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Chapter 8

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BIOL 359
Jonathan Witt

Chapter 8: Evolution at Multiple Loci: Linkage and Sex 8.3. The Adaptive Significance of Sex - Many species are capable of both sexual & asexual reproduction o Parthenogenesis: Offspring develop from unfertilized egg, no male participation, females produce live-born genetically identical to themselves - Which Reproduction Mode is Better: Sexual or Asexual? o The null model for population that reproduces sexually and asexually  1. Female’s reproductive mode does not affect the # of offspring  2. Female’s reproductive mode does not affect the probability of offspring survival o Asexual females produce 2x as many grandchildren as sexual females  Constitute larger fraction of the population every generation, population destined to be overwhelmed by asexuals  However, asexual takeovers do not seem to have happened very often  Null model is violated o 1. The # of offspring varies depend on whether the female is sexual or asexual  Fathers provide resources or other forms of parental care in minority of species, majority just provide genes o 2. The probability of survival does depend on whether she produces them asexually or sexually  E.g. Flour beetles - 2 strains: Red = Sexual & Black = Asexual  Not capable of asexual reproduction, for the asexual strain, each generation # was counted, and replaced each 1 of the black adults with 3 new black adults from a reservoir population (no competition)  Black strain with 3x reproductive success than the red strain, but there is no adaptation  no evolution  Red adults breed among themselves  evolution  Environmental selection: Spiked flours with malathion o 8 replications with different malathion concentrations  Population was prevented from mating with each other & kept for 30 generations o Asexual strain produce more offspring than sexual strain, so the only way asexual would fail to take over is if the second assumption is incorrect  Control: Neither population was allowed to evolve, but black strain has 3x reproductive advantage over the red strains o Black strain quickly eliminated the red strain  Experimental: Initially the black strain appears to be taking over, but the evolving red strain recovered & completely eliminated the black strain at the end o The speed elimination took over depends on the malathion concentration  High malathion concentration: Stronger selection for insecticide resistance, greater advantage for sexual reproduction, faster - Sex in Populations Means Genetic Recombination o Sex: Reproduction involving meiosis “crossing-over” and mating between unrelated individuals  Every generation, allele will be part of a different multilocus genotype  E.g. Artificial selection increases the rate of recombination o Genetic recombination reduces linage disequilibrium through shuffling of multilocus genotypes  In a population in linkage equilibrium, ,sex has no effect, thus no benefits - Genetic Drift, in Combination with Mutation, can Make Sex Beneficial o The removal of a deleterious mutation can be through back, counter mutation or by mating with an individual with no deleterious mutations o Muller’s ratchet: Asexual populations are doomed to accumulate deleterious mutations  Sex breaks the ratchet, mutations can be quickly reconstituted by outcrossing &
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