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Variation, fitness, and genetic diversity

Variation, fitness, and genetic diversity. Bengal tiger ( Panthera tigris tigris ). Premise 1 : evolution is important. Premise 1 : evolution is important

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Variation, fitness, and genetic diversity

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

  2. Premise 1: evolution is important

  3. 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

  4. Premise 2: genetic variation is valuable for fitness

  5. Premise 2: genetic variation is valuable for fitness So, what is fitness?

  6. Premise 2: genetic variation is valuable for fitness So, what is fitness? = relative ability of a genotype, or individual, to survive and reproduce

  7. 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

  8. 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

  9. 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

  10. What is variation? described at the individual level as homozygous, heterozygous AA Aa described at the population level as monomorphic, polymorphic

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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)

  17. 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

  18. 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

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

  20. 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)

  21. 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)

  22. 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.

  23. 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

  24. 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

  25. Devil facial tumor disease (DFTD) transmissible tumor, spread by biting tumors spread by allografts, genetically identical

  26. 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

  27. ‘Markers’ of low individual heterozygosity • developmental instability • fluctuating asymmetry

  28. 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! **

  29. 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

  30. ‘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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