Species and Their Formation - Laboratory of Visual Systems

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