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5/6/15 Ch 23: Evolution of Populations Population Species Why is a population important? relative fitness lineage genetic variation gene variability nucleotide variability new alleles sexual reproduction Non-heritable variation: diet-based variation on oak flowers leaves on oak 1 5/6/15 Temperature-based cline (graded change): mummichog fish enzyme Cold adaptation Population genetics: study of the distributions and changes of allele frequency in a population Modern Evolutionary Synthesis Sewall Wright R.A. Fisher J.B.S. Haldane Also: J. Huxley, T. Dobzhansky, E.B. Ford, E. Mayr, B. Rensch, S. Chetverikov, G.G. Simpson, G.L. Stebbins Hardy-Weinberg equilibrium If the population is not evolving, 5 conditions that need to be met: 1) No mutations 2) Random mating 3) No natural selection G. H. Hardy (Math) Wilhelm Weinberg (Ob-Gyn) 4) Extremely large population size 5) No gene flow 2 5/6/15 Hardy-Weinberg equilibrium: Defined relationships p+ =1 p = frequency of dominant allele = frequency of recessive allele p2 + 2p + =1 p2 = expected freq. of homozyg dom. genotype = expected freq. of homozyg rec. genotype 2p = expected freq. of heterozyg. genotype Prediction if population in HW equilibrium How is this applied? Example: Recessive allele (q) Data Gen 1 Gen1: 330/1100 Gen2: 573/1910 Is the above population in H-W Equil? Gen 2 Another example: 1/10,000 born with phenylketonuria (PKU) in US Have to inherit this as 2 recessives -> = q2 -> q = square root of 1/10,000 = 0.01 -> p = freq. of dominant allele = 1 - q = 0.99 -> freq. of heterozygotes = 2pq = 2 (0.99)(0.01) = 0.0198 or ~2 % ⇒ This is the predicted presence of PKU carriers in US population Another example to think about: Cystic fibrosis (CF) is found at a rate of 1/3500. How many of us are carriers? 3 5/6/15 How can the gene pool be changed? mutation non-random mating inbreeding Egyptian in-breeding eg: Akhenaten How can the gene pool be changed? mutation non-random mating inbreeding In-breeding depression 1.4" Weekly survival" 1.2" Non-inbred mice" 1.0" 0.8" 0.6" Inbred mice" 0.4" 0.2" 1" 2" 3" 4" 5" 6" 7" 8" Weeks after release" 9" 10" 11" SBM Fig. 18.02! How can the gene pool be changed? mutation non-random mating inbreeding assortative mating 4 5/6/15 How can the gene pool be changed? mutation non-random mating inbreeding assortative mating sexual selection How can the gene pool be changed? genetic drift How can the gene pool be changed? genetic drift bottleneck effect eg: N. elephant seals, cheetahs, humans 5 5/6/15 How can the gene pool be changed? genetic drift founder effect eg: more polydactyly than is seen in other populations How can the gene pool be changed? gene flow How can the gene pool be changed? natural selection 6 5/6/15 Natural selection moves the population toward better adaptation to the environment. Directional: finch beak depth Percent of infant death - log scale (red)" Stabilizing: human infant weight 100" 15" 30" 10" 10" Percent of infant population (blue)" 20" 5" 5" 2" 1" 2" 3" 4" 5" 6" 7" 8" 9"10"11" Body weight in pounds" http://www.hhmi.org/biointeractive/ origin-species-beak-finch Preservation of genetic variation diploidy balancing selection heterozygote advantage Preservation of genetic variation balancing selection frequency-dependent selection 7 5/6/15 Example already seen: Rock pocket mice At the DNA-level, some neutral variation: sometimes differences in DNA don t seem to matter (much). Futurama! Neutral President: I have no strong feelings one way or the other." Natural selection “tinkers” It does not make perfect organisms but it does help propagate those that are better than... 8