Chapter Learning Objectives Chapter 7: The Genetics of Populations ...
Chapter Learning Objectives Chapter 7: The Genetics of Populations 7.1 Individual-Level versus Population-Level Thinking • Describe how population-level thinking is an extrapolation of individual-level thinking and how it can lead to unexpected outcomes. Population level of thinking takes into account overall frequencies of alleles and phenotypes. The shift from individual level of thinking (prevalent in the study of genetics) to population level thinking (associate with ecology and evolution) is critical in understanding the process of evolutionary change. Example: a genotype that is dominant in individuals might not be dominant in a population of individuals because the frequency of the population that contains that trait 7.2 The Hardy-Weinberg Model: A Null Model for Population Genetics • Describe how the Hardy-Weinberg model serves as a null-model in population genetics o Hardy Weinberg serves as a null model because it tells what happens to genotype frequencies when natural selection and other important drivers of evolutionary change are not operating • List the assumptions that go along with the Hardy-Weinberg model o Natural selection is not operating on the trait or traits affected by the locus in question, Individuals have no preference for others with similar or dissimilar genotypes (random mating), No mutation is occurring, no migration into or out of the population, the population is effectively infinite in size such that genetic drift does not affect the allele frequency • Given a genotype or allele frequency, calculate the frequencies of the other genotypes and alleles o Genotype frequency ex: p^2,q^2, 2pq o Allele frequency ex: p,q o p+q=1 p^2 + 2pq + q^2 = 1 7.3 Natural Selection • Given certain survivorship statistics, calculate the fitness and selection coefficient (s) o Selection coefficient = s 1 – s = the fitness of a phenotype. o A1 dominant to A2. Fitness A1= 1. Fitness A2= 1 – s • Predict the rates of fixation under directional selection if the A1 allele is dominant, the A1 and A2 alleles are codominant, and if the A1 allele is recessive. o If A1 is dominant, its initial increase in frequency is the most rapid, but its pace slows once it is common in the population. If A1 and A2 are codominant, then A1 reaches fixation the fastest. If A1 is recessive, it takes much longer to increase in frequency, but once common it goes to fixation quickly • Predict allele frequencies in situations of overdominance and underdominance. o Overdominance – aka heterozygote advantage, the A1A2 heterozygote has a higher fitness than either the A1A1 or the A2A2 homozygotes. Due to overdominance, A1 will increase in frequency when rare and decrease in frequency when commonso that an intermediate frequency is met so long as both alleles are present in the population o Underdominance – is the reverse of overdominance. The A1A2 heterozygote has a lower fitness than either the A1A1 or the A2A2 genotype and natural selection will favor one allele over the other but which allele that becomes fixed depends on where the population starts. There is a threshold frequency at which an allele will be fixed or lost depending on whether it is above or below this line.
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Predict allele frequencies in situations of positive frequency-dependent selection and negative frequency-dependent selection. o Positive frequency dependent selection – the fitness associated with a trait increases as the frequency of the trait increases in a population. This means that each phenotype is favored once it becomes sufficiently common in the population. Ex: Snails shell coiling and mating; more common the style of coiling, the more mates are available o Negative frequency dependent selection – the fitness associated with a trait decreases as the frequency of the trait increases in a population. This means that each phenotype is favored when it is rare. Ex: African fish that attack prey from either the left or right; more rare gets more food.
7.4 Mutation • Describe how mutation can introduce new alleles and change gene frequencies in a population. o The process for mutation to change allele frequencies in a population takes thousands of years but can be mathematically calculated. If mutation is the only process operating to change allele frequency, then the equilibrium frequency of p = ? / (μ + ?) and q = μ / (μ + ?) where μ= the rate A1 mutates to A2 and ?= the rate A2 mutates to A1 • Explain how mutation-selection balance can maintain deleterious alleles within a population at low frequencies. o Mutation selection balance – the equilibrium at which the action of natural selection to increase the frequency of A1 is exactly balanced by the action of mutation to produce new A2 alleles. o If the deleterious allele is recessive then the wildtype allele si p=1 – √(μ/s) and the recessive deleterious allele is q=√(μ/s) o If the deleterious allele is dominant the wildtype allele is p=(s – μ)/s and the frequency for the dominant deleterious allele is q=μ/s • Calculate the equilibrium frequency of an allele if the deleterious allele is recessive or dominant. 7.5 Nonrandom Mating • Describe disassortative and assortative mating o Disassortative mating – when individuals tend to mate with those of different genotypes or phenotypes o Assortative mating – when individuals tend to mate with those of the same genotype or phenotype • Describe how a gene copy can be identical by descent and how inbreeding can affect allele frequencies. o Because of inbreeding, when individuals mate with genetic relatives, gametes are not paired at random but instead they are preferentially paired with gametes from close relatives. Thus, in inbreed populations a pair of gene copies may be identical by descent meaning they may be identical because of shared descent through a recent ancestor. 7.6 Migration • Describe what happens to allele frequencies as a consequence of migration.
When individuals migrate into a population they may bring new or previously uncommon alleles with them and alter the allele frequency previously established Given allele frequencies in a source and recipient population, calculate the change in allele frequency in the recipient population after one generation. o ONLY MIGRATION in effect. k=fraction of the island population made up by new migrants from the mainland. Pi = initial frequency of A1 allele on the island. Pm = initial frequency of A1 allele on the mainland. F[A1,A1] = (1 – k)Pi^2 + k(Pm^2). o Change of allele frequency on island as a result of migration. Pi’ = (1 – k)Pi + kPm o Net change in allele frequencies: deltaPi = Pi’ – Pi = k(Pm-Pi) o
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Predict allele frequencies in situations of positive frequency-dependent selection and negative frequency-dependent selection. o Positive frequency dependent selection – the fitness associated with a trait increases as the frequency of the trait increases in a population. This means that each phenotype is favored once it becomes sufficiently common in the population. Ex: Snails shell coiling and mating; more common the style of coiling, the more mates are available o Negative frequency dependent selection – the fitness associated with a trait decreases as the frequency of the trait increases in a population. This means that each phenotype is favored when it is rare. Ex: African fish that attack prey from either the left or right; more rare gets more food.
7.4 Mutation • Describe how mutation can introduce new alleles and change gene frequencies in a population. o The process for mutation to change allele frequencies in a population takes thousands of years but can be mathematically calculated. If mutation is the only process operating to change allele frequency, then the equilibrium frequency of p = ? / (μ + ?) and q = μ / (μ + ?) where μ= the rate A1 mutates to A2 and ?= the rate A2 mutates to A1 • Explain how mutation-selection balance can maintain deleterious alleles within a population at low frequencies. o Mutation selection balance – the equilibrium at which the action of natural selection to increase the frequency of A1 is exactly balanced by the action of mutation to produce new A2 alleles. o If the deleterious allele is recessive then the wildtype allele si p=1 – √(μ/s) and the recessive deleterious allele is q=√(μ/s) o If the deleterious allele is dominant the wildtype allele is p=(s – μ)/s and the frequency for the dominant deleterious allele is q=μ/s • Calculate the equilibrium frequency of an allele if the deleterious allele is recessive or dominant. 7.5 Nonrandom Mating • Describe disassortative and assortative mating o Disassortative mating – when individuals tend to mate with those of different genotypes or phenotypes o Assortative mating – when individuals tend to mate with those of the same genotype or phenotype • Describe how a gene copy can be identical by descent and how inbreeding can affect allele frequencies. o Because of inbreeding, when individuals mate with genetic relatives, gametes are not paired at random but instead they are preferentially paired with gametes from close relatives. Thus, in inbreed populations a pair of gene copies may be identical by descent meaning they may be identical because of shared descent through a recent ancestor. 7.6 Migration • Describe what happens to allele frequencies as a consequence of migration.
When individuals migrate into a population they may bring new or previously uncommon alleles with them and alter the allele frequency previously established Given allele frequencies in a source and recipient population, calculate the change in allele frequency in the recipient population after one generation. o ONLY MIGRATION in effect. k=fraction of the island population made up by new migrants from the mainland. Pi = initial frequency of A1 allele on the island. Pm = initial frequency of A1 allele on the mainland. F[A1,A1] = (1 – k)Pi^2 + k(Pm^2). o Change of allele frequency on island as a result of migration. Pi’ = (1 – k)Pi + kPm o Net change in allele frequencies: deltaPi = Pi’ – Pi = k(Pm-Pi) o
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