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Unformatted text preview: selection Adaptation • alleles that enhance survival and reproduction increase gradually in frequency • population becomes progressively better able to survive and reproduce in the environment Adaptation • EVOLUTIONARY ADAPTATION  progressive genetic improvement  increase in frequency through time of individuals superior in survival and reproduction due to inherited differences in the ability of the organism to survive and reproduce Selection • Unit of selection = individual not species not subpopulation not sibship Models of selection • Deterministic infinite population size no mutation random mating (no migration) discrete vs continuous generations diploid vs haploid haploid model • competing genotypes with different rates of potential increase haploid model • Initially use a discrete generation model fitness based on viability alone absolute fitness =  probability of genotype survivorship relative fitness =  fitness expressed relative to a particular genotype  (arbitrarily set = 1.0) haploid model • Change in allele frequency • p (w A ) • . p’ =  . p (w A ) + q (w a ) p (w A ) 0.5 (1) p’ =  =  = 0.6 p (w A ) + q (w a ) 0.5 (1) + 0.5 (0.67) haploid model • Since we are using an arbitrary “best” fitness for allele A [o] , (w A ) = 1 we can express change in terms of a selection coefficient against allele a s = 1  (w a ) • p 0.5 . p’ =  =  . 1  qs 1  (0.5)(0.33) haploid model • Change in allele frequency • p (w A ) • . p’ =  . p (w A ) + q (w a ) • we can also express the average fitness of the population as : w = p (w A ) + q (w a ) _ haploid model • We can predict the future frequency of the alleles as : • p . p t =  . p + q (1 – s) t haploid model • We can estimate the selection coefficient from information on allele frequencies in successive generations? • If s is small (actually can be just < 0.2) p t p . ln [  ] = ln [  ] + s t . q t q haploid model • We can estimate the selection coefficient from information on allele frequencies in successive generations? • If s is small (actually can be just < 0.2) p t p . ln [  ] = ln [  ] + s t . q t q Use slope to estimate s haploid model • estimating the selection coefficient from allele frequency changes? Consider two competing bacterial strains which differ at just one locus Slope = selection coefficient Selective changes in populations • Example: antibiotic resistance in bacteria antibiotic resistant allele has higher fitness in presence of antibiotics Penicillin resistant Streptococcus pneumoniae diploid model • We usually use a discrete generation model  viability selection absolute fitness =  probability of genotype survivorship relative fitness =  fitness expressed relative to a particular genotype (arbitrarily set = 1.0) Where does selection act in...
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 Spring '05
 FUERST
 Evolution, Population Genetics

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