Species and Their Formation - Laboratory of Visual Systems

(a)SPECIES AND THEIR FORMATION 483(b)Agelaius phoeniceus (male, NY) Agelaius phoeniceus (male, CA) Agelaius phoeniceus (female)24.2 Members of the Same Species Look Alike—or Not (a) Bothof these male red-winged blackbirds are obviously members of thesame species, even though one is from the eastern United States andthe other is from California. (b) Because red-winged blackbirds aresexually dimorphic (see Chapter 23), the female of the speciesappears quite different from the male.productively incompatible. Alternatively, they may becomereproductively incompatible before they evolve any noticeablemorphological differences. But how do these differencesbetween populations come about?How Do New Species Arise?Speciation is the process by which one species splitsinto two or more daughter species, which thereafter evolveas distinct lineages. Although Charles Darwin titled his bookThe Origin of Species, he did not extensively discuss speciation,a process he called “the mystery of mysteries.” He devotedmost of his attention to demonstrating that species areIncreasingGenetic distanceIncreasing01 A barrier isestablished.Interbreedingpopulation(parent species)2 The two populationsdiverge genetically butare still reproductivelycompatible.TimeDaughterspecies AReproductiveincompatibilityis established.Daughterspecies B24.3 Speciation May Be a Gradual Process In this hypotheticalexample, genetic divergence between two separated populationsbegins before reproductive incompatibility evolves.3altered by natural selection over time. But not all evolutionarychanges result in new species: A single lineage maychange over time without giving rise to a new species.The critical event in speciation is the separation of the genepool of the ancestral species into two or more separate andisolated gene pools. Subsequently, within each isolated genepool, allele and genotype frequencies may change as a resultof the action of evolutionary agents. If two populations areisolated from each other, and sufficient differences in theirgenetic structure accumulate during the period of isolation,then the two populations may not be able to exchange geneswhen they come together again. As we will see, the amountof genetic difference that is needed to prevent gene exchangeis highly variable. Gene flow among populations may be interruptedin two major ways, each of which characterizes amode of speciation.Allopatric speciation requires total genetic isolationSpeciation that results when a population is divided by aphysical barrier is known as allopatric (allo-, “different”; patris,“country”) or geographic speciation (Figure 24.4). Allopatricspeciation is thought to be the dominant mode ofspeciation among most groups of organisms. The physicalbarrier that divides the range of a species may be a waterbody for terrestrial organisms, dry land for aquatic organisms,or a mountain range. Barriers can form when continentsdrift, sea levels rise, glaciers advance and retreat, andclimates change. These processes continue to generate physicalbarriers today. The populations separated by such barriersare often large initially. They evolve differences for a varietyof reasons, especially because the environments inwhich they live are, or become, different.Allopatric speciation may also result when some membersof a population cross an existing barrier and found a new, isolatedpopulation. Populations established in this way usuallydiffer genetically from their parent populations because ofthe founder effect: A small group of founding individuals has

484 CHAPTER TWENTY-FOURTimeA single species is distributedover a broad range.Sea level rises and separates two populations.Populations adapt to differing environmentson opposite sides of the barrier.If the barrier is removed, the populationsmay recolonize the intervening area andmingle, but do not interbreed.Another example of allopatric speciation is found in thefinches of the Galápagos archipelago, 1,000 km off thecoast of Ecuador. Darwin’s finches (as they are usuallycalled, because Darwin was the first scientist to studythem) arose in the Galápagos by speciation from a singleSouth American species that colonized the islands. Todaythere are 14 species of Darwin’s finches, all of which differstrikingly from their closest mainland relative (Figure24.6). The islands of the Galápagos archipelago are sufficientlyisolated from one another that finches seldom dispersebetween them. Also, environmental conditions differamong the islands. Some are relatively flat and arid;others have forested mountain slopes. Populations offinches on different islands have differentiated enoughover millions of years that when occasional immigrants arrivefrom other islands, they either do not breed with theresidents, or, if they do, the resulting offspring usually donot survive as well as those produced by pairs composedof island residents. The genetic distinctness and cohesivenessof the different species is thus maintained.A physical barrier’s effectiveness at preventing geneflow depends on the size and mobility of the species inquestion. What is an impenetrable barrier to a terrestrialsnail may be no barrier at all to a butterfly or a bird. Popu-Range of overlapNumbers on the islands representthe number of species of picturewingedDrosophila found there.Picture-wingedDrosophila24.4 Allopatric Speciation Allopatric speciation may result whena population is divided into two separate populations by a physicalbarrier, such as rising sea levels.only an incomplete representation of the gene pool of its parentpopulation (see Chapter 23). For example, many of themore than 800 species of the fruit fly genus Drosophila in theHawaiian Islands are restricted to a single island. Thesespecies are almost certainly the descendants of new populationsfounded by individuals dispersing among the islands,because the closest relative of a species on one island is oftena species on a neighboring island rather than a species on thesame island. Biologists who have studied the chromosomesof picture-winged species of Drosophila believe that speciationamong this group of flies has resulted from at least 45such founder events (Figure 24.5).12OldestislandKauai1Oahu29Numbers on the arrows representthe number of proposedfounder events. The arrowwidths are proportional tothese numbers.51210Species dispersal is primarilyfrom older to younger islands.73Mauicomplex4015126YoungestislandHawaii24.5 Founder Events Lead to Allopatric Speciation Thelarge number of species of picture-winged Drosophila in theHawaiian Islands is the result of founder events: new populationsfounded by individuals dispersing among the islands.The islands, which were formed in sequence as Earth’s crust movedover a volcanic “hot spot,” vary in age.

PacificOceanNorth AmericaSeed eatersBills of seed eaters areadapted for harvestingand crushing seeds.Large ground finch(Geospiza magnirostris)Medium ground finch(G. fortis)SPECIES AND THEIR FORMATION 485Large-billed finchescan crush large,hard seeds.Cocos IslandGalápagosIslandsSouthAmericaSmall ground finch(G. fuliginosa)Sharp-billed ground finch(G. difficilis)Small-billed finchescannot crush largeseeds as well, butare more adept athandling small seeds.Large cactus finch(G. conirostris)Cactus finches areadapted to openingcactus fruits andextracting the seeds.Cactus finch(G. scandens)Bud eater The bud eater‘s heavy bill is adaptedfor grasping and wrenching buds from branches.Vegetarian finch(Platyspiza crassirostris)Small tree finch(Camarhynchus parvulus)The large tree finchuses its heavy bill totwist apart wood toreach larvae inside.ANCESTOR FINCHfrom South AmericanmainlandLarge tree finch(C. psittacula)Medium tree finch(C. pauper)The small andmedium tree finchesand mangrove finchpick insects fromleaves and branchesand explore crevicesfor hidden prey.Insect eatersThe bills of insect eaters varybecause they eat different typesand sizes of insects and theycapture them in different ways.Mangrove finch(C. heliobates)Woodpecker finch(C. pallidus)Warbler finch(Certhidea olivacea)The woopecker finchuses its long beak toprobe dead wood,crevices, and barkfor insects.The warbler finchuses quick motions tocapture insects onplant surfaces.24.6 Allopatric Speciation among Darwin’s Finches The descendantsof the ancestral finch that colonized the Galápagos archipelagoseveral million years ago evolved into 14 different species whosemembers are variously adapted to feed on seeds, buds, and insects.(The fourteenth species, not pictured here, lives in Cocos Island, farthernorth in the Pacific Ocean.)

486 CHAPTER TWENTY-FOURlations of wind-pollinated plants are isolated at the maximumdistance pollen can be blown by the wind, but individualplants are effectively isolated at much shorter distances.Among animal-pollinated plants, the width of the barrier isthe distance that animals can travel while carrying pollen orseeds. Even animals with great powers of dispersal are oftenreluctant to cross narrow strips of unsuitable habitat. For animalsthat cannot swim or fly, narrow water-filled gaps maybe effective barriers. However, gene flow can sometimes beinterrupted even in the absence of physical barriers.MeiosisHaploid gametes(one copy of eachchromosome)Sympatric speciation occurs without physical barriersAlthough physical isolation is usually required for speciation,under some circumstances speciation can occur withoutit. Such a partition of a gene pool is called sympatric speciation(sym-, “with”). The most common means of sympatricspeciation is polyploidy, the production within an individualof duplicate sets of chromosomes. Polyploidy can ariseeither from chromosome duplication in a single species(autopolyploidy) or from the combining of the chromosomesof two different species (allopolyploidy).An autopolyploid individual originates when (for example)cells that are normally diploid (with two sets of chromosomes)accidentally duplicate their chromosomes, resultingin a tetraploid (four sets of chromosomes) individual.Tetraploid and diploid plants of the same species are reproductivelyisolated because their triploid offspring are essentiallysterile.Even if triploid individuals survive to reproductive maturity,they cannot produce viable gametes because theirchromosomes do not synapse correctly during meiosis (Figure24.7). So a tetraploid plant cannot produce viable off-The diploidparent has twocopies of eachchromosomeMatingMeiosisMost gametes producedby the triploid hybridare not viable becausethey have an incorrectnumber of chromosomes.The F 1 offspring istriploid (three copiesof each chromosome)MeiosisDiploid gametes(two copies ofeach chromosome)The tetraploidparent has fourcopies of eachchromosome.24.7 Tetraploids Are Soon Reproductively Isolated from DiploidsEven if the triploid offspring of diploid and tetraploid parents reachessexual maturity, most of the gametes it produces have inviable numbersof chromosomes. Such triploid individuals are effectively sterile.(For simplicity, the diagram shows only three chromosomes; mostspecies have many more than that.)

SPECIES AND THEIR FORMATION 487spring by mating with a diploid individual—but it can do soif it self-fertilizes or mates with another tetraploid.Allopolyploids may be produced when individuals of twodifferent (but closely related) species interbreed, or hybridize.Allopolyploids are usually fertile because each of thechromosomes has a nearly identical partner with which topair during meiosis.New species arise by means of polyploidy much more easilyamong plants than among animals because plants ofmany species can reproduce by self-fertilization. In addition,if polyploidy arises in several offspring of a single parent, thesiblings can fertilize one another. Speciation by polyploidyhas been very important in the evolution of flowering plants.Botanists estimate that about 70 percent of flowering plantspecies and 95 percent of fern species are polyploids. Most ofthese arose as a result of hybridization between two species,followed by self-fertilization.How easily allopolyploidy can produce new species is illustratedby the salsifies (Tragopogon), members of the sunflowerfamily. Salsifies are weedy plants that thrive in disturbedareas around towns. People have inadvertentlyspread them around the world from their ancestral ranges inEurasia. Three diploid species of salsify were introduced intoNorth America early in the twentieth century: T. porrifolius,T. pratensis, and T. dubius. Two tetraploid hybrids—T. mirusand T. miscellus—between the original three diploid specieswere first discovered in 1950. The hybrids have spread sincetheir discovery and today are more widespread than theirdiploid parents (Figure 24.8).Studies of their genetic material have shown that both salsifyhybrids have formed more than once. Some populationsof T. miscellus—a hybrid of T. pratensis and T. dubius— havethe chloroplast genome of T. pratensis; other populations havethe chloroplast genome of T. dubius. Such differences amonglocal populations of T. miscellus show that this allopolyploidhas formed independently at least 21 times! Scientists seldomknow the dates and locations of species formation so well. T.mirus, a hybrid of T. porrifolius and T. dubius, has formed 12times. T. porrifolius prefers wet, shady places; T. dubius prefersdry, sunny places. T. mirus, however, can grow in partlyshaded environments where neither parent does well. Thesuccess of these newly formed hybrid species of salsifies illustrateswhy so many species of flowering plants originatedas polyploids.Polyploidy, as we have just seen, can result in a newspecies that is completely reproductively isolated from its parentspecies in one generation. Allopatric speciation proceedsmuch more slowly, and some populations separated by aphysical barrier may never acquire full reproductive isolation.Let’s see how reproductive isolation may become establishedonce two populations have been separated from each other.A tetraploid hybrid has an almostcontinuous range in an areaaround Spokane, Washington.The range oftetraploid hybrids ( )is broader than that ofdiploid parentalspecies ( ).24.8 Polyploids May Outperform Their Parent SpeciesTragopogon species (salsifies) are members of the sunflower family.The map shows the distribution of the three diploid parent speciesand of the two tetraploid hybrid species of Tragopogon in easternWashington and adjacent Idaho.Completing Speciation:Reproductive Isolating MechanismsOnce a barrier to gene flow is established, by whatevermeans, the separated populations may diverge geneticallythrough the action of the evolutionary agents we describedin Chapter 23. Over many generations, differences may accumulatethat reduce the probability that members of the twopopulations could mate and produce viable offspring. In thisway, reproductive isolation can evolve as an incidental byproductof genetic changes in allopatric populations.Geographic isolation does not necessarily lead to reproductiveincompatibility. For example, American sycamoresand European sycamores (also known as plane trees) havebeen physically isolated from one another for at least 20million years. Nevertheless, they are morphologically verysimilar (Figure 24.9), and they can form fertile hybrids,even though they never have an opportunity to do so innature.In other cases, however, genes that result in reproductiveisolation between two evolving lineages spreadquickly through populations as they diverge. In this section,we will examine the ways in which reproductive isolatingmechanisms may arise. In the following section, wewill explore what happens when reproductive isolation isincomplete.

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

490 CHAPTER TWENTY-FOURCOMPARATIVE METHODHypothesis: If reinforcement has occurred, sympatric species ofDrosophila should be more strongly isolated prezygotically thanallopatric species that have been separated for equal amounts of time.METHODRESULTSStrongPrezygotic isolationWeakLowAssess length of time Drosophila populations have beenevolving separately by the amount of genetic distancebetween them. Compare strength of prezygoticisolation between allopatric and sympatric speciespairs.Recently diverged pairs of sympatric species havemore prezygotic isolation than allopatric species do.Genetic distanceAllopatric species pairsSympatric species pairsGenetic distance, a measure of moleculargenetic differences between the species ina pair, is an indication of the length of timethe species have been evolving separately.HighConclusion: Reinforcement has resulted in particularly rapidevolution of prezygotic isolation among recently diverged sympatricspecies of Drosophila.24.12 Prezygotic Barriers Can Evolve Rapidly Reinforcement mayoccur when individuals who mate outside their own population leavefew or no viable offspring. In such a situation, prezygotic isolatingmechanisms can evolve extremely rapidly.as they fed on the fruits, and when they emerged from theirpupae, they used this cue to locate other hawthorn plants onwhich to mate and lay their eggs.About 150 years ago, large commercial apple orchardswere planted in the Hudson River valley. Apple trees areclosely related to hawthorns, and a few female Rhagoletis laidtheir eggs on apples, perhaps by mistake. Their larvae didnot grow as well as the larvae on hawthorn fruits, but manydid survive. These larvae recognized the odor of apples, sowhen they emerged as adults, they sought out apple trees,where they mated with other flies reared on apples.Today there are two types of Rhagoletis in the HudsonRiver valley that may be on the way to becoming distinctspecies. One feeds primarily on hawthorn fruits, the other onapples. The two incipient species are partly reproductivelyisolated because they mate primarily with individuals raisedon the same fruit and because they emerge from their pupaeat different times of the year. In addition, the apple-feedingflies have evolved so that they now grow more rapidly onapples than they originally did.Reproductive isolation does not develop at the same ratein all diverging populations. On the one hand, in somegroups, such as Darwin’s finches, there has been conspicuousmorphological evolution, but many of the 14 species stillinterbreed and produce fertile hybrid offspring. On the otherhand, reproductive isolation may develop rapidly, as it hasbetween diverging sympatric species of Drosophila. Generally,reproductive isolation evolves more rapidly in speciesthat have rapid reproductive rates (for example, most plants,fruit flies, and sea urchins) than in species that have slowerreproductive rates (for example, birds and mammals).Many species in nature form hybrids in areas where theirranges overlap, and they may continue to do so for manyyears. Let’s examine what happens in these situations.Hybrid Zones:Incomplete Reproductive IsolationIf contact is reestablished between formerly isolated populationsbefore complete reproductive isolation has developed,members of the two populations may interbreed. Three outcomesof such interbreeding are possible:If hybrid offspring are as successful as those resultingfrom matings within each population, hybrids mayspread through both populations and reproduce withother individuals. The gene pools would then combine,and no new species would result from the period of isolation.If hybrid offspring are less successful, complete reproductiveisolation may evolve as reinforcement strengthensprezygotic reproductive barriers.Even if hybrid offspring are at some disadvantage, a narrowhybrid zone may persist if, for one or more reasons,reinforcement does not happen.Hybrid zones are excellent natural laboratories for thestudy of speciation. When a hybrid zone first forms, most hybridsare offspring of crosses between purebred individualsof the two species. However, subsequent generations includea variety of individuals with different proportions of theirgenes derived from the original two populations. Thus, hybridzones contain recombinant individuals resulting frommany generations of hybridization. Detailed genetic studiescan tell us much about why hybrid zones may be narrow andstable for long periods of time.European toads of the genus Bombina have been the subjectof such studies. The fire-bellied toad (B. bombina) lives ineastern Europe. The closely related yellow-bellied toad (B.variegata) lives in western and southern Europe. The rangesof the two species meet in a narrow zone stretching 4,800 km

SPECIES AND THEIR FORMATION 491(a) Bombina bombina(b) Bombina variegata(c)B. bombina(fire-belliedtoad)24.13 Hybrid Zones May Be Long and Narrow Thenarrow zone in Europe where fire-bellied toads (a) meetand hybridize with yellow-bellied toads (b) stretchesacross Europe (c). This hybrid zone has been stable forhundreds of years, but has never expanded becausehybrid toads are much less fit than individuals of theparental species.B. variegata(yellow-belliedtoad)Area ofoverlapfrom eastern Germany to the Black Sea (Figure 24.13). Hybridsbetween the two species suffer from a range of defects,many of which are lethal. Those that survive often haveskeletal abnormalities, such as misshapen mouths, ribs thatare fused to vertebrae, and a reduced number of vertebrae.By following the fates of thousands of toads from the hybridzone, investigators have found that a hybrid toad is halfas fit as a purebred individual. The hybrid zone is narrow becausethere is strong selection against hybrids, and becauseadult toads do not move over long distances. It has persistedfor hundreds of years because individuals that move into ithave not previously encountered individuals of the otherspecies, so there has been no opportunity for reinforcementto occur.If two species hybridize, we know that they must be similargenetically, but the absence of interbreeding tells us nothingabout how dissimilar two species are. Not until modernmolecular genetic techniques were developed could biologistsmeasure genetic differences among species. These techniquesshow that the genetic differences that separate speciesare primarily differences among genes involved with reproductiveisolation. The extensive data gathered on Drosophilaindicate that fewer than ten, and often fewer than five, genesare responsible for reproductive isolation. Individuals of differentspecies of Hawaiian Drosophila share nearly all of theirmitochondrial DNA alleles. All of the hundreds of species ofDrosophila that have evolved in the Hawaiian Islands duringthe past 32 million years, even those that have diverged morphologically,are relatively similar genetically (Figure 24.14).Drosophila silvestrisDrosophila baliopteraDrosophila conspicua24.14 Morphologically Different, Genetically Similar Althoughthese fruit flies—a small sample of the hundreds of species foundonly on the Hawaiian Islands—vary greatly in appearance, they aregenetically similar.

492 CHAPTER TWENTY-FOURVariation in Speciation RatesSome lineages of organisms have many species; others haveonly a few. Hundreds of species of Drosophila evolved in theHawaiian Islands, but there is only one species of horseshoecrab, even though its lineage has survived for more than 300million years. Why do rates of speciation vary so widelyamong lineages?A number of factors are known to influence speciationrates:Species richness. The larger the number of species in a lineage,the larger the number of opportunities for newspecies to form. For speciation by polyploidy, the morespecies in a lineage, the more species are available tohybridize with one another. For allopatric speciation, thelarger the number of species living in an area, the largerthe number of species whose ranges will be bisected bya given physical barrier.Dispersal rates. Individuals of species with poor dispersalabilities are unlikely to establish new populations by dispersingacross barriers. Even narrow barriers are effectivein dividing species whose members are highlysedentary.Ecological specialization. Populations of species restrictedto habitat types that are patchy in distribution are morelikely to diverge than are populations that occupy relativelycontinuous habitats.Population bottlenecks. The changes in gene pools thatoften occur when a population passes through a bottleneckmay result in new adaptations.Type of pollination. Speciation rates in plants are correlatedwith pollination mode. Animal-pollinated plant familieshave, on average, 2.4 times as many species as closelyrelated families pollinated by wind. In addition, theswitch from animal to wind pollination is strongly associatedwith a reduction in the rate of speciation in a lineage.Sexual selection. Animals with complex behavior are likelyto form new species at a high rate because they makesophisticated discriminations among potential matingpartners. They distinguish members of their own speciesfrom members of other species, and they make subtlediscriminations among members of their own species onthe basis of size, shape, appearance, and behavior (seeFigures 23.16 and 23.17). Such discriminations can greatlyinfluence which individuals are most successful inproducing offspring and may lead to rapid reinforcementof reproductive isolation between species.Environmental changes. Oscillations of climates may fragmentpopulations of species that live in formerly continuoushabitats.Number of new antelope speciesappearing in the fossil record201510504 3 2 1Millions of years agoAfrican antelopes provide a striking example of the influenceof climate change on speciation rates. These animals experienceda burst of speciation and extinction between 2.5and 2.9 mya. During that period, the number of known antelopespecies doubled, and 90 percent of all known specieseither first appeared or became extinct (Figure 24.15). Thisburst coincided with a shift in Africa from a warm, wet climateto one that oscillated between warm and wet and coolerbut drier conditions. During this period, grassland and savannaenvironments increased and decreased, repeatedly coalescingand separating over much of Africa as the climateoscillated. The burst of speciation among antelopes resultedin many new species adapted to these environments.Evolutionary RadiationsMany new speciesappeared 2.5–2.9 mya.24.15 Climate Change Drove a Burst of Speciation amongAntelopes The excellent fossil record of African antelopes revealsthat there was a sudden burst of speciation between 2.5 and 2.9 millionyears ago. At that time, the climate of Africa shifted from beingconsistently warm and wet to oscillating between warm and wet andcool and dry.The fossil record reveals that, at certain times in certain lineages,speciation rates have been much higher than extinctionrates. The result is the proliferation of a large number ofdaughter species, as happened with finches in the Galápagosarchipelago (see Figure 24.6). Such an event is called an evolutionaryradiation. What conditions cause speciation ratesto be much higher than extinction rates?Evolutionary radiations are likely when a population colonizesa new environment that contains relatively fewspecies. As we saw in Chapter 22, evolutionary radiations occurredin many lineages on continents following mass extinctions.Evolutionary radiations occur on islands becauseislands lack many plant and animal groups found on themainland. The ecological opportunities that exist on islandsmay stimulate rapid evolutionary changes when a newspecies does reach them. Water barriers also restrict geneflow among the islands in an archipelago, so populations on

SPECIES AND THEIR FORMATION 49324.16 Rapid Evolution among HawaiianSilverswords The Hawaiian silverswords, threeclosely related genera of the sunflower family, arebelieved to have descended from a single commonancestor, similar to the tarweed (Madia sativa), thatcolonized Hawaii from the Pacific coast of NorthAmerica. The four plants shown here are moreclosely related than they appear to be based ontheir morphology.Madia sativa (tarweed)Argyroxiphium sandwicenseDubautia menziesiiWilkesia hobdyidifferent islands may evolve adaptations to their local environments.Together, these two factors make it likely that speciationrates of newly colonizing lineages on island archipelagoeswill exceed extinction rates.Remarkable evolutionary radiations have occurred in theHawaiian Islands, the most isolated islands in the world. TheHawaiian Islands lie 4,000 km from the nearest major landmass and 1,600 km from the nearest group of islands. The nativebiota of the Hawaiian Islands includes 1,000 species offlowering plants, 10,000 species of insects, 1,000 land snails,and more than 100 bird species. However, there were no amphibians,no terrestrial reptiles, and only one native mammal—abat—on the islands until humans introduced additionalspecies. The 10,000 known native species of insects onHawaii are believed to have evolved from only about 400 immigrantspecies; only 7 immigrant species are believed to accountfor all the native Hawaiian land birds.More than 90 percent of all plant species on the HawaiianIslands are endemic—that is, they are found nowhereelse. Several groups of flowering plants have more diverseforms and life histories on the islands, and live in a widervariety of habitats, than do their close relatives on themainland. An outstanding example is the group of Hawaiiansunflowers called silverswords (the genera Argyroxiphium,Dubautia, and Wilkesia). Chloroplast DNA sequencesshow that these species share a relatively recent commonancestor with a species of tarweed from the Pacific coast ofNorth America (Figure 24.16). Whereas all mainland tarweedsare small, upright, herbs (that is, nonwoody plants),the silverswords include prostrate and upright herbs,shrubs, trees, and vines. Silverword species occupy nearlyall the habitats of the Hawaiian islands, from sea level toabove timberline in the mountains. Despite their extraordinarymorphological diversification, however, the silver-

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.