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Transcript
Population Genetics and Evolution Forces of Evolution DETERMINISTIC: direction of change predictable Mutation Migration Natural Selection STOCHASTIC: direction of change unknowable (none exp.) Genetic Drift Forces of Evolution Mutation strength related to rate & genomic position of mutation Forces of Evolution Mutation strength related to rate & genomic position of mutation Forces of Evolution Migration strength related to directionality, rate of migration; genetic variability Forces of Evolution Migration strength related to directionality, rate of migration; genetic variability Forces of Evolution Natural Selection strength fitness differences; genetic variability Forces of Evolution Natural Selection strength fitness differences; genetic variability Forces of Evolution Genetic Drift strength related to population size (i.e., ‘sampling error’) Forces of Evolution Genetic Drift strength related to population size (i.e., ‘sampling error’) Forces of Evolution Genetic Drift Sampling error: more trials per sample… closer to expectations Forces of Evolution Genetic Drift &'$ &'# &'" &'! &'& 5062-72)819:).1&'% &'% Sampling error: more trials per sample… closer to expectations ! " # $ ()*+,-./(01,2304 % Hardy-Weinberg Principle Idealization of (less-than-ideal) populations… …allow for comparisons… Assumptions Random Mating No Mutation No Migration No Natural Selection No Genetic Drift (exceptionally large population size) Hardy-Weinberg Equilibrium Assuming no evolution of the populations… Hardy-Weinberg Equilibrium Assuming no evolution of the populations… … only need to keep track of allele frequencies heterozygote homozygotes allele Hardy-Weinberg Equilibrium Assuming no evolution of the populations… … only need to keep track of allele frequencies Alleles: come in singles Genotypes: come in doublets Hardy-Weinberg Equilibrium In meiosis, the alleles (B and b) segregate… … and randomly unite by fertilization. Hardy-Weinberg Equilibrium Hardy-Weinberg Equilibrium To find the genotypic, allelic, or phenotypic proportions in the next …use the Hardy-Weinberg formula! generation… p2 + 2pq + q2 = 1 p = frequency of B q = frequency of b random union of gametes Violations of Hardy-Weinberg Equilibrium ANY violation of HWE indicates evolution Industrial Melanism: natural selection in peppered moths Camouflaged organisms more apt to survive, reproduce Genetic variability existed (and exists) in some populations Habitat in some forests of UK modified by Industrial Revolution hypotheses Darker moths ought to be more common in heavily sooted forests Lighter moths ought to be more common in more pristine forests Industrial Melanism Kettlewell’s hypotheses (1950s) Darker moths more common in heavily sooted forests Lighter moths more common in more pristine forests Industrial Melanism Industrial Melanism Kettlewell’s experiments: a synopsis Natural Selection the principal violator of HWE Other forces of evolution (e.g., Drift, Migration) involved Industrial Melanism Kettlewell’s experiments: a synopsis Natural Selection the principal violator of HWE Other forces of evolution (e.g., Drift, Migration) involved Industrial Melanism No change in allele (or phenotypic) frequencies… … does NOT imply that evolutionary forces are absent!!! Chi-square test of HWE OBSERVE: EXPECT: 24 BB 0.25*100 = 25 BB 52 Bb 2(0.25)*100 = 50 Bb 24 bb 0.25*100 = 25 bb Find expected proportions: p2 + 2pq + q2 = 1 p = 0.5; q = 0.5 Chi-square test of HWE FIND DIFFERENCES: 24 BB - 25 BB = -1… squared = 1 52 Bb - 50 Bb = 2… squared = 4 24 bb - 25 bb = 1… squared = 1 TALLY DIFFERENCES, scaled by expected values: (1/25) + (4/50) + (1/25) = 0.6 Is the difference ‘real’? p = 0.74 (attribute difference to chance) Chi-square test of HWE OBSERVE: EXPECT: 49 BB 0.25*100 = 25 BB 1 Bb 2(0.25)*100 = 50 Bb 49 bb 0.25*100 = 25 bb Chi-square test of HWE FIND DIFFERENCES: 49 BB - 25 BB = 24… squared = 576 1 Bb - 50 Bb = 49… squared = 2401 49 bb - 25 bb = -24… squared = 576 TALLY DIFFERENCES, scaled by expected values: (biggish) + (big) + (biggish) = sizable (35.53) Is the difference ‘real’? p ~ 0.00 (Something wonky is going on!) Game plan… Two beads (alleles) is a single individual (genotype)! Sample with replacement! (put back individual) Do 4 (rather than 10) generations per simulation Exercises I. Do all; Chi-square test (Table 3, pp. 208); ASSUMES HWE (ask for help!) IIA. Choose either Bottleneck model (pp. 211) OR Founder effect model (pp. 214) IIB. Use 2 populations of 25 individuals each IIC. Choose either Industrial Melanism (pp. 218) OR Sickle-cell Anemia (pp. 219) PREPARE to DISCUSS RESULTS with CLASS