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Tamarin: Principles of Genetics, Seventh Edition
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21. Evolution and Speciation
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How does one species become two Basically, reproductive isolating mechanisms must evolve to prevent two subpopulations from interbreeding when they come into contact. Reproductive isolating mechanisms are environmental, behavioral, mechanical, and physiological barriers that prevent individuals of two species from producing viable offspring. Following is a modi cation of the classi cation system of isolating mechanisms suggested by evolutionary biologist G. L. Stebbins: 1. Prezygotic mechanisms prevent fertilization and zygote formation. a. Residential The populations live in the same region, but occupy different habitats. b. Seasonal or temporal The populations exist in the same region, but are sexually mature at different times. c. Ethological (in animals only) The populations are isolated by incompatible premating behavior. d. Mechanical Cross-fertilization is prevented or restricted by incompatible differences in reproductive structures. 2. Postzygotic mechanisms affect the hybrid zygotes after fertilization has taken place. a. F1 hybrid breakdown F1 hybrids are inviable or weak. b. Developmental hybrid sterility Hybrids are sterile because gonads develop abnormally or because meiosis breaks down before it is completed. c. Segregational hybrid sterility Hybrids are sterile because of abnormal distribution to the gametes of whole chromosomes, chromosome segments, or combinations of genes. d. F2 breakdown F1 hybrids are normal, vigorous, and fertile, but the F2 generation contains many weak or sterile individuals.
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Reproductive isolating mechanisms are barriers to gene ow, the spread of genes between populations. These isolating mechanisms can evolve in three different ways, each of which de nes a different mechanism of speciation. Usually, the mode of speciation is dictated by both the properties of the genetic systems of the organisms and stochastic (random) or accidental events. For example, vertebrates tend to have different speciation modes than phytophagous (plant-feeding) insects. The appearance of a geographic barrier, such as a river or mountain, through the range of a species physically isolates populations of the species. Physical isolation can also occur if migrants cross a particular barrier
and begin a new population (founder effect). The physically isolated populations can then evolve independently. If reproductive isolating mechanisms evolve, then two distinct species are formed, and if they come together in the future, they remain distinct species. Speciation that occurs because reproductive isolating mechanisms evolve during physical separation of the populations is called allopatric speciation ( g. 21.3). As evolutionary biologist Guy Bush pointed out, Although examples in nature are dif cult to substantiate . . . it [allopatric speciation] has been convincingly demonstrated in frogs . . . and lizards. Reproductive isolating mechanisms usually originate incidentally to the speciation process.That is, they arise incidentally during the process of evolution in isolated populations rather than being selected for. When isolated populations come together again, incomplete isolating mechanisms may allow hybrids to form. If the hybrids are normal and viable and can freely interbreed with individuals of each parent population, then no speciation has taken place. However, if the hybrids are at a disadvantage, natural selection may favor stronger isolating mechanisms. In this case, organisms that mate with individuals from the other population leave fewer offspring. The result is a more effective barrier to hybridization. Regions in which previously isolated populations come into contact and produce hybrids are called hybrid zones. Until recently, evolutionary biologists believed that allopatric speciation was the general rule. Many now believe that two other modes of speciation may occur frequently in certain groups of organisms. Parapatric speciation occurs when a population of a species that occupies a large range enters a new niche or habitat ( g. 21.3). Although no physical barrier arises, the new niche acts as a barrier to gene ow between the population in the new niche and the rest of the species. Here again, reproductive isolating mechanisms evolve to produce two species where there was only one before. Parapatric speciation is believed to have occurred often in relatively nonvagile animals such as snails, ightless grasshoppers, and annual plants. Sympatric speciation occurs when a polymorphism, which is the occurrence of alternative phenotypes in the same population, arises within an interbreeding population before a shift to a new niche.This mode of speciation may be common in parasites and phytophagous insects. For example, if a polymorphism arises within a parasitic species that allows an individual with a certain genotype to adapt to a new host, this genotype may be the forerunner of a new species. If the parasite not only feeds on the new host but also mates on the new host, a barrier to gene ow arises, although the parasite may be surrounded by other members of its species with the original genotype. Sympatric speciation can thus occur in the middle of a species range rather than at the edges ( g. 21.3).
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