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

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

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 (H o ) 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 individualLDHMDHGPIPGI

Measurement of variation locus individualLDHMDHGPIPGI # alleles2213

Measurement of variation locus individualLDHMDHGPIPGIH o = average H # alleles2213 H o = proportion of loci within an individual that are heterozygous

Measurement of variation locus individualLDHMDHGPIPGIH o = average H # alleles2213 P = 0.75 H o = 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) Mammalia Teleosts (fishes) Reptilia Plants Insecta Invertebrata from Nevo 1978

Plants Invertebrates Vertebrates Overall specialists generalists Genetic variation (H) present in specialists vs. generalists example: zebra musselscounter-example: Asian clam 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 Can. J. Fish. Aquat. Sci. 59:

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 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 Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. PNAS 104:

‘Markers’ of low individual heterozygosity developmental instability fluctuating asymmetry

What are the sources of variation? novel material - mutation: very rare!! approx 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 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 ) 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