Transcript third and last of Chapter 17, Molecular Evolution and Population

Chapter 17
Population
Genetics and
Evolution,
part 3
Jones and Bartlett Publishers © 2005
Bottlenecks and Founder Effects
• Temporary periods of small population size
= bottleneck.
• This results in a loss of alleles.
• Elephant seals – dropped to 20 animals, now
up to 30,000. Now there is no variation at
24 enzyme loci.
• Founder effect – new area of colonization.
Consequences of inbreeding for genotype & allele
frequencies at F = 1 and F = 0
Genotype frequencies under H-W
and with complete inbreeding (F = 1)
p = 0.4, q = 0.6
Decrease in heterozygosity with
successive generations of
inbreeding
Effect of inbreeding on the
genotype frequencies
F = Inbreeding
Coefficient.
Reduction in
heterozygosity due
to inbreeding
(HI) = 2pq (1-F)
The frequency of heterozygotes is reduced
as inbreeding increases
Calculating inbreeding coefficient using
allelic identity by descent in an inbred pedigree
A pedigree showing
inbreeding
A closed rectangle in a
pedigree indicates
inbreeding
Calculation of the probability that the alleles
indicated by the double-headed arrows
are identical by descent
The logic behind calculation of allelic
identity by descent in a pedigree
For example, the probability of producing 2 blue gametes for individual A is 1/2
x1/2 = 1/4. Similarly, the probability of producing 2 red gametes is also 1/4, but
the probability of producing a red and a blue gamete is 1/2 (1/4 + 1/4). FA is the
inbreeding coefficient of the individual producing the gametes.
A complex pedigree in which the individual I received genes
from different ancestors through multiple paths
Calculation of inbreeding coefficient in a complex pedigree is more
involved because each path contributes to the final inbreeding
coefficent.
Inbreeding increases the chance of having progeny
that are homozygous for a rare recessive trait
Effect of autozygosity on viability
Drosophila 2nd chromosome
Inbreeding
depression
in rats
inbreeding
depression in the
titmouse
Gene flow in corn
F = proportion of offspring of recessive
plants, grown at different distances from a
dominant strain, that were fathered by the
dominant strain
Three generalized forms of selection
Directional
Stabilizing
Disruptive
Natural selection increases adaptation
• Differential reproductive ability of
alternative genotypes is natural selection.
• Fitness describes the reproductive efficiency
of a genotype in relationship to others.
(W=fitness)
• Selection coefficient – S=1-W.
• Peppered moth example.
Positive frequency-dependent selection
on Heliconius color patterns
Frequency of melanic
moths of
A. Biston betularia
B. Gonodontis bidentata
Mutations
• The source of all genetic variation is
mutations.
• Random mutations occur at a background
level (replication error, radiation, etc.)
• Some mutations in response to stress may be
caused by transposons.
Effect of mutation (irreversible or reversible)
on allele frequency
Allele frequency is changed very slowly by mutation. In the case of
reversible mutation, an equilibrium state is reached where the allele
frequency becomes constant.
Effect of selection for a favored allele (A)
in a haploid (Escherichia coli)
Results of selection for a favored allele
in a diploid depends upon whether
the allele is dominant or recessive
Effect of the degree of dominance in a diploid on the
equilibrium frequency of a recessive lethal allele
h = degree of
dominance. If
the
deleterious
allele is
completely
recessive, then
h= 0
Geographic distribution of the diseases
sickle cell anemia and falciparum malaria
The heterozygote is favored over the homozygous
dominant genotype (overdominance) in areas where malaria is
prevalent. The homozygous recessive is usually lethal.
Speciation has a genetic basis
• Speciation may occur suddenly.
• Polyploidy is a good example of a sudden
reproductive barrier.
• Translocations also isolate populations.
• Neutralist vs. selectionist debate.