Evolution Genetic Basis of Variation
Genetic Polymorphism <ul><li>Two or more forms of a Mendellian character are often represented in a population </li></ul><...
Extent of Genetic Variation <ul><li>Genetic variation is extensive </li></ul><ul><li>Example:  Fruit flies  </li></ul><ul>...
Geographic Variation <ul><li>Natural selection contributes because of different environmental factors </li></ul><ul><li>Gr...
Sources of Genetic Variation <ul><li>Mutation & sexual recombination are sources of genetic variation </li></ul><ul><li>Mu...
Preservation of Variation <ul><li>Natural selection doesn’t cull unfavorable genotypes </li></ul><ul><li>Diploidy   preser...
Adaptive Evolution <ul><li>Darwinian fitness is measured only by the relative contribution an individual makes to the gene...
Pleiotropy <ul><li>Genes can have more than one effect </li></ul><ul><li>The ability to effect multiple  phenotypic charac...
Modes of Natural Selection <ul><li>There are three types of  natural selection: </li></ul><ul><li>Stabilizing Selection </...
Types of Natural Selection
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Evolution2

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Evolution2

  1. 1. Evolution Genetic Basis of Variation
  2. 2. Genetic Polymorphism <ul><li>Two or more forms of a Mendellian character are often represented in a population </li></ul><ul><li>Contrasting forms = morphs </li></ul><ul><li>The population is polymorphic if 2 or more morphs are commonly seen </li></ul>
  3. 3. Extent of Genetic Variation <ul><li>Genetic variation is extensive </li></ul><ul><li>Example: Fruit flies </li></ul><ul><ul><li>The gene pool has 2 or more alleles at 30% of loci </li></ul></ul><ul><ul><li>Each fly is heterozygous at 12% of loci </li></ul></ul><ul><ul><li>Therefore 700-1200 heterozygous loci per fly </li></ul></ul><ul><ul><li>Any 2 flies in a population differ in genotype at ~ 25% of loci </li></ul></ul><ul><li>Similar in humans </li></ul>
  4. 4. Geographic Variation <ul><li>Natural selection contributes because of different environmental factors </li></ul><ul><li>Graded change in a trait = cline </li></ul><ul><ul><li>e.g. body size increases with latitude in North American mammals </li></ul></ul>
  5. 5. Sources of Genetic Variation <ul><li>Mutation & sexual recombination are sources of genetic variation </li></ul><ul><li>Mutation </li></ul><ul><ul><li>rare & random </li></ul></ul><ul><ul><li>only mutations in gametes can be passed on </li></ul></ul><ul><ul><li>most point mutations are innocuous </li></ul></ul><ul><ul><li>those that alter a protein are more often harmful than beneficial </li></ul></ul><ul><li>Recombination </li></ul><ul><ul><li>most variation is due to recombination of existing alleles </li></ul></ul><ul><ul><li>sexual reshuffling </li></ul></ul><ul><ul><li>crossing over </li></ul></ul>
  6. 6. Preservation of Variation <ul><li>Natural selection doesn’t cull unfavorable genotypes </li></ul><ul><li>Diploidy preserves recessive alleles </li></ul><ul><ul><li>may not be favorable now, but maybe later </li></ul></ul><ul><li>Balanced polymorphism - ability to maintain diversity </li></ul><ul><ul><li>heterozygous advantage or hybrid vigor </li></ul></ul><ul><li>Frequency dependent selection </li></ul><ul><ul><li>reproductive success declines if that form becomes too common </li></ul></ul><ul><li>Some variation is neutral </li></ul>
  7. 7. Adaptive Evolution <ul><li>Darwinian fitness is measured only by the relative contribution an individual makes to the gene pool of the next generation </li></ul><ul><li>Selection coefficient: </li></ul><ul><ul><li>Relative fitness of most fertile variant set at 1 </li></ul></ul><ul><ul><li>So for wild flowers Aa or AA = 1 </li></ul></ul><ul><ul><li>If plants with white flowers average 80% as many offspring, their relative fitness = 0.8 </li></ul></ul><ul><ul><li>selection coefficient = 0.2 (1-0.8) </li></ul></ul>
  8. 8. Pleiotropy <ul><li>Genes can have more than one effect </li></ul><ul><li>The ability to effect multiple phenotypic characters = pleiotropy </li></ul><ul><li>Preserved if good outweighs the bad </li></ul>
  9. 9. Modes of Natural Selection <ul><li>There are three types of natural selection: </li></ul><ul><li>Stabilizing Selection </li></ul><ul><ul><li>eliminates the extremes </li></ul></ul><ul><ul><li>reduces variation </li></ul></ul><ul><ul><li>maintains characteristics that are successful </li></ul></ul><ul><li>Disruptive or Diversifying Selection </li></ul><ul><ul><li>selects against the average </li></ul></ul><ul><ul><li>favors opposite extremes over intermediate individuals </li></ul></ul><ul><li>Directional Selection </li></ul><ul><ul><li>moves characteristic in a particular direction   </li></ul></ul><ul><ul><li>shifts population by favoring variants at one extreme </li></ul></ul>
  10. 10. Types of Natural Selection

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