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Chapter 23: Population Genetics (Microevolution)  Microevolution Is a Change in Allele Frequencies Or Genotype Frequencies in a Population Over Time

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Chapter 23: Population Genetics (Microevolution)  Microevolution Is a Change in Allele Frequencies Or Genotype Frequencies in a Population Over Time Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time Genetic equilibrium in populations: the Hardy-Weinberg theorem Microevolution is deviation from Hardy- Weinberg equilibrium Genetic variation must exist for natural selection to occur . • Explain what terms in the Hardy- Weinberg equation give: – allele frequencies (dominant allele, recessive allele, etc.) – each genotype frequency (homozygous dominant, heterozygous, etc.) – each phenotype frequency . Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time Genetic equilibrium in populations: the Hardy-Weinberg theorem Microevolution is deviation from Hardy- Weinberg equilibrium Genetic variation must exist for natural selection to occur . Microevolution is a change in allele frequencies or genotype frequencies in a population over time population – a localized group of individuals capable of interbreeding and producing fertile offspring, and that are more or less isolated from other such groups gene pool – all alleles present in a population at a given time phenotype frequency – proportion of a population with a given phenotype genotype frequency – proportion of a population with a given genotype allele frequency – proportion of a specific allele in a population . Microevolution is a change in allele frequencies or genotype frequencies in a population over time allele frequency – proportion of a specific allele in a population diploid individuals have two alleles for each gene if you know genotype frequencies, it is easy to calculate allele frequencies example: population (1000) = genotypes AA (490) + Aa (420) + aa (90) allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600) freq[A] = 1400/2000 = 0.70 freq[a] = 600/2000 = 0.30 note that the sum of all allele frequencies is 1.0 (sum rule of probability) . Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time Genetic equilibrium in populations: the Hardy-Weinberg theorem Microevolution is deviation from Hardy- Weinberg equilibrium Genetic variation must exist for natural selection to occur . • Explain what terms in the Hardy- Weinberg equation give: – allele frequencies (dominant allele, recessive allele, etc.) – each genotype frequency (homozygous dominant, heterozygous, etc.) – each phenotype frequency . Make up and do some pop gen problems. Suggestions: Start with a population of either 100 or 10,000 individuals. Have the number of individuals with the recessive phenotype be the square of a whole number (so that q2 is easy to solve). Then answer the frequency questions below. What is the frequency in the population of: • the recessive phenotype? • the dominant phenotype? • the dominant allele? • the recessive allele? • homozygous recessive individuals? • homozygous dominant individuals? • heterozygous individuals? . The Hardy-Weinberg Theorem the Hardy-Weinberg theorem describes the frequencies of genotypes in a population based on the frequency of occurrence of alleles in the population that is in a state of genetic equilibrium (that is, not evolving) the usual case for calculations: if allele “A” is dominant to “a”, and they are the only two alleles possible at the A-locus, then p = freq[A] = the frequency of occurrence of the A-allele in the population q = freq[a] = the frequency of occurrence of the a-allele in the population Then p + q = 1 (following the sum rule for probability) . The Hardy-Weinberg Theorem Allele associations follow the product rule for probability, so you multiply to predict the genotype frequencies: ( p + q ) x ( p + q ) = p2 + 2 pq + q2 p2 = frequency of homozygous dominant genotypes 2 pq = frequency of heterozygous genotypes q2 = frequency of homozygous recessive genotypes note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so p2 + 2 pq + q2 = 1 . Hardy-Weinberg Equilibrium if the Hardy-Weinberg theorem can be used to accurately predict genotype frequencies from allele frequencies for a population then… the population is in Hardy-Weinberg equilibrium or genetic equilibrium in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next generation such populations are not evolving, but are static instead . Make up and do some pop gen problems. Suggestions: Start with a population of either 100 or 10,000 individuals. Have the number of individuals with the recessive phenotype be the square of a whole number (so that q2 is easy to solve). Then answer the frequency questions below. What is the frequency in the population of: • the recessive phenotype? • the dominant phenotype? • the dominant allele? • the recessive allele? • homozygous recessive individuals? • homozygous dominant individuals? • heterozygous individuals? . • Describe the assumptions of the Hardy- Weinberg equilibrium model. Hardy-Weinberg Equilibrium the assumptions of this model are: large population size (due to statistical constraints, to minimize genetic drift) no migration – no exchange of alleles with other populations (no gene flow) no mutations of the alleles under study occur random mating of all genotypes no natural selection . • Describe the assumptions of the Hardy- Weinberg equilibrium model. Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time Genetic equilibrium in populations: the Hardy-Weinberg theorem Microevolution is deviation from Hardy- Weinberg equilibrium Genetic variation must exist for natural selection to occur . • Describe conditions that can keep populations from establishing or maintaining genetic equilibrium. Microevolution is a deviation from Hardy-Weinberg equilibrium if allele and/or genotype frequencies in a population change over time, then it is by definition evolving (evolution means changing over time), undergoing microevolution things that can cause microevolution small population size: genetic drift migration – gene flow; individuals leave and/or join a population mutations nonrandom mating natural selection . Microevolution is a deviation from Hardy-Weinberg equilibrium consequences of small population size: genetic drift Consider taking a small sample of individuals from a larger population If only two individuals were picked they almost certainly won’t reflect the allele frequency in the larger population (in many cases, they can’t even possibly do so). The same holds true for 3, 4, or 5 individuals. As the selected sample gets larger it becomes more likely that the sample reflects the allele frequency in the larger population. Mating to produce the next generation is effectively sampling the population It takes a very large sampling size (thousands) to have a good chance of the sample essentially matching the allele frequencies and genotype frequencies of the population. Microevolution is a deviation from Hardy-Weinberg equilibrium genetic drift is a change in gene frequencies of populations because of small population size . Microevolution is a deviation from Hardy-Weinberg equilibrium genetic drift tends to decrease genetic variation within a population genetic drift tends to increase genetic variation between populations NOTE: genetic drift places a major factor in evolution, especially when populations are split, but does NOT involve natural selection . Microevolution is a deviation from Hardy-Weinberg equilibrium consequences of small population size: genetic drift two general mechanisms lead to small population sizes genetic bottlenecks are created by dramatic reduction in the population size – endangered species face a genetic bottleneck on a species-wide scale, and suffer lasting effects even if population size later recovers . Microevolution is a deviation from Hardy-Weinberg equilibrium consequences of small population size: genetic drift two general mechanisms lead to small population sizes founder effect – when a new population is established, typically only a few individuals (founders) are involved in colonizing the new area, essentially an “isolation bottleneck” for the new population; this is common for islands . Microevolution is a deviation from Hardy-Weinberg equilibrium migration – when individuals leave or join a population migrating individuals carry their alleles with them (gene flow), usually resulting in changes in allele frequencies gene flow tends to decrease genetic variation between populations . Microevolution is a deviation from Hardy-Weinberg equilibrium mutations increase variation in the gene pool of a species remember that mutations may be neutral, harmful, or beneficial even at the risk of harmful effects, mutations are necessary to increase variation in the population so that natural selection can produce organisms more suited to their environment . Microevolution is a deviation from Hardy-Weinberg equilibrium nonrandom mating if individuals do not mate at random, then Hardy-Weinberg equilibrium is not achieved the most common cases of nonrandom mating involve inbreeding – mating between individuals of similar genotypes, either by choice or due to environmental factors such as location . Microevolution is a deviation from Hardy-Weinberg equilibrium inbreeding does not change allele frequencies, but increases the frequency
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