Species and Their Formation - Laboratory of Visual Systems

More documents

Recommendations

Info

488 CHAPTER TWENTY-FOUR24.9 Geographically Separated,Morphologically Similar Althoughthey have been separated by theAtlantic Ocean for at least 20 millionyears, American and Europeansycamores have diverged very little inappearance.(a) Platanus occidentalis (American sycamore)(b) Platanus hispanica (European sycamore)Prezygotic barriers operate before fertilizationSeveral processes that operate before fertilization—prezygoticreproductive barriers—may prevent individuals of differentspecies from interbreeding:Spatial isolation. Individuals of different species mayselect different places in the environment in which to live.As a result, they may never come into contact duringtheir respective mating periods; that is, they are reproductivelyisolated by location.Temporal isolation. Many organisms have mating periodsthat are as short as a few hours or days. If the matingperiods of two species do not overlap, they will be reproductivelyisolated by time.Mechanical isolation. Differences in the sizes and shapes ofreproductive organs may prevent the union of gametesfrom different species.Gametic isolation. Sperm of one species may not attach tothe eggs of another species because the eggs do notrelease the appropriate attractive chemicals, or the spermmay be unable to penetrate the egg because the twogametes are chemically incompatible.Behavioral isolation. Individuals of a species may reject, orfail to recognize, individuals of other species as matingpartners.Two closely related species of crickets from the island ofHawaii, Laupala paranga and Laupala kohalensis, provide an exampleof behavioral isolation. These two species live in separateareas and do not hybridize in nature, but they will forminterspecific pairs in the laboratory. The males of the twospecies produce genetically determined songs that differ inthe number of pulses per second. The songs of hybrid maleshave intermediate numbers of pulses (Figure 24.10). Femalesare much more strongly attracted to the songs of conspecificmales (males of their own species) than they are to the songsof males of the other species. We know that this preference isgenetically determined because hybrid females are moststrongly attracted to the songs of hybrid males.Sometimes the mate choice of one species is mediated bythe behavior of individuals of other species. For example,whether two plant species hybridize may depend on thepreferences of their pollinators. Because pollinators visit flowersto gather nutritional rewards, not to pollinate plants, pollinatorbehavior can be influenced only by changes in floralstructures that affect the rewards the pollinators receive. Floraltraits can affect reproductive isolation either by influencingpollinator behavior or by altering where pollen is depositedon the bodies of pollinators.The evolution of floral traits that generate reproductiveisolation has been studied in columbines of the genus Aquilegia.Columbines have undergone recent and very rapid spe-Number of individuals producing songswith that pulse frequency5040302010LaupalaparanigraF 1 hybrid generationPulse frequency ofhybrid male songsdoes not overlapfrequencies of songsof parental males.Laupalakohalensis0 1.0 2.0 3.0 4.0 5.0Pulses per secondNarrow range ofpulse frequenciesindicates strongstabilizing selection.24.10 Songs of Male Crickets are Genetically DeterminedHybrid males produce songs with intermediate pulse frequencies.
SPECIES AND THEIR FORMATION 489ciation and, at the same time, have evolved long floral nectarspurs—tubular outgrowths of petals that produce nectarat their tips. Animals pollinate these flowers while probingthe spurs to collect nectar. The length of the spurs and the orientationof the flowers influence how efficiently pollinatorscan extract nectar. Two species, Aquilegia formosa and Aquilegiapubescens, that grow in the mountains of California canproduce fertile hybrids. A. formosa has pendant (hanging)flowers and short spurs (Figure 24.11a); it is pollinated byhummingbirds. A. pubescens has upright flowers and longspurs (Figure 24.11b); it is pollinated by hawkmoths.Investigators tested discrimination among these flowersby hawkmoths by turning A. formosa flowers so that theywere upright. Hawkmoths still visited mostly A. pubescensflowers (Figure 24.11c), probably because the flowers of thetwo species differ strongly in the color of light they reflect.Genetic analyses of hybrids between the two species showthat the color differences are caused by a small number ofgenes. Thus, although these two species are interfertile, hybridsrarely form in nature because the two species attractdifferent pollinators.Postzygotic barriers operate after fertilizationIf individuals of two different populations overcome prezygoticreproductive barriers and interbreed, postzygotic reproductivebarriers may still prevent gene exchange. Geneticdifferences that accumulated while the populations were isolatedfrom each other may reduce the survival and reproductionof the hybrid offspring in any of several ways:Hybrid zygote abnormality. Hybrid zygotes may fail tomature normally, either dying during development ordeveloping such severe abnormalities that they cannotmate as adults.Hybrid infertility. Hybrids may mature normally, but beinfertile when they attempt to reproduce. For example,the offspring of matings between horses and donkeys—mules—are strong, but sterile; they produce no descendants.Low hybrid viability. Hybrid offspring may simply surviveless well than offspring resulting from matingswithin populations.(a) Aquilegia formosa(c)Number of visits by hawkmoths140120100806040200A. pubescens(normalorientation)(b) Aquilegia pubescensA. formosa(upright)A. formosa(normalorientation)24.11 Hawkmoths Favor Flowers of One Columbine SpeciesFlowers of Aquilegia formosa are normally pendant (a), while those ofA. pubescens are normally upright (b). The plants can interbreed, butthe hawkmoths that pollinate A. pubescens distinguish between flowersof the two species, even when A. formosa flowers are experimentallymodified to be upright (c).If hybrid offspring survive poorly, more effective prezygoticbarriers may evolve, because individuals that mate with individualsof the other population will leave fewer surviving offspringthan individuals that mate only within their own population.This strengthening of prezygotic barriers is known asreinforcement. Reinforcement is difficult to detect experimentally,but it can be detected by using the comparativemethod (see Chapter 1). If reinforcement is occurring, thensympatric pairs of species should evolve prezygotic reproductivebarriers more rapidly than allopatric pairs of speciesdo. In a study of related sympatric and allopatric species ofDrosophila, this was shown to be the case (Figure 24.12).We can observe speciation in progressSince speciation is a gradual process, we can find many examplesof populations at different stages on the way to completereproductive isolation. A good example of speciation inprogress is found in a picture-winged fruit fly (Rhagoletispomenella) in New York State. Until the mid-1800s, these fruitflies courted, mated, and deposited their eggs only onhawthorn fruits. The larvae recognized the odor of hawthorn
Page 3 and 4: (a)SPECIES AND THEIR FORMATION 483(
Page 5 and 6: PacificOceanNorth AmericaSeed eater
Page 7: SPECIES AND THEIR FORMATION 487spri
Page 11 and 12: SPECIES AND THEIR FORMATION 491(a)
Page 13 and 14: SPECIES AND THEIR FORMATION 49324.1
Page 15: SPECIES AND THEIR FORMATION 4955. A

Species and Their Formation - Laboratory of Visual Systems

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?