The Evolution of The Evolution of Populations Populations Chapter 23 Chapter 23 The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve during their lifetime Evolutionary processes e g natural selection acts on individuals but populations evolve Concept 23 1 Genetic variation makes evolution possible Two main processes produce genetic variation 1 Mutation 2 Sexual Recombination Mutation Mutations changes in nucleotide sequence of DNA Source of new alleles and genes Point mutation change in one Chromosomal mutations delete nucleotide base in a gene disrupt or rearrange many genes on a chromosome Gene duplications duplication of whole segments of a chromosome Sexual Recombination In sexually reproducing organisms sexual recombination produces most of the variability in each generation Crossing over during prophase I of meiosis Variation within a Population Discrete characters classified on an either or basis e g flower color in pea plants Quantitative characters vary along a continuum within a population e g height weight Concept 23 2 The Hardy Weinberg equation can be used to test whether a population is evolving Populations Figure 23 6 Population a group of individuals of the same species living in a certain defined area Hardy Weinberg Equilibrium H W equilibrium describes a population that is not evolving i e allele frequencies don t change Five assumptions behind H W equilibrium 1 No mutations 2 Large population size 3 No gene flow 4 No natural selection 5 Random mating Allele Genotype Allele frequencies Frequencies p frequency of allele 1 q frequency of allele 2 p q 1 Genotype frequencies p2 frequency of homozygous dominant q2 frequency of homozygous recessive 2pq frequency of heterozygotes p2 2pq q2 1 Hardy Weinberg equation H W Equilibrium Figure 23 7 If a population has the following genotype frequencies AA 0 42 Aa 0 46 aa 0 12 what are the allele frequencies 1 A 0 42 a 0 12 2 A 0 6 a 0 4 3 A 0 65 a 0 35 4 A 0 76 a 0 24 5 A 0 88 a 0 12 0 of 5 If the frequency of the recessive allele is 30 the frequency of the heterozygous carriers would be 1 9 2 27 3 42 4 60 Concept 23 3 Natural selection genetic drift and gene flow can alter allele frequencies in a population Genetic Drift Statistically the smaller a sample the greater the chance of deviation from a predicted result With small population sizes allele frequencies can fluctuate unpredictably from one generation to the next Tends to reduce genetic variation Figure 23 9Genetic Drift CR CR CR CR CW CW CR CR CR CR CW CW CR CR CR CR CW CW CR CR CR CR CR CW CR CW CR CW CR CW CR CR CR CW CW CW CR CR CR CR CR CW Generation 1 p frequency of CR 0 7 Fig 23 8 3 q frequency of CW 0 3 CR CW CR CW Generation 2 p 0 5 q 0 5 CR CR CR CR CR CR CR CR CR CR CR CR CR CR Generation 3 p 1 0 q 0 0 Founder effect Geneti c Drift Population bottleneck Chance events Gene Flow Genetic additions to or subtractions from a population resulting from the movement of fertile individuals or gametes Migration among populations Tends to reduce variation among populations over time little genetic variation between populations Migration high Population 1 Population 2 more genetic variation between populations Migration low Concept 23 4 Natural selection is the only mechanism that consistently causes adaptive evolution Natural selection differential success in the reproduction of different phenotypes resulting from their interactions with the environment Evolutionary Fitness Fitness the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals From the chart below who is the most fit Size Children Comments Tom 6 3 200 lbs 2 Awesome at life Absolutely crushing it Dick 5 10 185 lbs 3 Slightl y better than you at life Harry 5 5 125 lbs 5 Huge loser Really bad at life 3 Modes of Selection Directional favors individuals at one end of a phenotypic range Disruptive favors individuals at both extremes of a phenotypic range Stabilizing favors intermediate phenotypes and acts against extremes Directional Disruptive Stabilizing Selection Figure 23 13 A population of seedcracker finches has small and large billed birds specializing in soft and hard seeds respectively If climatic change resulted in a loss of the soft seeded plants what type of selection would then operate on the finch population 1 Disruptive 2 Directional 3 Stabilizing 4 Sexual Sexual Selection Type of selection that favors a trait giving an individual a competitive edge in attracting or keeping a mate Intrasexual selection direct competition among individuals of one sex for mates of the opposite sex Intrasexual selection Sexual Selection Intersexual selection individuals of one sex usually females are choosy in selecting their mates from individuals of the other sex Figure 23 15 What provides the selective pressure in artificial selection 1 The environment 2 A large number of offspring 3 Humans 4 Nothing it is a random process 0 of 5 IF the frequency of the dominant allele is 60 the frequency of the heterozygous carriers would be 48
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