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Berkeley BIOLOGY 1B - Lecture Notes

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1 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) Relative fitnesses: Without malaria: HbbAA > HbbAS > HbbSS anaemia With malaria: HbbAS > HbbAA > HbbSS Example of Heterozygote advantage sickle-cell anaemia More resistant to malaria Note - fitness of HbbAS depends on environment (+ malaria)2 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. 22.133 Genomic signatures of recent selection 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 Variation Variable site high high high low Storz 2005 Genomic signatures of selection; localized reductions in diversity Sutter et al. 2007 Science 316:112!What’s with my crazy dog? Coevolution species 1 species 2 Mutualistic • Symbioses, mutualisms; eg. attine ants fungi Antagonistic • Host pathogen • Predator prey selection selection Garter snake and poisonous pacific newt Leaf-cutter ants (Fig 31.22)4 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 Cost of sex Apomictic dandelion Parthenogenetic whiptail lizards Daphnia - asexual in good times, sexual in harsh conditions 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 – Advantage in variable environment – Increases ability to resist pathogens & parasites (coevol “arms race” => Red Queen hypothesis Long-term and only if sexual populations are large (weak drift) 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 Recombination5 Evolution & consequences of parthenogenesis in an Australian gecko (Heteronotia binoei) Rapid spread, but more parasites Sexual population Parthenogenetic population Mite


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Berkeley BIOLOGY 1B - Lecture Notes

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