Variation, fitness, and genetic diversity Bengal tiger (Panthera tigris tigris)

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  • Variation, fitness, and genetic diversityBengal 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 fitnessSo, what is fitness?

  • Premise 2: genetic variation is valuable for fitnessSo, 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 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 factorsmore premises:

  • What is variation?described at the individual level as homozygous, heterozygous AA Aa

    described at the population level as monomorphic, polymorphic

  • Measurement of variationAt 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 locusindividualLDHMDHGPIPGI111111111212121112322121123422111133511221133

  • Measurement of variation locusindividualLDHMDHGPIPGI111111111212121112322121123422111133511221133# alleles2213

  • Measurement of variation locusindividualLDHMDHGPIPGIHo1111111110.02121211120.753221211230.54221111330.05112211330.00.25 = average H# alleles2213

    Ho = proportion of loci within an individual that are heterozygous

  • Measurement of variation locusindividualLDHMDHGPIPGIHo1111111110.02121211120.753221211230.54221111330.05112211330.00.25 = average H# alleles2213 P = 0.75

    Ho = proportion of loci within an individual that are heterozygousP = 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 HAves (birds)0.100.043 Mammalia0.150.036Teleosts (fishes)0.150.051Reptilia0.220.047Plants0.260.071Insecta0.330.081Invertebrata0.400.100

    from Nevo 1978

  • Plants Invertebrates Vertebrates Overallspecialists0.04 0.06 0.04 0.05generalists0.08 0.15 0.07 0.11Genetic variation (H) present in specialists vs. generalistsexample: zebra musselscounter-example: Asian clamEvidence that variability is important?

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

  • 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

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

  • 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

    Evidence that variability is important?

  • Tasmanian devil (Sarcophilus harrisii)currently ~ 10,000-100,000Eliminated from mainland Australia ~ 600 yrs agoHigh mortality from car strikes, dogsProtected in Tasmania in 1941

  • Devil facial tumor disease (DFTD)transmissible tumor, spread by bitingtumors spread by allografts, genetically identical

  • Devil facial tumor disease (DFTD)transmissible tumor, spread by bitingtumors 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 heterozygositydevelopmental instabilityfluctuating 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 generationrearranged material - sexual reproduction

    blending of genes, and rearrangements

  • Markers of low individual heterozygositycutthroat 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 fin3 fish missing 2 finsmany had deformed vertebral columns

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