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Transcript
Genetical theory of natural selection Reminders Natural selection ≠ evolution Natural selection ≠ evolution by natural selection Natural selection can have no effect unless phenotypes differ in genotype Feature cannot evolve by natural selection unless it affects survival or reproduction Fitness Fitness of a genotype is the average lifetime contribution of individuals of that genotype to the population Many times measured as reproductive success (number of offspring that survive) Absolute fitness (R) versus relative fitness (W) Rate of genetic change under selection depends on relative fitness of genotypes Mean fitness Average fitness (w) of individuals in a population relative to the fittest genotype Suppose W A = 0.75, WB = 1.0, p = 0.2, q = 0.8 Then w = (0.2)(0.75) + (0.8)(1.0) = 0.95 Coefficient of selection Amount by which the fitness of one genotype is reduced relative to the reference genotype WA = 0.75, s = 0.25 Overall fitness Fitness depends not only on reproductive success, especially when species reproduce sexually and have more than one reproductive event Age of reproduction Selection during sexual reproduction Phenotype and fitness Relationship described by modes of selection Directional Stabilizing (normalizing) Diversifying (disruptive) What are effects of selection? Fixation of one allele/loss of genetic variation (directional selection) Maintenance of genetic variation (balancing selection) Assumptions for now Population is very large Mutation and gene flow do not occur Selection at each locus independent Natural section is differential survival No overlapping generations Directional selection Assume fitness of heterozygote is intermediate between the two homozygotes Fixation by natural selection Advantageous allele increases in frequency per generation according to: Δp = 1/2spq w Δp is positive whenever p and q are greater than zero Rate of evolutionary change increases as variation at locus increases Δp is positive as long as s is greater than zero Number of generations to fixation Depends on: Initial frequency Selection coefficient Degree of dominance Time to fixation If initial frequency low, recessive mutation increases very slowly (rarely exposed in homozygous form); when common, recessive alleles go to fixation quickly (selection against deleterious dominant fast) Dominant mutations increase in frequency rapidly, but approach fixation slowly (selection against rare recessive is slow) w increases as natural selection proceeds Directional selection Directional selection Why do we see deleterious alleles in a population? Frequency of a deleterious allele is the balance between the rate at which it is eliminated by natural selection and the rate at which it is introduced by mutation or gene flow Directly proportional to mutation rate and inversely proportional to strength of selection Gene flow Gene flow in mussels Gene flow can reduce adaptiveness How does selection maintain variation? Balancing selection Heterozygote advantage/disadvantage Varying selection Frequency dependent selection If selection coefficient of two homozygotes are equal… If selection coefficient of two homozygotes are not equal… Overdominance Sickle cell anemia and malaria Single amino acid substitution leads to crystallization of hemoglobin at low oxygen concentrations Heterozygotes have mild condition, homozygotes often don’t survive Malaria Malaria caused by Plasmodium falciparum (a protist) that also infects rbc’s Individuals “normal” at the sickle cell locus get very bad malaria, heterozygotes less bad, blood cells sickle preventing population growth of protist Heterozygote advantage arises from balance of opposing selective factors – anemia and malaria (antagonistic selection) Anemia/malaria Heterozygote advantage (overdominance) Heterozygote disadvantage (underdominance) Varying selection Temporal variation Spatial variation Multiple-niche polymorphism Frequency-dependent selection The fitness of a genotype depends on the genotype frequencies in the population Inverse-frequency dependent selection Rarer the phenotype, the greater its fitness Scale eating fish Inverse-frequency dependent selection Rewardless orchids Positive frequency dependent selection Fitness of a genotype is greater the more frequent it is in a population Mullerian mimicry Adaptive landscapes Adaptive landscapes