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Published byBrandon Clarke Modified over 9 years ago
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Evolution of Populations
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The Smallest Unit of Evolution Natural selection acts on individuals, but only populations evolve – Genetic variations contribute to evolution
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Population genetics – study of how populations change genetically over time Mendelian genetics with the Darwinian theory populations as units of evolution
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Gene Pools and Allele Frequencies Population localized group of individuals capable of interbreeding and producing fertile offspring gene pool – total aggregate of genes in a population at any one time – all gene loci in all individuals of the population
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The Hardy-Weinberg Theorem population that is not evolving frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work preservation of genetic variation in a population
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Hardy-Weinberg Equilibrium The five conditions for non-evolving populations are rarely met in nature: – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection
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Hardy-Weinberg Equilibrium If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p 2 + 2pq + q 2 = 1 – And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
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LE 23-4 Generation 3 25% C R C R Generation 4 50% C R C W 25% C W C W 50% C W gametes 50% C R come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions gametes Generation 2 Generation 1 CRCRCRCR CWCWCWCW genotype Plants mate All C R C W (all pink flowers) 50% C R 50% C W gametes come together at random X
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Evolutionary Change Three major factors alter allele frequencies and bring about most evolutionary change: – Mutations – Natural selection – Nonrandom Mating – Genetic drift – Gene flow
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Variations that make Natural Selection Possible Mutation – changes in the nucleotide sequence of DNA – new genes and alleles to arise – Point Mutations change in one base in a gene usually harmless may impact on phenotype
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Mutations Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Gene duplication is nearly always harmful
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Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
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3 conditions for natural selection to occur and to result in evolutionary change 1.Variation must exist among individuals in a population 2.Variation among individuals must result in differences in the number of offspring surviving in the next generation 3.Variation must be genetically inherited 14
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Sexual Recombination far more important than mutation produces the genetic differences that make adaptation possible
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Nonrandom mating – Assortative mating Phenotypically similar individuals mate Increases proportion of homozygous individuals – Disassortative mating Phenotypically different individuals mate Produces excess of heterozygotes
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Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result allele frequencies fluctuate unpredictably from one generation to the next reduces genetic variation through losses of alleles
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Genetic Drift The Bottleneck Effect – sudden change in the environment that may drastically reduce the size of a population – gene pool may no longer be reflective of the original population’s gene pool
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Genetic Drift The Founder Effect – a few individuals become isolated from a larger population – affects allele frequencies
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Gene Flow genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes gain or loss of alleles reduce differences between populations over time
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A Closer Look at Natural Selection From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations
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Evolutionary Fitness Misleading – “struggle for existence” – “survival of the fittest” Fitness – contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness – contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus
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Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: – Directional favors individuals at one end of the phenotypic range – Disruptive favors individuals at both extremes of the phenotypic range – Stabilizing favors intermediate variants and acts against extreme phenotypes
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The Preservation of Genetic Variation Diploidy – maintains genetic variation in the form of hidden recessive alleles Balancing selection – natural selection maintains stable frequencies of two or more phenotypic forms
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Heterozygote Advantage – Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes – Natural selection will tend to maintain two or more alleles at that locus – Sickle cell and malaria
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Sexual selection – natural selection for mating success – sexual dimorphism differences between the sexes in secondary sexual characteristics Intrasexual selection – competition among individuals of one sex for mates of the opposite sex Intersexual selection – individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex
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Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations
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