rav65819_ch20_395-414

rav65819_ch20_395-414 - part IV evolution 20 Genes Within...

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;;;;;;;;;;; part IV evolution 20 Genes Within Populations
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chapter introduction NO OTHER HUMAN BEING is exactly like you (unless you have an identical twin). Often the particular characteristics of an individual have an important bearing on its survival, on its chances to reproduce, and on the success of its offspring. Evolution is driven by such consequences, as different alleles rise and fall in populations. These deceptively simple matters lie at the core of evolutionary biology, which is the topic of this chapter and chapters 21 through 25. 20.6 Maintenance of Variation Frequency-dependent selection may favor either rare or common phenotypes In oscillating selection, the favored phenotype changes as the environment changes In some cases, heterozygotes may exhibit greater ±tness than homozygotes concept outline 20.1 Genetic Variation and Evolution Natural selection is an important mechanism of evolutionary change 20.2 Changes in Allele Frequency The Hardy–Weinberg principle describes stable populations Hardy–Weinberg predictions can be applied to data to ±nd evidence of evolutionary processes 20.3 Five Agents of Evolutionary Change Mutation changes alleles Gene flow occurs when alleles move between populations
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Nonrandom mating shifts genotype frequencies Genetic drift may alter allele frequencies in small populations Selection favors some genotypes over others 20.4 Fitness and Its Measurement A phenotype with greater Ftness usually increases in frequency ±itness may consist of many components 20.5 Interactions Among Evolutionary Forces Mutation and genetic drift may counter selection Gene flow may promote or constrain evolutionary change 20.7 Selection Acting on Traits Affected by Multiple Genes Disruptive selection removes intermediates Directional selection eliminates phenotypes on one end of a range Stabilizing selection favors individuals with intermediate phenotypes 20.8 Experimental Studies of Natural Selection Guppy color variation in different environments suggests natural selection at work Experimentation reveals the agent of selection 20.9 The Limits of Selection Genes have multiple effects Evolution requires genetic variation Gene interactions affect Ftness of alleles 395 rav65819_ch20_395-414.indd 395 rav65819_ch20_395-414.indd 395 1/2/07 6:22:06 PM 1/2/07 6:22:06 PM 20.1 Genetic Variation and Evolution Genetic variation, that is, differences in alleles of genes found within individuals of a population, provides the raw material for natural selection, which will be described shortly. Natural populations contain a wealth of such variation. In plants, insects, and vertebrates, many genes exhibit some level of variation. In this chapter, we explore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in natural populations to change. The word
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rav65819_ch20_395-414 - part IV evolution 20 Genes Within...

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