variation, fitness, and genetic diversity bengal tiger (panthera tigris tigris)

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 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

more premises:

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

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

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

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

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

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) 0.10 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

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?

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 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