Chapter 25__upload_Genetics of Populations


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Unformatted text preview: lection pressure (alcohol) showed a decline in AdhS allele • The populations evolved because of selection for better ability to break down alcohol Impact of selection on real animals animals Mutation Mutation Mutation alone is not a potent evolutionary force Model mice population • • • • • Locus A Frequency of allele A = 0.9 Frequency of allele a = 0.1 a is recessive loss of function mutation Copies of A are converted to a at a rate of 1 copy per 10,000 generations Very high mutation rate Mutation Mutation Model mice population • Back mutations to A are negligible • Assume all mutations happen in gametes in gene pool • Adult genotype frequencies AA = 0.81 Aa = 0.18 aa = 0.01 In Hardy­Weinberg proportions • Alleles in gametes are still 0.9 and 0.1 Mutation Mutation Model mice population • Now 1 of 10,000 A alleles mutates to “a” • New frequency of A (p) is old frequency minus fraction lost to mutation p = 0.9 ­ (0.0001)(0.9) • New frequency of a (q) is old frequency plus fraction gained by mutation q = 0.1 + (0.0001)(0.9) • When gametes make zygotes AA = 0.80984 Aa = 0.18014 aa = 0.01002 Migration Migration Movement of alleles among populations Not the same as seasonal migration of animals Gene flow • Transfer of alleles from one gene pool to another gene pool of a different population Dispersal of adults or any other life stage Migration Migration One­island model • Gene flow is effectively one way • Locus A with two alleles: A1 and A2 • Before migration frequency of A1 is 1.0 A is fixed in island population 1 No A A or A A 12 22 • 800 zygotes before migration • Continental population fixed for A2 • 200 individuals migrate to island • New genotype frequencies after mating A = 0.8 1 A = 0.2 2 Genetic Drift Genetic Population Genetics shows us that natural selection is not the only mechanism for evolution Evolution can happen by random chance events This kind of evolution is not adaptive but does lead to changes in allele frequencies Violate assumption of infinite population size Genetic Drift Genetic Random discrepancy between theoretical expectations and actual results can be called sampling error Sampling error in production of zygotes is genetic drift Random luck causes evolution Drift is a result of finite population size If the population is larger, drift is less important Random Fixation of Alleles Random Effects of genetic drift over many generations can be powerful In a computer simulation, three different populations sizes with two alleles for one locus were allowed to mate randomly • Because of drift, one allele rose to fixation after time • Smaller populations went to fixation faster Genetic Drift Genetic Fixation of alleles is consequence of genetic drift Which allele becomes fixed is random Sewall Wright proved that the probability of fixation for a particular allele is the same as its original frequency • If the initial frequency of an allele is 0.8, there is 80% it will drift to fixation Violations of H-W assumptions Violations Violation due to sampling error • Drift Systematic changes • Mutation • Selection • Gene flow / migration Violations of H-W assumptions assumptions Factors that increase variation w/in pop. • • • • Mutation Gene flow Selection Drift • • • Selection Drift Inbreeding Factors that decrease variation w/in pop. Estimate Frequency of Carriers Estimate Frequency of carriers: 2pq/(p2 + 2pq) = 0.28/0.97 = 0.29 or about 30% of normal birds are carriers of this disease. Why is q so high in such a deadly disease? Now can you answer this question???...
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