Species and Their Formation - Laboratory of Visual Systems

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.