Bio1B Evolution 4 Applications of HWE Last lecture More history Darwin Mendel the neodarwinian synthesis Mechanisms of evolution A null model for evolution Evolution in populations population genetics Allele genotype and phenotype frequencies Predicting genotype freq s Hardy Castle Weinberg Equilibrium Deviations from expected proportions indicate something interesting but what Application Null model for evolution Today Predicting genotype freq s Hardy Castle Weinberg Equilibrium Application Predicting heterozygote frequencies for recessive traits Common mutation F508 is recessive and at p 0 02 in caucasian population F het 2pq 0 04 carriers F hom p2 0 0004 affected Predicting frequency of heterozygotes for recessive alleles e g cystic fibrosis Evolutionary processes Sampling effects genetic drift Relevance in evolution loss of variation bottlenecks Cystic fibrosis Mapped to chloride transport gene on chromosome 7 Mutation as the ultimate source of variation effects on fitness Selection fitness Forms and consequences 1 Hardy Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles 2 Effect of small population size genetic drift Sampling gametes zygotes Small population have greater sampling error larger fluctuations in allele frequency reduced variation within populations Fig 23 8 Futuyma 2nd Ed 3 4 Mutations forms Population bottlenecks Changes in Habitat loss or over harvesting Colonization of new areas eg islands humans Out of Africa Loss of genetic diversity Rapid change in allele frequencies divergence Coding sequence Gene regulation Gene copy number Chromosome number structure Regulation Fig 23 10 Fig 23 9 5 Coding Fig 17 22 HbbA HbbS 6 Fig 18 8 Exons mRNA aa Some key genes in melanin production pathway Genetic fitness Agouti melanistic mutations Selection acts through the phenotype Fitness Survival and reproduction relative to other phenotypes or genotypes in the population Relative fitness can be environment dependent Recessive regulatory coding X Mc1r Melanistic mutations Dominant structural So what Mc1r melanoma 7 Sorry Arnie 8 Example of Heterozygote advantage sickle cell anaemia Forms of selection Fig 23 13 Relative fitnesses Without malaria HbbAA HbbAS HbbSS anaemia With malaria HbbAS HbbAA HbbSS 9 More resistant to malaria Note fitness of HbbAS depends on 10 environment malaria Association between melanic phenotype and Mc1r allele in rock pocket mice from Pinacates lava flow Hopi Hoekstra in the field 11 12 Geographic cline Genomic signatures of recent selection Variable site New mutation arises that increases fitness Under directional selection increases to p 1 drags linked sites with it What s with my crazy dog Results in a region of low variation relative to others Storz 2005 Variation Genomic signatures of selection localized reductions in diversity high high low 13 high Sutter et al 2007 Science 316 112 14
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