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Section 4
Evolution in Large Populations:
Mutation, Migration & Selection
Genetic diversity lost by chance and selection
regenerates through mutation.
When genetic diversity is lost in small threatened
populations, it can be recovered by migration from
other genetically distinct populations.
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.
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.
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
selection.
Mutation is the ultimate source of genetic diversity
while recombination can produce new combinations
of alleles.
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.
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.
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.
The most important mutations are those at loci
affecting fitness traits, most notably, lethal or
deleterious mutations.
G
A
Transition
Substitutions
Transversion
Substitutions
C
T
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.
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).
Mutation is normally a recurrent process where
mutations continue to arise over time.
Mutation Rate:
Initial Allele Frequency:
p = -p0
A1
p0

A2
p1
The time taken to regenerate genetic diversity is
a major issue in conservation biology because it
may take thousands to millions of generations to
regenerate genetic diversity at a single locus.
Time to regenerate genetic diversity due to
mutation:
pt = p0(1 - )t or p0e-t
t = (lnp0 - lnpt)/
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!
Mutations typically occur in both directions and
since there are two opposite forces, this usually
results in an equilibrium.
A1


Stable Equilibrium:
A2
ˆq =  / ( + )
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.
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.
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.
Deleterious alleles increase due to mutation rate
(p) and are removed by selection at a rate of:
(-spq2)/(1-sq2) therefore:
q is approximately p - spq2
At equilibrium q = 0, so p ≈ spq2 and q2 ≈ /s
Therefore, the equilibrium frequency is:
ˆ ≈ (/s)0.5
q
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.
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?
n = 1,000
qm = 0.75
200 migrate
n = 150
q0 = 0.40
q = m(qm - q0)
qm = 0.75
q0 = 0.40
m = migration coefficient = 200 migrate but only
100 breed thus, m = 100/250 = 0.4
q = 0.4(0.75 - 0.4) = 0.14
q1 = q0 + q = 0.4 + 0.14 = 0.54
Rearrangement of this equation allows examination
ff the effect of Introgression.
Example: 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.
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”)
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.
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.
At equilibrium q = 0 and:
qˆ = (2m + s) ± [{2m + s)2 - (8s m qm)}/2s]0.5
Although there are 2 solutions to this equation,
because the allele frequency has to be between
0 and 1, only one solution will be correct.
Migration-selection balance can arise between
wild and captive populations when there is regular
movement of wild individuals into captivity or
vice versa.