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GENETICS & EVOLUTION: POPULATION GENETICS Chapter 3.3 Overview Microevolution and mutation Hardy-Weinberg Principle and Equilibrium Natural Selection Genetic drift Gene flow Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a population over generations Fig. 23-1 Mutation and sexual reproduction produce the genetic variation that makes evolution possible Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Gene Pools and Allele Frequencies • • The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population 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 Fig. 23-5 Porcupine herd MAP AREA Beaufort Sea Porcupine herd range Fortymile herd range Fortymile herd Fig. 23-5a MAP AREA Beaufort Sea Porcupine herd range Fortymile herd range The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 For example, p + q = 1 The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population Fig. 23-6 Alleles in the population Frequencies of alleles p = frequency of CR allele = 0.8 q = frequency of CW allele = 0.2 Gametes produced Each egg: 80% chance 20% chance Each sperm: 80% chance 20% chance Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Fig. 23-7-1 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) 64% (p2) CRCR 16% (pq) CRCW 16% (qp) CRCW 4% (q2) CW CW Fig. 23-7-2 64% CRCR, 32% CRCW, and 4% CWCW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q Fig. 23-7-3 64% CRCR, 32% CRCW, and 4% CWCW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q Genotypes in the next generation: 64% CRCR, 32% CRCW, and 4% CWCW plants Fig. 23-7-4 20% CW (q = 0.2) 80% CR ( p = 0.8) Sperm CR (80%) CW (20%) 64% ( p2) CR CR 16% ( pq) CR CW 16% (qp) CR CW 4% (q2) CW CW 64% CR CR, 32% CR CW, and 4% CW CW Gametes of this generation: 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW = 20% CW = 0.2 = q + 16% CW Genotypes in the next generation: 64% CR CR, 32% CR CW, and 4% CW CW plants Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population In real populations, allele and genotype frequencies do change over time The five conditions for nonevolving populations are rarely met in nature: No mutations Random mating No natural selection Extremely large population size No gene flow Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci Applying the Hardy-Weinberg Principle We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: The PKU gene mutation rate is low Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions The population is large Migration has no effect as many other populations have similar allele frequencies The occurrence of PKU is 1 per 10,000 births q2 q = 0.01 The frequency of normal alleles is p = 0.0001 = 1 – q = 1 – 0.01 = 0.99 The frequency of carriers is 2pq or = 2 x 0.99 x 0.01 = 0.0198 approximately 2% of the U.S. population Required conditions are rarely (if ever) met Changes in gene pool frequencies are likely When gene pool frequencies change, microevolution has occurred Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place Hence, microevolution (via genetic mutations) The raw material for evolutionary change Provides new combinations of alleles Some might be more adaptive than others Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow You should now be able to: 1. 2. 3. 4. Explain why the majority of point mutations are harmless Explain how sexual recombination generates genetic variability Define the terms population, species, gene pool, relative fitness, and neutral variation List the five conditions of Hardy-Weinberg equilibrium 5. 6. 7. Apply the Hardy-Weinberg equation to a population genetics problem Explain why natural selection is the only mechanism that consistently produces adaptive change Explain the role of population size in genetic drift Allele Frequencies Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies? a. b. c. d. red = 0.36, white = 0.16 red = 0.6, white = 0.4 red = 0.5, white = 0.5 Allele frequencies cannot be determined unless the population is in equilibrium. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Hardy-Weinberg Equilibrium Determination Which of these populations are in HardyWeinberg equilibrium? a. b. c. d. A B both A and B neither A nor B Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Cystic Fibrosis 1 The frequency of cystic fibrosis, a recessive genetic disease, is 1 per 2,500 births among Northern Europeans. Assuming random mating, what is the frequency of carriers? a. b. c. d. 1/2,500 1/50 1/25 The frequency cannot be calculated because selection violates Hardy-Weinberg assumptions. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Cystic Fibrosis 2 Until the 1950s, infants born with cystic fibrosis did not survive longer than a few months. If the frequency of carriers was 4% in the year 1900, what proportion of CF alleles was eliminated in one generation? a. b. c. d. e. 100% 50% 4% 2% <0.1% Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.