Bio1B Evolution 6 Last lecture Evolutionary processes Selection Fitness Forms of selection Today Evolutionary processes Selection Heterozygote advantage eg sickle cell anaemia Directional selection eg coat color in mice experimental evidence genome signatures Coevolution mutualistic antagonistic Why have sex Figure 15 12 Natural Selection Can Operate in Several Ways Eg Disruptive selection beak width in African finch population Fig 15 15 Example of Heterozygote advantage sickle cell anaemia Relative fitnesses Without malaria HbbAA HbbAS HbbSS anaemia With malaria HbbAS HbbAA HbbSS More resistant to malaria Note fitness of HbbAS depends on environment malaria 1 Association between melanic phenotype and Mc1r allele in rock pocket mice from Pinacates lava flow Hopi Hoekstra in the field Geographic cline Experimental evidence for rapid evolution due to selection Eg guppy color field experiments text pp460 Others microbial evolution etc etc Fig 22 13 2 Genomic signatures of recent selection Variable site New mutation arises that increases fitness Under directional selection increases to p 1 drags linked sites with it Results in a region of low variation relative to others Storz 2005 Variation high high low high Genomic signatures of selection localized reductions in diversity What s with my crazy dog Sutter et al 2007 Science 316 112 Coevolution species 1 selection selection Mutualistic Symbioses mutualisms eg attine ants fungi Antagonistic Host pathogen Predator prey species 2 Leaf cutter ants Fig 31 22 Garter snake and poisonous pacific newt 3 Why have sex Alternatives asexual parthenogenesis in animals pp 639 apomixis in plants pp 568 569 Why not sex Inefficient risky breaks up good gene combinations Daphnia asexual in good times sexual in harsh conditions Cost of sex Parthenogenetic whiptail lizards Apomictic dandelion Hypotheses for advantages of sex pp 998 999 1 Reduces accumulation of disadvantageous mutations Mueller s ratchet 2 Brings together independent mutations that together increase fitness 3 Generates genetically diverse offspring Long term and only if sexual populations are large weak drift Advantage in variable environment Increases ability to resist pathogens parasites coevol arms race Red Queen hypothesis Sex and genetic variation Sexual reproduction produces genetically variable offspring through Random mating Independent assortment across loci Recombination between loci See pp 138 139 Independent assortment Recombination 4 Evolution consequences of parthenogenesis in an Australian gecko Heteronotia binoei Sexual population Parthenogenetic population Mite load Rapid spread but more parasites 5
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