Species and Their Formation - Laboratory of Visual Systems

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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-
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