Migration often reverses effects of inbreeding.

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Migration often reverses effects of
inbreeding. Many rare species are being
hybridized out of existence by crossing with
common related species. Mutation and migration
are often important determinants in the
maintenance of genetic diversity.
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Balance between deleterious mutations
selection results in an ever-present but changing
gene pool of rare deleterious mutations (mutation
load) in the population. Inbreeding exposes
these mutations, resulting in reduced
reproduction survival which in turn increases
the extinction risk in threatened species.
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Genetic diversity is the raw material required
for adaptive evolutionary change. However,
genetic diversity is lost by chance in small
populations and as a result of directional selecti
on. Mutation is the ultimate source of genetic
diversity while recombination can produce new
combinations of alleles.
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If genetic diversity is lost, it can be
regenerated via mutation, but this is a very slow
process. Alternatively, genetic diversity can be
restored by natural or artificial immigration
between populations with different allelic
content.
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Mutations are sudden changes in an allele or
chromosome. All genetic diversity originates
from mutations. Patterns of genetic diversity in
populations are the result of a variety of forces
that act to eliminate or increase disperse
mutations among individuals and populations.
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Conservation Concerns with regards to
mutations How rapidly mutations add genetic
diversity to populations. How mutations affect
the adaptive potential and reproductive fitness
of populations. How important are the
accumulation of deleterious alleles to fitness
decline in small populations.
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Silent Substitution Base substitution that
DOES NOT change an amino acid. These probably
have little or no impact on fitness and
therefore are also referred to as Neutral
Mutations. Neutral mutations are important as
molecular markers and clocks that provide
valuable information on genetic differences
among individuals, populations, species.
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Rate of mutation is critical to its role in
evolution. Mutation rates differ for different
classes of loci. Although spontaneous mutations
are considered to be nearly constant over time,
mutation rates may be elevated under stressful
conditions and by particular environmental agents
(radiation, mutagens).
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Example How long will it take a microsatellite
locus to regenerate a frequency of 0.5 for an
allele that has been lost? p0 1.00 pt
0.5 ? 1 X 10-4 t ln 1.00 - ln 0.50/1 X
10-4 6,931 generations!
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Most mutations not occurring in functional
loci are expected to be neutral or nearly
neutral. Mutations within functional loci will
predominantly be deleterious and some are
lethal. While selection can remove deleterious
alleles from the population, the time taken is
so long that new deleterious mutations will arise
before previous deleterious mutations have been
removed, especially for recessive alleles.
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Eventually, an equilibrium is reached between
the addition of deleterious alleles by mutation
and their removal by selection. This is known
as mutation - selection balance. Consequently,
low frequencies of deleterious alleles are found
in all naturally outbreeding populations and this
is known as the mutation load.
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Mutation Loads Mutational loads are found in
essentially ALL species, including several
threatened endangered. Deleterious alleles are
normally found only at low frequencies, typically
much less than 1 at any locus. Deleterious
alleles are found at many loci.
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Migration Gene pools of populations
diverge over time due to chance events and
selection. Such divergence may be reduced by
migration which can have very large effects on
allele frequencies. Change in allele frequency
due to migration ?q m(qm - q0) Where m
migration coefficient, qm allele freq. in
migrant population, q0 allele frequency
in original population.
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Example You have a mainland population of
1,000 bats with an allele frequency (qm) of
0.75. 200 individuals from the mainland migrate
to a nearby island that contains a population of
150 individuals with an allele frequency (q0) of
0.40. Of the 200 migrants, only 100 are able to
breed. What is the new allele frequency in the
island population in the generation following
the migration event?
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Rearrangement of this equation allows
examination ff the effect of Introgression. Examp
le Ethiopian wolves are genetically
distinct from domestic dogs but hybridization
occurs in areas where they co-occur, as in Web
Valley, Ethiopia. The population for the Sanetti
Plateau is relatively pure.
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Extent of admixture from domestic dogs in the web
population can be estimated using
allele frequencies at a particular microsatellite
locus. Dogs lack the J allele while pure
Ethiopian wolves are homozygous for it. Sanetti
population q0 1.00 (old) Web
population q1 0.78 (new -- contains dog
) Domestic Dog qm 0.00 (migrants)
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m (q1 - q0)/(qm - q0) (0.78 - 1.0)/(0 - 1.0)
0.22 Based on this, the Web Valley population
of Ethiopian wolves contains about 22 of its
genetic composition from Domestic dog. It is
important to realize that this is an accumulated
contribution, not a per generation estimate.
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Migration-selection equilibrium depends only
upon the migration rate (m), the selection
coefficient (s) and the allele frequency in the
migrants (qm). Thus, equilibrium is NOT
dependent upon the allele frequency in the
initial population. When migration rates are
high and selection is weak, migration dominates
the process and can erase local
adaptation. Conversely, when migration rates are
low and selection is strong, there will be local
adaptation.
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