Genetical theory of natural selection

Transcription

1 Reminders Genetical theory of natural selection Chapter 12 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 Phenotype and fitness Relationship described by modes of selection Directional Stabilizing (normalizing) Diversifying (disruptive) Overdominant Underdominant Negative selection = purifying selection Normalizing Disruptive 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 1

2 Mean fitness Wb = 1.0 Wa = 0.75 Average fitness (w) of individuals in a population relative to the fittest genotype Suppose W A = 0.75, W B = 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 W A = 0.75, s = 0.25 Measures the selective advantage of the fitter genotype Rate of evolutionary change depends on relative fitness of genotypes 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 2

3 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 Mating is random Mutation and gene flow do not occur Selection at each locus is independent Natural section is differential survival No overlapping generations Directional selection Directional selection Homozygous form of beneficial allele has highest fitness Adventageous allele can invade a population Assume fitness of heterozygote is intermediate between the two homozygotes Fixation by natural selection Number of generations to fixation 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 Depends on: Initial frequency Selection coefficient Degree of dominance 3

4 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 recessive 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 4

5 Gene flow Gene flow in mussels Gene flow can reduce adaptiveness Maintenance of variation Mutation producing deleterious alleles subject to weak selection Selective neutrality (genetic drift) Gene flow Maintenance by natural selection How does selection maintain variation? Balancing selection Heterozygote advantage/disadvantage Varying selection Frequency dependent selection If selection coefficient of two homozygotes are equal 5

6 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 prevent population growth of protist Heterozygote advantage arises from balance of opposing selective factors anemia and malaria (antagonistic selection) Anemia/malaria Genotype AA AS SS Phenotype Initial frequency Relative fitness Normal blood, susceptible to malaria Slight anemia, less susceptible to malaria Severe anemia p 2 2pq q 2 1 s 1 1 t Estimates: s = 0.12 t = 0.86 Heterozygote advantage (overdominance) 6

7 Varying selection Multiple niche polymorphism Spatial variation Temporal 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 7

8 Inverse-frequency dependent selection Positive frequency dependent selection Fitness of a genotype is greater the more frequent it is in a population Mullerian mimicry Heterozygote disadvantage (underdominance) New variants are selected against in H. erato Adaptive landscapes Adaptive landscapes 8

9 Interaction of selection and drift Effect of drift is negligible if selection is strong or effective population size is large Deleterious mutations can be fixed by drift A slightly advantageous allele is less likely to be fixed by selection if population is small Peak shift Strength of natural selection 9