1 Bio1B Evolution 3 Last lecture: • Natural selection - principles, lines of evidence in the “Origin” • Descent with modification • Estimation & interpretation of phylogeny Today • Phylogenetics - molecular data and 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 (Castle) Weinberg Equilibrium • Application: Null model for evolution • Application: Predicting heterozygote frequencies for recessive traits Fitting observed patterns of sequence variation at homologous (aligned) sites to a phylogenetic hypothesis DNA sequence variation The “molecular clock” – dating divergence events Hemoglobin and divergence of chimpanzee & human lineages (Fig 16.9) Dating the emergence of HIV (Fig. 16.10)2 Mendel’s principles of inheritance (1865) [see Ch 14] • Alternative versions of genes (alleles) account for variation in inherited characters • For each character, an organism inherits 2 alleles, one from each parent • If the 2 alleles at a locus differ, then the dominant allele determines phenotype • The 2 alleles for a heritable character segregate during gamete formation (Law of Segregation) • Each pair of alleles segregates independently of others during gamete formation [for unlinked genes] Mendel’s garden; Brno Dominance of purple (P) over white (p) flower color: see also F 8.1 in text Appearance: P Generation Genetic makeup: Gametes F1 Generation Appearance: Genetic makeup: Gametes: F2 Generation Purple flowers Pp P p 1 2 1 2 P p F1 sperm F1 eggs PP Pp Pp pp P p 3 : 1 Purple flowers PP White flowers pp P p Red CRCR Gametes P Generation CR CW White CWCW Pink CRCW CR Gametes CW F1 Generation F2 Generation Eggs CR CW CR CRCR CRCW CRCW CWCW CW Sperm 1 2 1 2 1 2 1 2 1 2 1 2 Co-dominance - heterozygote is intermediate (pink) in snapdragons: see also Fig. 8.10 in text Genotype and allele frequencies for a locus with two alleles: see also Figs. 15.3 & 15.10 in text Futuyma, 2nd Ed.3 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 in expected proportions in a single generation • 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) 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)4 Hardy-Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles Futuyma, 2nd
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