Evolutionary Biology BIO 3350 Clemson
Evolutionary Biology BIO 3350 Clemson 12050 - BIOL 3350 - 001
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12050 - BIOL 3350 - 001
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This 43 page Class Notes was uploaded by Emily Emmons on Tuesday July 12, 2016. The Class Notes belongs to 12050 - BIOL 3350 - 001 at Clemson University taught by Dr. Michael Sears in Fall 2016. Since its upload, it has received 6 views. For similar materials see Evolutionary Biology in Biological Sciences at Clemson University.
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Date Created: 07/12/16
Evolution at Multiple Loci: Linkage and Sex produced asexually buds then makes copy of itself where do new alleles come from and how are they maintained in a population? adaptive significance of sexual reproduction Evolution at Two Loci two loci that are physically linked goal: track allele frequencies and chromosome frequencies Does selection at the A locus interfere with our ability to use the models of earlier chapters to make predictions about evolution at the B locus? Populations can have identical allele frequencies but different chromosome frequencies. calculate frequency of B on chromosomes carrying a and A allele. If in equilibrium the frequencies should be equal. The coefficient of linkage disequilibrium, symbolized by D, is defined as g Ag ab−g Ag aB typo in book gAB = ps, gAb = pt, gaB = qs, and gab = qt D =psqt−ptqs = 0 D ranges between -0.25 and 0.25 Two loci in a population are in linkage equilibrium when the genotype of a chromosome at one locus is independent of its genotype at the other locus. Two loci are in linkage disequilibrium when there is a nonrandom association between a chromosome's genotype at one locus and its genotype at the other locus. Two Loci are in Linkage Equilibrium if: 1. The frequency of B on chromosomes carrying allele A is equal to the frequency of B on chromosomes carrying allele a. 3. The frequency of any chromosome haplotype can be calculated by multiplying the frequencies of the constituent alleles. For example, the frequency of AB chromosomes can be calculated by multiplying the frequency of allele A and the frequency of allele B. 5. The quantity D, known as the coefficient of linkage disequilibrium, is equal to zero Hardy–Weinberg analysis for two loci 10 possible zygote genotypes frequencies recombination rates must be incorporated We can now write an expression for gAB′, the frequency of AB chromosomes in the new gene pool: when D = 0—the chromosome frequencies do not change from one generation to the next. Under Hardy–Weinberg assumptions, chromosome frequencies remain unchanged from one generation to the next, but only if the loci in question are in linkage equilibrium. If the loci are in linkage disequilibrium, the chromosome frequencies move closer to linkage equilibrium each generation. Selection on multilocus genotypes can create linkage disequilibrium to see this, calculate the frequency of allele a and allele b and use criterion 2 Genetic Drift Can Create Linkage Disequilibrium genetic drift is the mechanism because disequilibrium occurs only in a finite population Population admixture can create linkage disequilibrium combination of populations with different allele and chromosome frequencies created a new population with an excess of AB and ab chromosomes With sexual reproduction and random mating, linkage disequilibrium falls over time the rate of decline in linkage disequilibrium between a pair of loci is proportional to the rate of recombination between them. linkage disequilibrium falls over time The Bad News about Linkage Disequilibrium single-locus population genetics model looking only at locus B will make inaccurate predictions about evolution. elevated concentrations of ergothioneine may contribute to Crohn's disease the apparent connection between Crohn's and ergothioneine is a spurious result of linkage disequilibrium The Good News about Linkage Disequilibrium if locus A and locus B are in linkage equilibrium—then selection on locus A has no effect whatsoever on allele frequencies at locus B. On human chromosome 22, most pairs of loci are in linkage equilibrium situated near enough to each other on the same chromosome that crossing over between them is rare. Practical Reasons to Study Linkage Disequilibrium the rate of crossing over between the loci to estimate the rate at which the disequilibrium is decaying Practical Reasons to Study Linkage Disequilibrium depending on the recombination rate, one can determine the mechanism of evolution Glucose-6-phosphate dehydrogenase deficiency is common a locus in linkage disequilibrium with nearby markers may be young and if common under positive selection The signature of recent positive selection neutral alleles evolving by drift can have a high frequency, or high linkage disequilibrium, but not both Sexual reproduction is complicated, costly, and dangerous offspring develop from unfertilized eggs is called parthenogenesis Which Reproductive Mode Is Better? A null model 1. A female's reproductive mode does not affect how many offspring she makes. 2A female's reproductive mode does not affect the probability that her offspring will survive. A null model And yet such asexual takeovers do not seem to have happened very often Males can persist in a population of facultatively sexual females if they have sufficiently high fertilization success, if they produce offspring that survive at a sufficiently elevated rate, or both. Sex, to a population geneticist, means allelic segregation and genetic recombination. In a population genetics analysis, sex does only two things: It restores Hardy–Weinberg equilibrium, and it restores linkage equilibrium. Elevated mutation rate selects for outcrossing in C. elegans Outcrossing is, at least in part, an adaptation for maintaining fitness in the face of deleterious Elevated mutation rate selects for outcrossing in C. elegans A higher mutation rate selected for more frequent outcrossing, and thus more males. Sex breaks Muller's ratchet asexual species accumulated far more nonsynonymous, deleterious mutations Pathogens select for outcrossing in C. elegans Sex is adaptive because it recreates these missing genotypes through segregation and recombination A host parasite arms race can make sex beneficial Genes for sex ride to high frequency in the currently more fit genotypes they help to create. The frequency of sexual individuals in snail populations Males are more frequent in populations where more snails are infected. We have learned that by reducing disequilibrium, sex both helps maintain fitness despite deleterious mutation and facilitates evolution in response to selection re-creates favorable multilocus genotypes that were recently eliminated by selection.
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