Transcript Adaptive evolution

Evolution
Genetic variation
There can be no evolution without variation
Mutations are the ultimate source of new
variation in DNA
Germ-line mutations are important for
evolution
Genetic variation in natural populations
Natural populations exhibit great variation.
Heterozygosity is approximately 10% for all loci.
Maintaining genetic variation
Neutral genetic diversity
Balance between origin
of variation by mutation
and loss by drift
Adaptive genetic
diversity
Balanced between origin
of variation by mutation
and the loss by selection
Overdominance
Selection favoring
overdominance
o favors heterozygotes
o results in stable gene frequencies
fitness
SS
SA
AA
Overdominance: Sickle Cell
Anemia
Overdominance: Sickle Cell
Anemia
SS = sickle-cell anemia
SA = malaria resistance
AA = Malaria susceptability
Sickle-cell anemia
Balancing Selection
Case in which natural selection maintains genetic variation at
frequencies above levels of mutation.
…in this case, balancing selection makes the population as a
whole more resistant to malaria
Prion disease in Fore
Tribespeople
Kuru (prion disease related to Mad Cow
disease) resulted from cannibalism
Only heterozygotes for the Prion Protein
Gene (PRNP) are resistant
Underdominance
Underdominance
o heterozygotes have a lower fitness
o results in an unstable equilibrium
o Uncommon and few natural examples
fitness
SS
SA
AA
Other forms of selection….
Frequency-dependent selection
male
andromorph
gynomorph
andromorph
Rare male
effect
(negative frequency
dependent mating
success)
Rare male effect
Stabilizing Selection
Selection that eliminates the extremes of a
distribution and favors the centre
Stabilizing Selection
Stabilizing
selection is
common in
quantitative traits.
Disruptive Selection
Selection that favors the extremes and
eliminates the middle of a frequency
distribution of trait values
Dull yearlings allowed to nest on
territory
Bright yearlings can hold their
own
Disruptive Selection
Selection that favors the extremes and
eliminates the middle of a frequency
distribution of trait values
Expression of
variation
Expression of
variation
Feather modification
Feather modification
Environmentally-induced
phenotypic responses
Induced response: One genotype produces multiple phenotypes
Environmentally induced response
Daphnia can
produce helmets
and spines if
predators are
present
Other responses
Barnacles will
change shape of
growth if a predator
snail present
Snails change
offspring number,
shell size, shell
organic content in
utero in presence of
crayfish
And Matt’s favorite….
Phenotypes and reaction
norms
GENOTYPE
PHENOTYPE
Phenotypes and reaction
norms
GENOTYPE
X
ENVIRONMENT
PHENOTYPE
Phenotypes and reaction
norms
GENOTYPE
X
ENVIRONMENT
PHENOTYPE
No genetic variation
underlying number of digits,
so no sensitivity to population
density
Fecundity is much more
sensitive to fecundity, and
some genotypes more than
others
So, is phenotypic plasticity
adaptive?
The reaction norms themselves can be subject to
selection
Selection to increase sensitivity to population density
would lead to the increase in frequency of G1
Selection to decrease sensitivity to density would
increase the frequency of G3