Population Genetics 2
(To Dr. Robert Wyatt: Thanks for
the inspiration!)
Inbreeding = breeding between relatives
Self-fertilization is the most extreme form of inbreeding.
The coefficient of inbreeding = F = prob. two genes
at same locus are identical by common descent = loss of heterozygosity.
F = Ho-H/Ho
where Ho = 2pq and H =
real frequency of heterozygotes
Genotypic Frequencies with Inbreeding
|
Genotype |
Inbreeding coefficient F |
F=0 random mating |
F=1 |
|
AA |
P2(1-F)+pF |
p2 |
p |
|
Aa |
2pq(1-F) |
2pq |
0 |
|
aa |
q2(1-F)+qF |
q2 |
q |
Consequences of inbreeding:
a. inbreeding depression.
Outbreeding depression caused by loss of
well-adapted alleles is also a risk. It may lead to a decrease in local
adaptation. A decision was made to augment the Tatra Mountain, Czechoslovakia
population of Ibex (Capra ibex) with imported animals from Turkey. The hybrid
young were born in February, the coldest month of the year. The entire
population went extinct as a result of this outbreeding (Greig 1979). However,
populations tend to recover fitness lost from outbreeding depression more
rapidly than from inbreeding depression (Templeton 1983).
b. genetic drift: is a change in the gene pool resulting from random
events.
Genetic drift is unlikely in a large
population. However, in a small population alleles can be lost and the
population can become homozygous for certain traits. Drift is random unlike
selection; therefore, it does not tend to lead toward adaptation to the
environment.
Founder Effect - is genetic drift that occurs when only a few
individuals
representing a small fraction of the global
gene pool invade a new area.
Examples include the California Cypress and
Bottlenecks - periods of near extinction due to disaster
Cheetahs
The northern elephant seal (Mirounga
angustirostris) was hunted for their
blubber, which was rendered for oil. By 1910, only 100 northern elephant seals
remained on Guadalupe Island, Baja, Mexico. Their population now exceeds
150,000. Northern elephant seals exhibit a polygynous breeding strategy, in
which males establish a dominance hierarchy. Alpha males typically reside over
harem of 50 females.
Can we calculate reduction of heterozygosity?
FST=HT-HS/HT
Measures solely inbreeding due to
population subdivision
FST =fixation index = reduction of hets. Due to drift
where HS=heterozygosity of a random mating sub pop.
And HT=heterozygosity
of random mating total pop.
Fixation Index in subpopulations in generation t
Ft=1-(1-1/2N)
= 0 for no homozygosity
=1 for complete homozygosity
Effective Population Size with
Unequal Sex Ratios
Ne = 4NmNf/Nm+Nf
Fixation over Time
1-FST = (1-1/2Ne)t
where 1/Ne = (1/t) (1/N1 +1/N2+…1/Nt)
Migrants come to the Rescue
F = 1/ 4Nm+1 = change in
homozygosity in subpopulations
The Big Picture
The Tempo and Mode of Evolution S.J. Gould
Natural Selection and The Red Queen
Van Valen 1973
Adaptive topography (Sewall Wright) -Fitness does not always change
linearly with changes in allele frequency. Changes in phenotype may lead to
local optima. If evolution moves in small steps then species can be trapped on
lower peaks. Alteration of the environment alters the adaptive topography.
"Beware the Panglossian Paradigm" S. J. Gould
Two Sites for you to play at: