Species and Their Formation - Laboratory of Visual Systems

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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.)
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