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Transcript
BIOE 109 Evolution Summer 2009 Lecture 3- Part I Natural selection – theory and definitions Some important principles of natural selection Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. 4. Natural selection is not necessarily progressive. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. 4. Natural selection is not necessarily progressive. 5. Product of selection is a “compromise”. What is fitness? What is fitness? Natural selection and the concept of fitness Darwinian fitness: the number of gene copies (i.e., offspring) a phenotype places into the next generation. Natural selection and the concept of fitness Darwinian fitness: the number of gene copies (i.e., offspring) a phenotype places into the next generation. Relative fitness: a phenotype’s Darwinian fitness relative to other phenotypes. http://www.blackwellpublishing.com/ridley/video_gallery/LP_What_is_fitness.asp What is fitness? 1. Fitness is a description not an explanation What is fitness? 1. Fitness is a description not an explanation 2. Fitness is an average property What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components: What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components: Total fitness = viability + fecundity + longevity + mating success Natural selection at a single locus 1. Purifying selection Natural selection at a single locus 1. Purifying selection • a form of selection acting against deleterious (harmful) alleles. Natural selection at a single locus 1. Purifying selection • a form of selection acting against deleterious (harmful) alleles. • the majority of deleterious alleles are recessive. Natural selection at a single locus 1. Purifying selection • a form of selection acting against deleterious (harmful) alleles. • the majority of deleterious alleles are recessive. • purifying selection drives deleterious recessives to low frequencies where they are maintained at mutation-selection balance: Natural selection at a single locus 1. Purifying selection • a form of selection acting against deleterious (harmful) alleles. • the majority of deleterious alleles are recessive. • purifying selection drives deleterious recessives to low frequencies where they are maintained at mutation-selection balance: rate of introduction = rate of removal by mutation by selection e.g., Tay-Sachs disease, cystic fibrosis, etc. Natural selection at a single locus 2. Directional selection Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. • the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. • the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example: Genotype: AA Aa aa Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. • the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example: Genotype: Fitness: AA wAA Aa wAa aa waa Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. • the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example: Genotype: Fitness: AA wAA 1.0 Aa wAa aa waa 1.005 1.010 Natural selection at a single locus 2. Directional selection • a form of selection acting on advantageous mutations. • the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example: Genotype: Fitness: AA wAA 1.0 Aa wAa aa waa 1.005 1.010 • here, the small a allele would reach fixation in about 3,000 generations. Natural selection at a single locus 3. Balancing selection Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations. Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations. - alleles are said to be “balanced” because a stable equilibrium state is reached. Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations. - alleles are said to be “balanced” because a stable equilibrium state is reached. - if allele frequencies are perturbed from this equilibrium, selection will return them back to that state. Forms of balancing selection 1. Overdominance Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype: AA Aa aa Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype: AA Aa aa Fitness: wAA wAa waa Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype: AA Aa aa Fitness: wAA wAa waa 0.88 1 0.14 Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype: AA Aa aa Fitness: wAA wAa waa 0.88 1 0.14 Example: Sickle cell hemoglobin in west-central Africa Example: Sickle cell hemoglobin in west-central Africa Alleles: HbA = normal allele HbS = sickle cell allele Example: Sickle cell hemoglobin in west-central Africa Alleles: HbA = normal allele HbS = sickle cell allele Genotypes: HbAHbA: susceptible to malaria HbAHbS: resistant to malaria, mild anemia HbSHbS: susceptible to severe anemia Example: Sickle cell hemoglobin in west-central Africa Alleles: HbA = normal allele HbS = sickle cell allele Genotypes: HbAHbA: susceptible to malaria HbAHbS: resistant to malaria, mild anemia HbSHbS: susceptible to severe anemia • results in stable polymorphic equilibrium where HbA = 0.89 and HbS = 0.11 Forms of balancing selection 2. Frequency-dependent selection Forms of balancing selection 2. Frequency-dependent selection • the relative fitness of genotypes are not constant but vary with their frequencies in the population. Forms of balancing selection 2. Frequency-dependent selection • the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: Fitness: AA wAA Aa wAa aa waa 1-p2 1-2pq 1-q2 Forms of balancing selection 2. Frequency-dependent selection • the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: Fitness: AA wAA Aa wAa aa waa 1-p2 1-2pq 1-q2 Example: Self-incompatibility (S) loci in flowering plants S loci in flowering plants S loci in flowering plants ● leads to obligate out-crossing S loci in flowering plants ● leads to obligate out-crossing ● at equilibrium, all S alleles occur at equal frequencies Forms of balancing selection 3. Spatially or temporally varying selection - some genotypes are more fit than others in some habitats, or under some environmental conditions, than others. Environment A Genotype: Fitness: AA wAA 1 Aa wAa 0.95 aa waa 0.91 gene flow Environment B Genotype: Fitness: AA wAA 0.84 Aa wAa 0.93 aa waa 1 2-year female morph cycle: Uta stansburiana Orange females • small eggs • large clutches Yellow females • large eggs • small clutches 2-year female cycle: Uta stansburiana Orange female frequency Number of Breeding Females 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 Population status 0.05 0.00 8990 91 92 93 94 95 96 97 98 99 00 01 Year (Modified from Sinervo et al., 2000) Orange common Yellow common Fitness of rare O strategy 1.00 1.61 Fitness of rare Y strategy 1.60 1.00 Convergent evolution Parallel evolution Convergent evolution Parallel evolution Convergent evolution Parallel evolution Convergent evolution: the evolution of similar traits independently in distantly related taxa from different ancestral features or from different developmental pathways Example: marsupial and placental mammals (common ancestor ~ 170 mya) Convergent evolution between placental and marsupial mammals Convergent evolution in crocodiles and hippos Both have eyes on top of their heads Parallel evolution: the evolution of similar traits independently in closely related taxa involving the same genes or developmental pathways Parallel evolution: the evolution of similar traits independently in closely related taxa involving the same genes or developmental pathways Example: hemoglobins in high altitude geese Bar-headed goose, Anser indicus Andean goose, Chloephaga melanoptera Lives > 4,000 m in Himalayas Lives > 3,500 m in Andes 3D structure of hemoglobin x x 119 55 x x 119 Bar-headed goose proline alanine 55 leucine x x 119 55 Bar-headed goose proline alanine leucine Andean goose proline leucine serine Recap • Concept of fitness: Darwinian and relative fitness • Types of selection: Purifying selection Directional selection Balancing selection over-dominance frequency-dependent • Convergent and parallel evolution spatially/ temporally varying