Bio1B Evolution 4 Last lecture Molecular phylogenetics the molecular clock More history Darwin Mendel the neodarwinian synthesis Mechanisms of evolution Evolution in populations population genetics Allele genotype and phenotype frequencies Predicting genotype freq s Hardy Weinberg Equilibrium Today Predicting genotype freq s Hardy Weinberg Equilibrium Application Null model for evolution Predicting heterozygote frequencies for recessive traits Evolutionary processes Sampling effects genetic drift Relevance in evolution loss of variation bottlenecks Mutation as the ultimate source of variation effects on fitness Migration gene flow spreading mutations clines Hardy Weinberg Equilibrium general case Gametes for each generation are drawn at random from the gene pool of the previous generation 80 CR p 0 8 20 CW q 0 2 male gametes Expected genotype frequencies qp A2A1 q2 A2A2 A1A1 p2 A2A2 q2 pq 64 C RC R qp p2 A1A2 2pq CW 20 CR 80 A1A1 pq A1A2 Sperm CR 80 Eggs p2 CW 20 female gametes f A2 q f A1 p f A1 p f A2 q 16 C RC W 4 CWCW 16 C RC W q2 See also Fig 15 11 in text for dominant case Hardy Weinberg Equilibrium Predicts genotype phenotype frequencies from allele frequencies Genotype frequencies are at expected proportions in a single generation fig 15 11 in text Allele genotype frequencies constant across generations inheritance alone does not cause evolution Assumptions Random mating for this gene trait No mutation selection migration Large population no drift 1 Applications of HWE A null model for evolution Deviations from expected proportions indicate something interesting but what Cystic fibrosis Mapped to chloride transport gene on chromosome 7 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 Hardy Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles Futuyma 2nd Ed Mutations forms Changes in Coding sequence Gene regulation Gene copy number Chromosome number structure Regulation Fig 18 8 Coding Fig 17 22 HbbA HbbS Exons mRNA aa 2 Example light dark mice cf lab Some key genes in melanin production pathway Agouti melanistic mutations Recessive regulatory coding X Mc1r Melanistic mutations Dominant coding So what Mc1r melanoma 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 Population bottlenecks 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 Fig 15 7 3 Migration as gene flow Gene flow movement of genes among populations Arises from net movement birth reproduction or gamete dispersal zygotes Spreads new mutations maintains variation Opposes effects of genetic drift or local selection Spread of genes from GMO crops is a concern 4
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