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Lecture 14

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Biology (Sci)
BIOL 202
Daniel Schoen

MIDTERM EXAM 6-730, IN THEADAMS AUDITORIUM Dr Schoen's last lecture Population genetics III • Natural Selection • Interaction of forces • Linkage disequilibrium and association mapping The forces of evolutionary change (forces that effect allele frequency of a population of a given gene, allusion= change of allele frequency?) • natural selection • mutation • migration • genetic drift Adaptations: weird and wonderful -Bats are nocturnal animals, with an adaptation of echolocation. -Bullshorn Acacia=a tree that goes in the tropics that is very palatable to mammals, but its adapted to live with ants, providing a home for them, the ants live in the home and get food from it. The ants defend the plant in return. -Nematode parasite that an ant gets a berry on its ass, bird eats this berry like ant, and the parasite gets into the birdie Natural Selection Principles 1. Organisms produce offspring wt different traits, which can be inherited (can be brought into the population by migration or mutation) 2. organisms produce more offspring than can survive because the word has limited resources 3. offpspring whose traits are best suited to their environment survive and pass on those variations to their offpsring ex: light form and dark form of moth (dark form is better hidden from predators), caused by mutations of genes involved in pigmentation the peppered moth has 2 different forms, the light form and the dark form, originally the light form was the dominant one. They used to use the light on the surface as the tree as camouflage. In the middle of the 20 century, the trees they were chilling on was darkened by coal in the industrial revolution. White phenotype was now exposed to predators, and dark was preferably selected. This changed the allele frequency of the population....for our purposes we assume that the colour is determined by a single gene. Fitness (absolute, expressed as # of progeny per offspring. and relative fitness) and natural selection Fitness is dtmnd by the joint effects of genetic make-up and environment Can see the genotype, and the phenotype, and their relative fitness ( for the dark phenotype, w=1, for the recessive phenotype, w=1-s)......w=fitness of ___, and s= selection coefficient, and that's just the reduction in the overall fitness. relative fitness = the fitness of an individual genotype wt respect to the most fit genotype in the population (take the absolute fitness and divivde by the msot fit genotype) Life cycle of the moth eggs hatch in early summer (caterpillars) [ASSUME HW PROPORTIONS]--->flying moths in forest (get selected for by Natural selection, birdies eat the s*** out of them, throws HW out of whack) ----->moths mating (no preference for colour)---->eggs layed for next generation [HW REESTABLISHED] How will natural selection act to alter allele frequency in a forest environment? Natural selection drives genotype frequencies out of HW proportions, then H-W proportions reestablished by random mating Environmentally-dependant fitness in Peppered moths (how will natural selection act to alter allele frequency in a forest environment) How will natural selection act to alter the frequency of allele a (parameter q) in a forest environment (see the slide for the example) How wil natural selection act to alter frequency of allele a in forest environment in the next generation (see the slide) AVERAGE FITNESS = sum of the frequencies of the different genotypes each multiplied by the fitness of that genotype TO CALCULATGE THE FREQUENCY OF THE GENOTYPES AFTER SELECTION, WE MULTIPLY THE FITNESS OF THAT GENOTYPE * THE FREQUENCY OF THAT GENOTYPE AND DIVIDE BY THE AVERAGE FITNESS Now when we start out the next generation, we take the new allele frequency and put it back into regular HW proportions for the next generation (will register a drop in the unfavorable allele frequency) SEE THE SLIDES FOR THE EXAMPLE Natural selection is a lot faster, it amplifies mutation variation. Wherease mutation took 200, 000 generations to fully permeate, natural selection only took 200 gens NATURAL SELECTION=POSITIVE SELECTION (changes variation in the population) MHC gene complex and balancing selection (rare genotypes are favored,) this is shown at mutations at the MHC complex..its advantageous for us to have more alleles at this locus, that way we recognize more pathogens.....selection might want
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