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 Migration strength related to directionality, rate of migration; 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 Sampling error: more trials per sample closer to expectations

Forces of Evolution Genetic Drift Sampling error: more trials per sample closer to expectations 5062-72)819:).1&'% &'& &'! &'" &'# &'$ &'%! " # $ % ()*+,-./(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 generation use the Hardy-Weinberg formula! p 2 + 2pq + q 2 = 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 No change in allele (or phenotypic) frequencies does NOT imply that evolutionary forces are absent!!!

Chi-square test of HWE OBSERVE: 24 BB 52 Bb 24 bb EXPECT: 0.25*100 = 25 BB 2(0.25)*100 = 50 Bb 0.25*100 = 25 bb Find expected proportions: p 2 + 2pq + q 2 = 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: 49 BB 1 Bb 49 bb EXPECT: 0.25*100 = 25 BB 2(0.25)*100 = 50 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