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Microevolution lecture notes.doc

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Jon Houseman

MicroevolutionSlide 3combination of Darwin and Wallacemust look at variation within populations rather than individualsin population there is a range of variation and in many cases the variation is genetic and heritable when we want to see natural selection we need to measure and monitor variation within the population Slide 5Mendel gives us an heritable basis for variationSlide 6Huxley combines genetics with evolution and gives us calculations in population genetics that become predictable for changes in variation we need mechanisms to measure change and to quantify change the Synthetic Theory of Evolution populations geneticsnatural selection based on Mendelian genetics allows us to measure changeSlide 7Microevolutionchanges in alleles within the populationchange in allele frequenciesvariationmicroevolution occurs when there is change in allelic frequencies Slide 8allelecarries traitstraits present in two forms associated with a homologous chromosome phenotypevisual expression of an underlying gene does not tell allele frequency not an expression of genotype genotypeunderlying frequency of the alleles homozygous identical alleles of the gene are present on both homologous chromosomesheterozygous two different alleles of a genedominant and recessive Slide 112 2p 2pqq1if everything is random and pooled together you can predict what the reassembly should bep and q represent allelic frequencies Slide 13HardyWeinberg Principleno matter the initial frequency you can predict the outcomeHardyWeinberg Equilibriuma way to understand population genetics tells you if frequencies have changedallelic frequency changemicroevolution makes assumptionsno natural selection no allele combinations are favourable and all combinations are neutral doesnt increase fitness no mutation no allele has been turned into something elseno genetic drift the population is sufficiently large that we dont have small size sampling needs a large number samples for accuracyno gene flow nobody is migrating in and out to change allelic frequency random mating there is no preference for mating other than the opposite sex tells us whenever there is no microevolution if you see allele frequencies changing microevolution is occurring and we try to figure out what is changing and why it is changing Slide 18 HardyWeinberg Principles Effects of Selection fixationfrom selective pressure even after a thousand generations there is little change in allele frequenciesselective pressure has no impact all alleles remainwe need a severe selective pressure in order to change allele frequencyunder severe selection ie 12 selection the allele that is being selected for disappears out of the populationthe population becomes fixed on the other allele under severe selection a situation where there are a minimal number of survivors and a maximal number of organisms that dont survive we lose genetic variability within a population if we are selecting an agricultural crop and breeding for certain traits we may also losing other variability through selection ie
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