Species and Their Formation - Laboratory of Visual Systems

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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.
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