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 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
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
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
individual
1
2
3
4
5
locus
LDH MDH GPI
11
11
11
12
12
11
22
12
11
22
11
11
11
22
11
PGI
11
12
23
33
33
Measurement of variation
individual
1
2
3
4
5
locus
LDH MDH GPI
11
11
11
12
12
11
22
12
11
22
11
11
11
22
11
PGI
11
12
23
33
33
# alleles
2
3
2
1
Measurement of variation
individual
1
2
3
4
5
locus
LDH MDH GPI
11
11
11
12
12
11
22
12
11
22
11
11
11
22
11
PGI
11
12
23
33
33
# alleles
2
3
2
1
Ho
0.0
0.75
0.5
0.0
0.0
0.25 = average H
Ho = proportion of loci within an individual that are heterozygous
Measurement of variation
individual
1
2
3
4
5
locus
LDH MDH GPI
11
11
11
12
12
11
22
12
11
22
11
11
11
22
11
PGI
11
12
23
33
33
# alleles
2
3
2
1
Ho
0.0
0.75
0.5
0.0
0.0
0.25 = average H
P = 0.75
Ho = 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
Aves (birds)
Mammalia
Teleosts (fishes)
Reptilia
Plants
Insecta
Invertebrata
P
0.10
0.15
0.15
0.22
0.26
0.33
0.40
H
0.043
0.036
0.051
0.047
0.071
0.081
0.100
from
Nevo 1978
Evidence that variability is important?
Genetic variation (H) present in specialists vs. generalists
specialists
generalists
Plants
0.04
0.08
example: zebra mussels
Invertebrates
0.06
0.15
Vertebrates
0.04
0.07
Overall
0.05
0.11
counter-example: Asian clam
Evidence that variability is important?
• heterosis – enhancement of fitness due to increased
heterozygosity
(heterosis can be present in non-hybrids)
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)
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
Arkush, D. K., et al. 2002. Can. J. Fish. Aquat. Sci. 59:159-167.
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
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
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. 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