BS110 Evolutionary Processes

BS110 Evolutionary Processes - Evolutionary Processes...

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Evolutionary Processes Population Genetics BS 110 29 Feb 2008
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Evolution Change in allele frequencies over time Evolutionary change occurs in populations Natural selection operates at level of individual Population: Group of interbreeding individuals of same species in same area at same time
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Evolutionary processes and their consequences Natural selection Increases frequency of alleles that improve fitness Only mechanism consistently leading to adaptation Genetic drift Change in allele frequencies due to random chance Causes random changes in allele frequency Mostly in small populations Gene flow Immigrating individuals introduce new alleles Alleles may be lost through emigration Mutation Ultimate cause of all variation Continually introduces new alleles Beneficial, harmful, or neutral
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But what’s the null model? What happens to allele frequencies when evolution is NOT occurring? Hardy-Weinberg Principle What happens when ALL individuals (all genotypes) in population mate and produce offspring Shows that processes of sexual reproduction (meiosis and fertilization) will not change proportions of alleles in population » Some previously thought that dominant Some previously thought that dominant alleles would spread alleles would spread » others thought that two alleles for same gene others thought that two alleles for same gene would stabilize at 50% would stabilize at 50%
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H-W Principle: If all gametes produced in each generation go into same gene pool Gene pool: all alleles for all loci in population To get H-W Principle, calculated what happens when Many random samples taken from gene pool Each gamete pair combined to form offspring » NOT specific pair of genotypes but all possible NOT specific pair of genotypes but all possible genotypes in population genotypes in population Calculations would predict genotypes of offspring produced Also, frequency of each genotype
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Simplest possible situation: Only two alleles for a particular gene in a population A 1 , A 2 p = frequency of A 1 alleles in gene pool q = frequency of A 2 alleles in same gene pool Two frequencies must add to 1 p + q = 1
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So, in hypothetical example in book, • If initial frequency of A 1 in gene pool is 0.7, initial frequency of A 2 must be 0.3 – 70% of gametes in parental generation are A 1 , 30% are A 2
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Next, • Figure out the possible combinations of A 1 and A 2 in offspring Alleles join together at fertilization to form offspring in combinations: A 1 A 1 A 1 A 2 A 2 A 2
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Then what? What are the frequencies of each of the possible genotypes in the offspring? – (Remember, in parental generation:
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This note was uploaded on 03/19/2008 for the course BS 110 taught by Professor S.lawrence during the Spring '07 term at Michigan State University.

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BS110 Evolutionary Processes - Evolutionary Processes...

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