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Genome Evolution. Amos Tanay 2012 Genome evolution Lecture 5: Selection vs. mutation/recombination. Species Slide 2 Genome Evolution. Amos Tanay 2012 Mutation-Selection balance When an allele is weakly deleterious, mutations can play a major role in driving allele frequencies Genotype Fitness Frequency (HW) New allele frequency, without mutation New allele frequency, assuming mutation A a ignore (q0, and we can estimate the mutation rate at the Huntington locus, as hsq = 10 -6 (1-0.81) = 1.9x10 7 to 70x10 -6 (1-0.81) = 1.3x10 -6 Slide 4 Genome Evolution. Amos Tanay 2012 Mutation-Selection balance: Haldane-Muller The average fitness of the population, given recurrent mutations in rate at a locus with negative fitness s. Assume perfect recessivity (h=0): Assuming partial dominance (h>0) The Haldane-Muller principle: the effect of mutation on the average population fitness depends only on the mutation rate, not on the fitness of the alleles!! Slide 5 Genome Evolution. Amos Tanay 2012 (historical) Malaria distribution Sickle-cell anemia Overdominance A SNP affecting the beta-globin gene make the encoded protein defected. The resulted red blood cells are curved and elongated, and are removed from the circulation Homozygous for the mutation will usually die from anemia without intensive care Heterozygous individual will have mild anemia, but will deal better with the malaria parasite Plasmodium fliciparum (maybe because infected red cells become sickled) wiki Slide 6 Genome Evolution. Amos Tanay 2012 Other types of selection Different fitness for different individuals. e.g., male vs. female For example male genes that take up female resources in mammals This was suggested to lead to the phenomenon of imprinting where cells are expressing only the maternal or paternal allele Imprinted genes are much like haploids Slide 7 Genome Evolution. Amos Tanay 2012 Frequency-, Density-dependent selection: when the fitness depend on the frequency of the allele or the population size. Fecundity selection: different reproductive potential for mating pairs. Effects of heterogeneous environment Effects that apply directly to the haplotype: gametic selection/meiotic drive (e.g., killing your homologous chromosome reproductive potential) Sexual selection: male advertising the reproductive potential, or confronting other males Kin selection: (origin of altruism) Other types of selection Slide 8 Genome Evolution. Amos Tanay 2012 Recombination and selection Slide 9 Genome Evolution. Amos Tanay 2012 Linkage and selection Beneficial Weakly deleterious Beneficial Linkage interfere with the purging of deleterious mutations and reduce the efficiency of positive selection! Selective sweep or Hitchhiking effect or genetic draft (Gillespie) Hill-Robertson effect Slide 10 Genome Evolution. Amos Tanay 2012 Linkage and selection The variance in allele frequency is used to define the effective population size Simplistically, assume a neutral locus is evolving such that a selective sweep is affecting a fully linked locus at rate A sweep will fixate the allele with probability p, and we further assume that the sweep happens instantly: This is very rough, but it demonstrates the basic intuition here: sweeps reduce the effective selection in a way that can be quantified through reduction in the effective population size. C the average frequency of the neutral allele after the sweep Slide 11 Genome Evolution. Amos Tanay 2012 Cost of sex Wasting half of your genes on non-reproductive individuals Selective advantage of an asexual gene = 2 fold! Still sex is prevalent among complex species It even persists when both asexual and sexual reproduction is available as in S. cerevisae: Mating locus MAT type a and alpha Haploids are growing quickly when all is well Mating is occurring when time is rough Meiosis take the diploid back to haploids Slide 12 Genome Evolution. Amos Tanay 2012 Benefits of sexual reproduction Fighting genetic draft: clearing deleterious mutations Buffering variation DNA repair through recombination (even in somatic tissues) Fighting mutation interference: more effective/rapid adaptation The red queen hypothesis Can this add up to a factor of 2? (Alexey Kondrashov theory: epistatsis of deleterious alleles make sex beneficial) Slide 13 Genome Evolution. Amos Tanay 2012 Moran et al., Running with the Red Queen: Host- Parasite Coevolution Selects for Biparental Sex Science 8 July 2011: vol. 333 no. 6039 216-218 Slide 14 Genome Evolution. Amos Tanay 2012 What is a species? Multiple definitions.. free flow of genetic information within population Weak (or zero) flow of information across species barriers Species 1 Species 2 Strain 1 Strain 2 We change wright-fischers or Moran model, by removing the assumption of random mixing. Instead, we can assume subpopulations are more likely to mate among themselves. Different models are possible, all end up increasing the genetic distance between subpopulations Slide 15 Genome Evolution. Amos Tanay 2012 Speciation Allopatric speciation occurs through geographical separation Parapatric speciation occurs without geographical separation but with weak flow of genetic information Sympatric speciation occurs while information is flowing Barriers can genetic, physical, and behavioral The Phenomenon of new species emergence is called speciation It is well accepted that speciation is driven by the formation of reproductive barriers Slide 16