WZ BISC 120 Sept. 14

WZ BISC 120 Sept. 14 - Biological S nce 120 cie s Le cture7...

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Evolution of Populations Biological Sciences 120 Lecture 7, Sept. 14 Wiebke Ziebis Chapter 23 short
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Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
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Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component
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Fig. 23-2 (a) (b)
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Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Quantitative characters vary along a continuum within a population
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Variation Between Populations Most species exhibit geographic variation , differences between gene pools of separate populations or population subgroups
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Fig. 23-3 13.17 19 XX 10.16 9.12 8.11 1 2.4 3.14 5.18 6 7.15 9.10 1 2.19 11.12 13.17 15.18 3.8 4.16 5.14 6.7 XX
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Some examples of geographic variation occur as a cline , which is a graded change in a trait along a geographic axis
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Fig. 23-4 1.0 0.8 0.6 0.4 0.2 0 46 44 42 40 38 36 34 32 30 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency
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Mutation Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring
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Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrangemany loci aretypically harmful Duplication of largechromosomesegments is usually harmful Duplication of small pieces of DNA is sometimes less harmful and increases thegenomesize Duplicated genes can takeon new functions by further mutation
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Sexual Reproduction Sexual reproduction can shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible
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Concept 23.2: The Hardy-Weinberg equation can beused to test whether a population is evolving The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
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Gene Pools and Allele Frequencies A population is a localized group of individuals capable of interbreeding and producing fertile offspring A gene pool consists of all the alleles for all loci in a population A locus is fixed if all individuals in a population are homozygous for the same allele
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Fig. 23-5 Porcupine herd Porcupine herd range Beaufort Sea NORTHWEST TERRITORIES MAP AREA ALASKA CANADA Fortymile herd range Fortymile herd ALASKA YUKON One species Two populations
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The frequency of an allelein a population can be calculated For diploid organisms, thetotal number of alleles at a locus is thetotal number of individuals x 2
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