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UNC-Chapel Hill BIOL 201 - Genetic Drift

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BIOL 201 1st Edition Lecture 6Outline of Previous LectureI. Mutation-selection balance/equilibriumII. Chart of frequenciesIII. EquationsIV. Cystic fibrosisV. Example ProblemOutline of Current LectureI. Genetic Drift and Natural Selectiona. Similaritiesb. DifferencesII. ExamplesIII. Genetic Drifta. Definitionb. Sampling errorIV. FixationV. Bottlenecks and Founder’s Effecta. Bottlenecksb. Founder’s EffectVI. Effective Population Sizea. Definitionb. Equationsc. ExamplesVII. Group QuestionsVIII. Coalescent TheoryCurrent LectureI. Genetic Drift and Natural Selectiona. Similarities: both mechanisms of evolution and use population as the unitb. Differences: absence of selective pressure/agent in genetic drift and presence of the same in natural selection.i. Genetic drift is more being in the wrong place at the wrong time. II. Examplesa. Person is walking along the sidewalk unaware that there are insects underfoot and over time there are less green beetles that brown ones. This is genetic drift because there was not specific reason that the green ones died more.b. Predator eats the more easily seen beetle resulting in an increase in brown gene frequency  natural selection and the selective pressure is predation.III. Genetic Drifta. Definition- moving from one allelic variation to another without regard to environmental pressure/force; random change in allelic frequency in a populationi. Small populations drift more readily than larger ones.ii. There’s more fluctuation in smaller populations than large ones.iii. Alleles can become fixed in a population.b. Sampling error – result of random drawing from a population; caused by observing a sample instead of a whole population so not looking at group representative of whole populationi. Difference between what you sample and what is the actual but unknown valueii. Of greater magnitude in smaller populations; larger margin of erroriii. If a population is small then you don’t get the normal bell curve distribution of traitsIV. Fixation: when have only AA or only aa in a population; easier to be reached in smaller populationsi. Smaller population loses genetic diversity and fluctuates more than larger populationii. All members of population have that one allele when an allele becomes fixediii. Consequences = lose genetic variation and have issue if natural selection comes in then may lose long term stability1. Less able to adapt to potential change in environmentV. Bottlenecks and Founder’s Effecta. Bottlenecks: when a population is dramatically reduced in size and later expands in numbersb. Founder’s Effect: a few individuals from the original population start a new colony/population and offer a new allele frequencyi. In both cases, the remaining individuals and their alleles influence future generations.ii. Populations should maintain genetic variation to maximize long termstability as a viable population and heterozygosity is key. Larger populations are better and there is an ideal size.VI. Effective Population Sizea. Definition: size of a totally random mating population that would generate the same amount of variation as observed in a real population that is undergoing drift; calibrates evolutionary change over timeb. Equationsi. Ne = 4 NfNm/ (Nf + Nm)1. Nf= # of parental females and Nm = # of parental malesii. Ne = m/[1/N1 + 1/N2 + 1/N3+ ……. 1/Nm]; for when population size changes over time1. m= harmonic meanc. Examplesi. A population has a total of 1000 animals where 3 males mated with 300 females to produce the next generation?1. 4(300)(3)/303 = 11 [always round down]ii. If there is a breeding population of 10,000 loggerhead turtles of which 8000 are female and 2000 are male, what is the effective population size?1. 4(8000)(2000)/10,000 – 64002. has been reduced to less than 2/3 of original population so drift will act more strongly on the new generationiii. Suppose over 20 yrs a population of mice spends 12 years at size 14500 and 8 years at size 400. What’s the effective population size?1. 20 divided by 1/14500  add this together 12 times  12/14500 and then 1/400 added together 8 times  8/4002. Ne = 9602VII. Group Questionsi. Can drift and selection occur in a population at the same time? YESii. Does genetic drift happen in all populations at all times? YES becauseit’s random so it can’t really be stopped.iii. Does drift reduce, increase, or maintain genetic variation in a population? REDUCES in small populations and MAINTAINS in largerpopulationsiv. What about between different populations? INCREASES variation between different populationsVIII. Coalescent Theoryi. Coalescence: things coming togetherii. Want to trace the ancestry of gene copies backward from the present through finite populations until 2 or more distinct gene copies at some point are descended from the same ancestral gene copyiii. Primitive doesn’t mean simpler; it means older.iv. If we focus on a neutral locus, much of the coalescing happens shortly before the present with the final coalescent even taking a very long timev. Demography affects coalescence.vi. The average coalescent time for a large population is 4N with the lastcoalescent even between 2 lineages taking another 2N generationsvii. If a population is shrinking then coalescence is recent.viii. If a population is growing then coalescence occurs further


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