variation, fitness, and genetic diversity bengal tiger (panthera tigris tigris)
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Variation, fitness, and genetic diversity
Bengal tiger (Panthera tigris tigris)
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Premise 1: evolution is important
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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
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Premise 2: genetic variation is valuable for fitness
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Premise 2: genetic variation is valuable for fitness
So, what is fitness?
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Premise 2: genetic variation is valuable for fitness
So, what is fitness?
= relative ability of a genotype, or individual, to survive and reproduce
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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
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• 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:
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• 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
more premises:
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What is variation?described at the individual level as homozygous, heterozygous
AA Aa
described at the population level as monomorphic, polymorphic
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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
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Measurement of variation
locusindividual LDH MDH GPI PGI
1 11 11 11 112 12 12 11 123 22 12 11 234 22 11 11 335 11 22 11 33
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Measurement of variation
locusindividual LDH MDH GPI PGI
1 11 11 11 112 12 12 11 123 22 12 11 234 22 11 11 335 11 22 11 33
# alleles 2 2 1 3
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Measurement of variation
locusindividual LDH MDH GPI PGI Ho
1 11 11 11 11 0.02 12 12 11 12 0.753 22 12 11 23 0.54 22 11 11 33 0.05 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
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Measurement of variation
locusindividual LDH MDH GPI PGI Ho
1 11 11 11 11 0.02 12 12 11 12 0.753 22 12 11 23 0.54 22 11 11 33 0.05 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 heterozygousP = proportion of loci that are polymorphic in a population
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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)
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Measurement of variation
P HAves (birds) 0.10 0.043 Mammalia 0.15 0.036Teleosts (fishes) 0.15 0.051Reptilia 0.22 0.047Plants 0.26 0.071Insecta 0.33 0.081Invertebrata 0.40 0.100
from Nevo 1978
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Plants Invertebrates Vertebrates Overallspecialists 0.04 0.06 0.04 0.05generalists 0.08 0.15 0.07 0.11
Genetic variation (H) present in specialists vs. generalists
example: zebra mussels counter-example: Asian clam
Evidence that variability is important?
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Evidence that variability is important?
• heterosis – enhancement of fitness due to increased heterozygosity
(heterosis can be present in non-hybrids)
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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?
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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?
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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.
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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?
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Tasmanian devil (Sarcophilus harrisii)currently ~ 10,000-100,000Eliminated from mainland Australia ~ 600 yrs agoHigh mortality from car strikes, dogsProtected in Tasmania in 1941
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Devil facial tumor disease (DFTD)transmissible tumor, spread by bitingtumors spread by allografts, genetically identical
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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
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‘Markers’ of low individual heterozygosity
• developmental instability
• fluctuating asymmetry
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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! **
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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
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‘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