TOPIC 13 NATURAL SELECTION NATURAL SELECTION TYPES OF NATURAL

							BIO 1120                                         Topic 13 - Slide 1

        TOPIC 13: NATURAL SELECTION

  1.   Stabilizing
  2.   Directional
  3.   Disruptive
  4.   Balancing
  5.   Sexual selection: a form of natural selection




BIO 1120                                         Topic 13 - Slide 2

                NATURAL SELECTION

 For the Hardy-Weinberg equilibrium to hold, no allele must
 provide an advantage over another, if an allele does confer
 some advantage over other alleles then the equilibrium will
 be broken and evolution will occur
 – thus, in addition to increasing the adaptiveness of an
 organism, natural selection changes allele frequencies and
 causes evolution
     1. – natural selection works on existing phenotypes
     2. – natural selection operates on what is available
     3. – natural selection does not produce perfection
     4. – natural selection does not always increase
           complexity




BIO 1120                                         Topic 13 - Slide 3

        TYPES OF NATURAL SELECTION
   (1) Stabilizing (also called normalizing)
   (2) Directional
   (3) Disruptive
   (4) Balancing (this is not listed in your text)
         a. heterozygote advantage (hybrid vigor or heterosis)
         b. frequency-dependent selection (rare-mate
             advantage)
   (5) Sexual selection




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BIO 1120                                                      Topic 13 - Slide 4

                                      Stabilizing Selection
  Lungfish, Natrix




                         A                    B               C
  # with phenotype




                         phenotypes
                                              time




BIO 1120                                                      Topic 13 - Slide 5

                              DIRECTIONAL SELECTION


          ã Shifts gene frequencies in one direction only
          ã Changes one phenotypic property towards another
          ã Peppered moth, Darwin’s finches and drought

                     A                    B               C




                                              time




BIO 1120                                                      Topic 13 - Slide 6

                               DISRUPTIVE SELECTION
  ã Shifts allele frequencies in more than one direction
  ã Variant of directional but outcome is increased
    diversity
  ã Bent grass on mine tailings


                     A                    B               C




                                              time




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BIO 1120                                               Topic 13 - Slide 7
               BALANCING SELECTION
  ã Prevents loss of variant alleles (maintains variation)
  ã Heterozygote advantage (heterosis)
     ã heterozygote has greater overall fitness
         ã sickle cell disease, cystic fibrosis
  ã Frequency-dependent selection
     ã When relative fitness of phenotype varies with frequency
         ã ‘search image’
         ã rare mate advantage


     A                       B                     C




                                                   no change!
                              time




BIO 1120                                               Topic 13 - Slide 8
                  SEXUAL SELECTION
   ã We have seen this already. In many cases it is quite
     simply the survival of the most flamboyant.
   ã e.g.’s
   ã A peacock’s tail
   ã Sticklebacks – Good gene model
   ã East African widowbird’s tail feather – Runaway
     sexual selection
   ã Can you think of any examples in humans?




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