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Chapter 14 The Origin of Species RAPID EVOLUTION • One example of rapid evolution occurred among mosquitoes who migrated into the London underground • In less than 150 years, Culex pipiens evolved into a new mosquito species, Culex molestus • The origin of new species is called speciation • The isolated mosquitoes adapted to their new underground environment – They altered their prey, mating habits, and breeding patterns • Environmental barriers that isolate populations are just one of many mechanisms in the evolution of species CONCEPTS OF SPECIES What is a species? • Linnaeus used physical appearance to identify species when he developed the binomial system of naming organisms – This system established the basis for taxonomy • But appearance alone does not always define a species – Example: eastern and western meadowlarks Figure 14.1A • Similarities between some species and variation within a species can make defining species difficult – Humans exhibit extreme physical diversity Figure 14.1B • A ring species may illustrate the process of speciation 1 OREGON POPULATION Sierra Nevada COASTAL POPULATIONS Yelloweyed Yellowblotched 2 Gap in ring Monterey INLAND POPULATIONS Largeblotched 3 Figure 14.1C • The biological species concept is not applicable to fossils or asexual organisms • Most organisms are classified based on observable phenotypes – The morphological species concept • The genealogical species concept defines a species as a cluster of organisms representing a specific evolutionary lineage • The ecological species concept defines a species by its ecological role • Reproductive barriers keep species separate Prezygotic and postzygotic reproductive barriers prevent individuals of different species from interbreeding Table 14.2 • Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation • Many plant species have flower structures that are adapted to specific pollinators – This is an example of mechanical isolation, another prezygotic barrier Figure 14.2A, B • Hybrid sterility is one type of postzygotic barrier – A horse and a donkey may produce a hybrid offspring, a mule – Mules are sterile Figure 14.2C MECHANISMS OF SPECIATION Geographic isolation can lead to speciation • When a population is cut off from its parent stock, species evolution may occur – An isolated population may become genetically unique as its gene pool is changed by natural selection, genetic drift, or mutation – This is called allopatric speciation Figure 14.3 Islands are living laboratories of speciation • On the Galápagos Islands, repeated isolation and adaptation have resulted in adaptive radiation of 14 species of Darwin’s finches Figure 14.4A • Adaptive radiation on an island chain 1 A Species A from mainland 2 B B 3 B C B 4 C C D C C D 5 Figure 14.4B New species can also arise within the same geographic area as the parent species • In sympatric speciation, a new species may arise without geographic isolation – A failure in meiosis can produce diploid gametes – Self-fertilization can then produce a tetraploid zygote Parent species Zygote Meiotic error Selffertilization 2n = 6 Diploid Offspring may be viable and self-fertile 4n = 12 Tetraploid Unreduced diploid gametes Figure 14.5A • Sympatric speciation by polyploidy was first discovered by Dutch botanist Hugo de Vries in the early 1900s Figure 14.5B Connection: Polyploid plants clothe and feed us • Many plants are polyploid – They are the products of hybridization – The modern bread wheat is an example Figure 14.6A • The evolution of wheat AA BB Wild Triticum (14 chromosomes) Triticum monococcum (14 chromosomes) AB Sterile hybrid (14 chromosomes) Meiotic error and self-fertilization AABB DD T. turgidum EMMER WHEAT (28 chromosomes) T. tauschii (wild) (14 chromosomes) ABD Sterile hybrid Meiotic error and self-fertilization AA BB DD T. aestivum BREAD WHEAT (42 chromosomes) Figure 14.6B Reproductive barriers may evolve as populations diverge • This has been documented by Initial sample of fruit flies Starch medium Maltose medium Results of mating experiments Female populations Same Different 8 20 Mating frequencies in experimental group Same 9 Different 22 Male populations Female Starch Maltose Male Maltose Starch – laboratory studies (fruit flies) 18 15 12 15 Mating frequencies in control group Figure 14.7A – examples in natural populations (pupfish in Death Valley) Figure 14.7B The tempo of speciation can appear steady or jumpy • According to the gradualist model of the origin of species – new species evolve by the gradual accumulation of changes brought about by natural selection • However, few gradual transitions are found in the fossil record Figure 14.8A • The punctuated equilibrium model suggests that speciation occurs in spurts – Rapid change occurs when an isolated population diverges from the ancestral stock – Virtually no change occurs for the rest of the species’ existence Figure 14.8B