrative of |
| a. the influence of environment on an individual’s development. |
| b. paedomorphosis, or retention of ancestral juvenile structures in an adult organism. |
| c. a change in a developmental gene or in its regulation that altered the spatial organization of body parts. |
| d. punctuated equilibrium. |
| e. the origin of a new species due to allopolyploidy. |
For Self-Quiz answers, see Appendix A.
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Go to the website or CD–ROM for more quiz questions.
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Evolution Connection
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In the margin of one of his notebooks, Darwin scrawled a note to remind himself never to apply the terms higher or lower to species. It was, and still is, very common for people to think of some species or species groups as more or less evolved than others. This probably stems from a notion of evolutionary “progress.” Is there such a thing as evolutionary progress? Why or why not? Defend your position as if you were debating someone holding the opposite view.
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Scientific Inquiry
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| Cultivated American cotton plants have a total of 52 chromosomes (2n = 52). In each cell, 13 pairs of chromosomes (26 chromosomes) are smaller than the other 13 pairs. Old World cotton plants have a total of 26 chromosomes (2n = 26), all large. Wild American cotton plants have 26 chromosomes, all small. Propose a hypothesis to explain how cultivated American cotton may have originated. How could you test your hypothesis? |
Investigation How Do New Species Arise by Genetic Isolation?
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Science, Technology, and Society
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What is the biological basis for assigning all human populations to a single species? Can you think of a scenario in which a second human species could arise in the future by cladogenesis?
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