1 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 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) Sperm CR (80%) CW (20%) pq p2 16% CRCW 64% CRCR Eggs CW (20%) CR (80%) 16% CRCW qp 4% CWCW q2 p2 A1A1 pq A1A2 qp A2A1 q2 A2A2 male gametes f(A1) = p f(A2) = q female gametes f(A2) = q f(A1) = p Hardy-Weinberg Equilibrium general case A1A1 = p2 A1A2 = 2pq A2A2 = q2 Expected genotype frequencies 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)2 Applications of HWE • A null model for evolution – Deviations from expected proportions indicate something interesting - but what? • Predicting frequency of heterozygotes for recessive alleles, e.g. cystic fibrosis 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) Hardy-Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles Futuyma, 2nd Ed. Mutations - forms Fig. 17.22 HbbA HbbS Fig 18.8 Regulation Coding Changes in: Coding sequence Gene regulation Gene copy number Chromosome number & structure Exons -> mRNA -> aa3 Agouti melanistic mutations: Recessive [regulatory & coding] Mc1r Melanistic mutations Dominant [coding] So what? … Mc1r & melanoma! X Example: light & dark mice (cf. lab). Some key genes in melanin production pathway 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. 15.74 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
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