Species and Their Formation - Laboratory of Visual Systems

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494 CHAPTER TWENTY-FOURswords have differentiated very little in their chloroplastgenes.The island silverswords are more diverse in size andshape than the mainland tarweeds because the original colonizersarrived on islands that had very few plant species. Inparticular, there were few trees and shrubs, because suchlarge-seeded plants rarely disperse to oceanic islands. In fact,many island trees and shrubs have evolved from nonwoodyancestors. On the mainland, however, tarweeds live in ecologicalcommunities that contain tree and shrub lineagesolder than their own—that is, where opportunities to exploitthe “tree” way of life have already been preempted.The processes we have discussed in this chapter, operatingover billions of years, have produced a world in which life isorganized into millions of species, each adapted to live in aparticular environment and to use environmental resourcesin a particular way. How these millions of species are distributedover the surface of Earth and organized into ecologicalcommunities will be a major focus of Part Eight of this book.Chapter SummaryWhat Are Species? Species are independent evolutionary units. A commonlyaccepted definition of species is “groups of actually or potentiallyinterbreeding natural populations which are reproductivelyisolated from other such groups.” Because speciation is often a gradual process, it may be difficultto recognize boundaries between species. Review Figure 24.3How Do New Species Arise? Not all evolutionary changes result in new species. Allopatric (geographic) speciation is the most importantmode of speciation among animals and is common in othergroups of organisms. Review Figures 24.4, 24.5, 24.6.See Web/CD Tutorial 24.2 Sympatric speciation may occur rapidly by polyploidybecause polyploid offspring are sterile in crosses with membersof the parent species. Polyploidy is a major factor in plant speciationbut is rare among animals. Review Figures 24.7, 24.8See Web/CD Tutorial 24.1Completing Speciation: Reproductive IsolatingMechanisms Once two populations have been separated, reproductive isolatingmechanisms may prevent the exchange of genes betweenthem. Prezygotic reproductive barriers operate before fertilization.Some prezygotic barriers affect mate choice; others work byinfluencing pollinator behavior. Review Figure 24.10, 24.11 Postzygotic reproductive barriers operate after fertilization byreducing the survival or fertility of hybrid offspring. If hybrid offspring survive poorly, more effective prezygoticreproductive barriers may evolve. This process is known asreinforcement. Review Figure 24.12Hybrid Zones: Incomplete Reproductive Isolation Hybrid zones may develop if barriers to gene exchange fail todevelop while diverging species are isolated from each other.Review Figure 24.13 Species may differ from one another in very few genes.Variation in Speciation Rates Rates of speciation differ greatly among lineages. Speciationrates are influenced by the number of species in a lineage, theirdispersal rates, ecological specialization, experience of populationbottlenecks, pollinators, and behavior, as well as by climaticchanges.Evolutionary Radiations Evolutionary radiations occur when speciation rates exceedextinction rates. High speciation rates often coincide with low extinction rateswhen species invade islands or other environments that containfew other species. As a result of speciation, Earth is populated with millions ofspecies, each adapted to live in a particular environment and touse resources in a particular way.See Web/CD Activity 24.1 for a concept review of this chapter.Self-Quiz1. A species is a group ofa. actually interbreeding natural populations that arereproductively isolated from other such groups.b. potentially interbreeding natural populations that arereproductively isolated from other such groups.c. actually or potentially interbreeding natural populationsthat are reproductively isolated from other such groups.d. actually or potentially interbreeding natural populationsthat are reproductively connected to other such groups.e. actually interbreeding natural populations that arereproductively connected to other such groups.2. Allopatric speciation may happen whena. continents drift apart and separate previously connectedlineages.b. a mountain range separates formerly connected populations.c. different environments on two sides of a barrier causepopulations to diverge.d. the range of a species is separated by loss of intermediatehabitat.e. all of the above3. Finches speciated in the Galápagos Islands becausea. the Galápagos Islands are not far from the mainland.b. the Galápagos Islands are arid.c. the Galápagos Islands are small.d. the islands of the Galápagos Archipelago are sufficientlyisolated from one another that there is little migrationamong them.e. the islands of the Galápagos Archipelago are closeenough to one another that there is considerablemigration among them.4. Which of the following is not a potential prezygoticreproductive barrier?a. Temporal segregation of breeding seasonsb. Differences in chemicals that attract matesc. Hybrid infertilityd. Spatial segregation of mating sitese. Sperm cannot survive in female reproductive tracts
SPECIES AND THEIR FORMATION 4955. A common means of sympatric speciation isa. polyploidy.b. hybrid infertility.c. temporal segregation of breeding seasons.d. spatial segregation of mating sites.e. imposition of a geographic barrier.6. Sympatric species are often similar in appearance becausea. appearances are often of little evolutionary significance.b. the genetic changes accompanying speciation are oftensmall.c. the genetic changes accompanying speciation are usuallylarge.d. speciation usually requires major reorganization of thegenome.e. the traits that differ among species are not the same as thetraits that differ among individuals within species.7. Narrow hybrid zones may persist for long times becausea. hybrids are always at a disadvantage.b. hybrids have an advantage only in narrow zones.c. hybrid individuals never move far from their birthplaces.d. individuals that move into the zone have not previouslyencountered individuals of the other species, so reinforcementof isolating mechanisms has not occurred.e. Narrow hybrid zones are artifacts because biologistsgenerally restrict their studies to contact zones betweenspecies.8. Which statement about speciation is not true?a. It always takes thousands of years.b. It often takes thousands of years, but may happen withina single generation.c. Among animals, it usually requires a physical barrier.d. Among plants, it often happens as a result of polyploidy.e. It has produced the millions of species living today.9. Speciation is often rapid within lineages in which specieshave complex behavior becausea. individuals of such species make fine discriminationsamong potential mating partners.b. such species have short generation times.c. such species have high reproductive rates.d. such species have complex relationships with theirenvironments.e. none of the above10. Evolutionary radiationsa. often happen on continents, but rarely on islandarchipelagos.b. characterize birds and plants, but not other taxonomicgroups.c. have happened on continents as well as on islands.d. require major reorganizations of the genome.e. never happen in species-poor environments.11. Speciation is an important component of evolution becauseita. generates the variation upon which natural selection acts.b. generates the variation upon which genetic drift andmutations act.c. enabled Charles Darwin to perceive the mechanisms ofevolution.d. generates the high extinction rates that drive evolutionarychange.e. has resulted in a world with millions of species, eachadapted for a particular way of life.For Discussion1. The snow goose of North America has two distinct colorforms: blue and white. Matings between the two color formsare common. However, blue individuals pair with blue individualsand white individuals pair with white individualsmuch more frequently than would be expected by chance.Suppose that 75 percent of all mated pairs consisted of twoindividuals of the same color. What would you concludeabout speciation processes in these geese? If 95 percent ofpairs were the same color? If 100 percent of pairs were thesame color?2. Suppose pairs of snow geese of mixed colors were found onlyin a narrow zone within the broad Arctic breeding range ofthe geese. Would your answer to Question 1 remain thesame? Would your answer change if mixed-color pairs werewidely distributed across the breeding range of the geese?3. Although many butterfly species are divided into local populationsamong which there is little gene flow, these speciesoften show relatively little morphological variation amongpopulations. Describe the studies you would conduct todetermine what maintains this morphological similarity.4. Evolutionary radiations are common and easily studied onoceanic islands, but in what types of mainland situationswould you expect to find major evolutionary radiations?Why?5. Fruit flies of the genus Drosophila are distributed worldwide,but most of the species in the genus are found on theHawaiian Islands. What might account for this distributionpattern?6. Evolutionary radiations take place when speciation ratesexceed extinction rates. What factors can cause extinctionrates to exceed speciation rates in a lineage? Name some lineagesin which human activities are increasing extinction rateswithout increasing speciation rates.
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