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Evolution Notes

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Western University
Biology 3466B
Yolanda Morbey

Evolution NotesNov 2011 If a trait is controlled by a single gene with two alleles there could be 1 2 or 3 phenotypic variants in a population Natural selection Four necessary conditions1 Phenotypic variation among individuals 2 Variation is partly heritable 3 More individuals are born than will survive and reproduce individuals have different fitness 4 Fitness is nonrandom and depends on phenotype If all of these conditions are met populations will evolve Natural selectionexample 1 Geospiza fortis medium ground finch Is variation in beak shape within medium ground finch due to natural selectionPostulate 1phenotypic variation There is lots of phenotypic variation normal distribution It is not controlled by just two alleles Postulate 2heritable variation One way to measure heritability is to look at parent trait values and offspring trait values If there is positive correlation it means the beak depths of parents and offspring are similar Parents with deep beak depth have offspring with deep beak depth Postulate 3differential fitness After a drought in 1977 the number of finches decreased Most birds died of starvation 84 population decline This shows that they had different fitnesses Drought is the selection factor Drought affects availability of food After the drought late 1977 there was a severe reduction in the number of available seeds and during the drought only large and hard seeds were available Postulate 4nonrandom fitness Individuals with deeper bills were able to survive better finches with shallow beaks werent able to eat the large hard seeds Before the drought in 1976 there were 751 finches but after the drought in 1978 there were only 90 finches left Also the average beak depth had increased This is evidence for nonrandom fitness due to variations in beak depth Did the population evolve Look at beak depth for birds born before and after drought The average beak depth increased There has been evolution in one generation due to natural selection Selection occurs within generations evolution occurs between generations Natural selection doesnt necessarily lead to adaptation Natural selectionexample 2 HIVdid resistance to drug AZT evolve by natural selection HIVs reverse transcriptase uses the viral RNA as a template to construct a complementary strand of DNA using nucleotides present in the host cell AZT is a drug used to treat HIV It fools reverse transcriptase into integrating AZT into the strand This ceases further DNA synthesis because AZT has N3 instead of OH and so new nucleotides cannot be added on they cant bind to N3 Nonrandom fitness For a patient that received AZT for two months the higher the concentration of the drug the lower the resistance of HIV HIV evolves more resistance to AZT the longer amount of time it is used for treatment Over time more AZT is needed to kill HIV evolution of drug resistance AZT resistance evolves in most patients within six months There is an increase in resistance as the months of therapy increase Postulates 14 Random errors in transcription produce mutations that may affect the ability of reverse transcriptase to be tricked by AZT mutants vary in their resistance to AZT Another mutation may occur to variants resistant to AZT Resistance is passed on to offspring resistance is heritable During treatment with AZT some virionsvariants are better able to survive and reproduce than others The variants that persist in the presence of AZT are the ones with mutations in their reverse transcriptase genes that confer resistance Variation due to mutation inheritance and differences in survival due to AZT resistance results in changes in the composition of the population over time
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