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Lecture 14

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Western University
Biology 1001A
Tom Haffie

Lecture 14:  Conditions necessary for Hardy-Weinberg equilibrium
 whether a population is in HWE, given observed genotype or phenotype frequencies
 relative vs. absolute fitness:  Hardy-Weinberg assumptions:  No selection  Very large population size  No mutation  No gene flow  Random mating  In a large, random-mating population, where mutations are rare enough to be ignored, in the absence of immigration or emigration, and if there is no selection…  We cannot recognize a population that is evolving unless we have something we are comparing it to (something that is not evolving or when mutation is so low it is negligible)  Hardy-W doesn‟t apply when there is selection pressure  How to calculate relative fitness: • Absolute fitness (W): Usually defined as # surviving offspring produced over an individual‟s lifetime: -influenced by survival to adulthood, longevity, attracting mates, fertility, offspring survival… *Sometimes we have to estimate absolute fitness (W) based on one or two of these components of fitness (and assume the others are similar across genotypes) “Relative fitness (w)”: • standardize absolute fitness to get relative fitness (by comparing it to other absolute fitness of other individuals in the same population “ex. It does not make sense to compare a human fitness to a fruit fly‟s !) • by definition, fittest genotype in a population has w = 1 • all other genotypes w = W/W max – W = AA offspring; W Aa and W aa= 8 offspring; what is w ?aa we devide 8 by 10 = 80) • difference in w (relative fitness) indicates whether selection is weak or strong ( the greater the magnitude of the difference of fitness in genotype and the other genotypes in a population, the stronger the selection is acting on the population)  How to quantify strength of selection: -Strong selection operates more quickly than weak selection ( like the pigs) -The bigger the difference in relative fitness, the stronger the selection pressure  Relationship between dominance/recessiveness of alleles and response to selection: -Sometimes when there is a selection going on in a population it actually matters because it does make a difference if selection favors a dominant allele or recessive allele! -So as long as selection isn‟t occurring, then it doesn„t matter if the allele is dominant or not -It matters in determining phenotype of offspring b.c. those phenotypes are associated with differences of success in reproductive fitness but allele fitness is not going to change. -There is nothing special about being dominant  We use black and brown pigs( black is dominant !) and add wolfs to make it like the wolfs like to eat black pigs but not the brown ones: The black pigs frequency go down although it is dominant! If we increase the death rate more (stronger selection pressure) it disappears faster  THE LESSON : When selection is acting in favour of one allele (one beneficial and one harmful allele in the population), selection removes the harmful alleles, but the speed in which it happens depends on the magnitude of fitness ( ex. The black pigs had higher chances of death than brown ones! The wolfs ate the black ones) • Now we try recessive alleles to be the harmful and put a selection pressure: Dominant is beneficial, recessive harmful ( the opposite of the last one) The dominant allele gets higher frequencies and the recessive dicrease but doesn‟t reach zero !! In this situation for example it makes a difference whether an allele is dominant or recessive!  Why can we not get rid of the harmful recessive allele It is shielded by the heterozygous gene (Bb) who have normal fitness So it exists in low frequencies in a population (2pq bigger than p^2) There is a few dominant gene disease, it is mostly recessive genetic diseases Because selection pressure can eliminate the harmful dominant allele, but not all of the recessive allele (heterozygotes wont be harmed) When we have recessive allele beneficial it eventually reaches a frequency of 1 (fixation) Dominant never reach complete fixation ! (a frequency of 1)  Effect of heterozygote advantage on genetic variation:  Effect of positive and negative frequency-dependent selection
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