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					Variation, fitness, and genetic diversity




       Bengal tiger (Panthera tigris tigris)
Premise 1: evolution is important
Premise 1: evolution is important
Fundamental theorem of natural selection (Fisher 1930): rate
of evolutionary change is proportional to the amount of genetic
diversity available
Premise 2: genetic variation is valuable for fitness
Premise 2: genetic variation is valuable for fitness
                 So, what is fitness?
Premise 2: genetic variation is valuable for fitness
                 So, what is fitness?


= relative ability of a genotype, or individual, to
  survive and reproduce
  more premises:

• more offspring are produced than will survive or reproduce
          (death happens)
• individuals differ in their ability to survive and reproduce
          (death is not entirely random)
• some of these differences are genetically based
  more premises:

• more offspring are produced than will survive or reproduce

• individuals differ in their ability to survive and reproduce

• some of these differences are genetically based

• at reproductive age, genotypes that promote survival, or
  production of more offspring, will be more abundant in the
  population and will be passed on disproportionately
   more premises:

• more offspring are produced than will survive or reproduce

• individuals differ in their ability to survive and reproduce

• some of these differences are genetically based

• at reproductive age, genotypes that promote survival, or
  production of more offspring, will be more abundant in the
  population and will be passed on disproportionately

• It is very difficult to distinguish differences in fitness
  among genotypes from ‘accident’ or other factors
                  What is variation?
described at the individual level as homozygous, heterozygous
                                     AA                Aa

described at the population level as monomorphic, polymorphic
                  Measurement of variation

At the level of the gene:
   # alleles per locus

At the level of the individual:
   proportion of loci within an individual that are heterozygous (Ho)

At the level of the population:
   proportion of loci that are polymorphic in a population (P)
   = # polymorphic loci
     number loci examined
               Measurement of variation
                    locus
individual   LDH MDH GPI      PGI
       1     11  11     11    11
       2     12  12     11    12
       3     22  12     11    23
       4     22  11     11    33
        5    11  22     11    33
                 Measurement of variation
                    locus
individual   LDH MDH GPI        PGI
       1     11  11     11      11
       2     12  12     11      12
       3     22  12     11      23
       4     22  11     11      33
        5    11  22     11      33

# alleles    2      2     1     3
                   Measurement of variation
                      locus
individual     LDH MDH GPI           PGI     Ho
       1       11  11     11         11      0.0
       2       12  12     11         12      0.75
       3       22  12     11         23      0.5
       4       22  11     11         33      0.0
        5      11  22     11         33      0.0
                                             0.25 = average H
# alleles      2      2       1      3


Ho = proportion of loci within an individual that are heterozygous
                   Measurement of variation
                      locus
individual     LDH MDH GPI           PGI     Ho
       1       11  11     11         11      0.0
       2       12  12     11         12      0.75
       3       22  12     11         23      0.5
       4       22  11     11         33      0.0
        5      11  22     11         33      0.0
                                             0.25 = average H
# alleles      2      2       1      3       P = 0.75


Ho = proportion of loci within an individual that are heterozygous
P = proportion of loci that are polymorphic in a population
rare alleles – frequency usually less than 5%

private alleles – present in only one population

fixed alleles – population is monomorphic for an allele
                (due to loss of other alleles)
       Measurement of variation


                      P      H
Aves (birds)        0.10   0.043
Mammalia            0.15   0.036
Teleosts (fishes)   0.15   0.051
Reptilia            0.22   0.047
Plants              0.26   0.071
Insecta             0.33   0.081
Invertebrata        0.40   0.100



                                   from
                                   Nevo 1978
              Evidence that variability is important?

   Genetic variation (H) present in specialists vs. generalists

                 Plants    Invertebrates   Vertebrates    Overall
specialists      0.04         0.06           0.04         0.05
generalists      0.08         0.15           0.07         0.11
example: zebra mussels                 counter-example: Asian clam
     Evidence that variability is important?

• heterosis – enhancement of fitness due to increased
  heterozygosity
 (heterosis can be present in non-hybrids)
           Evidence that variability is important?


Metabolic, developmental fitness:
  – growth rate of Coot clam decreased after genetic bottleneck
    caused loss of variation              (Koehn et al. 1988)


   – efficiency of oxygen intake in American oyster decreased
                                          (Koehn and Shumway 1982)
            Evidence that variability is important?


Metabolic, developmental fitness:
  – Florida panther: sperm defects, cowlicks, kinked tails,
    cryptorchidism – reduced after increasing diversity through
    outbreeding                              (Pimm et al. 2006)
            Evidence that variability is important?


Disease resistance:
  - 82% of outbred Chinook salmon resistant to whirling disease
  - 56% of inbred salmon resistant
  - absence of 3 alleles resulted in complete susceptibility to
     whirling disease




                          Arkush, D. K., et al. 2002. Can. J. Fish. Aquat. Sci. 59:159-167.
            Evidence that variability is important?


Disease resistance:
• MHC (major histocompatibility complex) :
   immune system protects by recognition of ‘non-self’ proteins
       (e.g., graft rejection)
   most highly variable portion of genome
Tasmanian devil (Sarcophilus harrisii)
     currently ~ 10,000-100,000
     Eliminated from mainland Australia ~ 600 yrs ago
     High mortality from car strikes, dogs
     Protected in Tasmania in 1941
Devil facial tumor disease (DFTD)
       transmissible tumor, spread by biting
       tumors spread by allografts, genetically identical
 Devil facial tumor disease (DFTD)
        transmissible tumor, spread by biting
        tumors spread by allografts, genetically identical

 DFTD is recent (~10 yrs), clonal –
        but not recognized as non-self by MHC
       - severe loss of variability at MHC compared w. other species

Siddle et al. 2007. Transmission of a fatal clonal tumor by biting occurs due to depleted
MHC diversity in a threatened carnivorous marsupial. PNAS 104:16221-16226
      ‘Markers’ of low individual heterozygosity

• developmental instability
• fluctuating asymmetry
             What are the sources of variation?

novel material - mutation: very rare!!

       approx. 10-6 mutations per gamete per generation

       most of these mutations do not affect the phenotype

       > 100 to 1,000 generations to restore variability via mutation

       ** lost alleles are not regained! **
             What are the sources of variation?

novel material - mutation: very rare!!

       approx. 10-6 mutations per gamete per generation

rearranged material - sexual reproduction

       blending of genes, and rearrangements
        ‘Markers’ of low individual heterozygosity

cutthroat trout in hatchery vs. wild (Leary et al. 1985)
  57% reduction in # polymorphic loci
   29% reduction in average # alleles per locus
   21% reduction in average heterozygosity per locus


   of 51 fish:
    – 10 fish missing one pectoral fin
    – 3 fish missing 2 fins
    – many had deformed vertebral columns

				
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posted:3/31/2013
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