Transcript Document

Mutation as an Evolutionary Force
1. It occurs when errors are made in
duplicating alleles in producing the gametes.
2. It is one of the weaker evolutionary forces,
because errors are relatively rare. The error
rate or mutation rate, m, in copying an allele
of a nuclear gene is ~ 1 x 10-6 to 1 x 10-9.
3. It changes allele frequencies in a population
and this change in the genetic composition
of a population from parents to offspring is
what we mean by evolution.
No Mutation
AA Parents produce only ‘A’ bearing gametes.
Aa Parents produce ½ ‘A’ and ½ ‘a’ bearing gametes
aa Parents produce only all ‘a’ bearing gametes.
With Mutation
AA Parents produce some ‘a’ bearing mutant gametes.
Aa Parents produce ½ ‘A’ and ½ ‘a’ gametes
aa Parent produce some ‘A’ bearing mutant gametes.
= A alleles
= a alleles
Parent
population
Reproduction
With Mutation
Offspring
population
How strong is mutation as an
evolutionary force?
Calculate how much the frequency of an
allele changes in the population as a result
of mutation.
Mechanism of
Mutation
μ
A
Allele in the
Parent
a
Allele in the
Parent
u
a
Mutant Allele in the
Gamete and then
In the Offspring
A
Mutant Allele in the
Gamete and then
In the Offspring
Change in allele frequency, DPa, as a
result of mutation
Mechanism of
Mutation
μ
A
Parent Frequencies:
{PA, Pa}
a
u
Reproduction
With Mutation
Offspring Frequencies:
{PA’, Pa’}
How similar are PA’ and PA?
The change in allele frequency, DPa,
caused by mutation
Parent Frequencies:
{PA, Pa}
Freq of a
allele in
offspring
after
mutation
Pa’
Reproduction
With Mutation
Offspring Frequencies:
{PA’, Pa’}
Mutation rate
from A to a times
the Freq of A
before mutation
Non-Mutation
rate times the
Freq of a
before mutation
=
(1- v) Pa
+
μPA
ΔPa = Pa’– Pa = μ – (u + m)Pa
An Evolutionary Equilibrium occurs when
DP = 0
• Evolution: change in allele frequency in a
population from one generation to the next.
DP > 0 the allele is increasing in frequency
DP < 0 the allele is decreasing in frequency
• No Evolution: No change in allele frequency in a
population from one generation to the next.
DP = 0 the allele is neither increasing nor decreasing
in frequency
• The Equilibrium Allele Frequency, P*, is the
frequency that makes DP = 0
The change in allele frequency, DPa,
caused by mutation
ΔPa = Pa’– Pa = μ – (u + m)Pa
What is the Equilibrium Allele Frequency, Pa*?
Set ΔPa = 0 = μ – (u + m)Pa*
Solve for Pa*
The Equilibrium Allele Frequency,Pa*, that
makes DPa = 0
At the Mutation Equilibrium, ΔPa = 0.
0 = μ – (u + m)Pa*
P*a = μ/(u + m)
The Equilibrium Allele Frequency =
Rate at which A is wrongly copied as a,
Relative to all errors of copying at this gene.
Parent Population
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Offspring Population
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Yy YY yy
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Yy YY yy
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Yy yy
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Female Parents
Male Parents
{GYY, GYy, Gyy},
{PY, Py}
YY
Yy
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YY
Yy
yy
Offspring
{GYY, GYy, Gyy},
{PY, Py}
Parent Population
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Offspring Population
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Yy
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Yy YY yy
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Y y yy
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{GYY, GYy, Gyy},
{PY, Py}
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Yy YY yy
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Yy
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Yy yy
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Yy
{GYY, GYy, Gyy},
{PY, Py}
Y
yy
Gametes are mixed by Ocean Currents
For pelagic spawners and broadcast spawners
Pelagic eggs - eggs that float in the water column, like most deep
water marine fishes and coral reef fishes.
Pelagic Scatterers – spawn by scattering eggs and sperm into the water
in schools near surface. For example:sardines, mackerel, tuna, whitefish.
Broadcast Spawners: another name for forming zygotes by
scattering eggs and sperm into the water
Examples:
Thunnus thynnus, atlantic blue fin tuna
10 feet long, weigh up to 1,400 pounds, lifespan longer than 20 years.
the largest bony fish in the world; they migrate up to 5,000 miles.
0.8 to 2.6 million eggs per spawning female (roughly 100,000 eggs/kg).
Eggs are ~ 1 mm in diameter and buoyant, enclosed in an oil droplet.
LABRIDAE (WRASSES) All species are proto-gynous
hermaphrodites: Females have the capacity to turn into males.
Males are either born as males or are older sex-reversed females.
Star Fishes.
Parent Population
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Offspring Population
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Yy
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Yy YY yy
Yy
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Yy
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Y
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y y
y
y Y y Y Y yy
Y y yy
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Y
Y
Y
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{GYY, GYy, Gyy},
{PY, Py}
Y
Y
yy
Yy YY yy
YY
Yy
YY Yy
Yy yy
Y
Y
yy
YY
Yy
{GYY, GYy, Gyy},
{PY, Py}
Y
yy
Gametes are mixed by Ocean Currents
For pelagic spawners and broadcast spawners
Parent Population
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Offspring Population
YY
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Yy
YY yy yy
Yy YY yy
Yy
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Yy
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Yy
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Yy YY yy
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Yy yy
Yy
Female Gametes
Male Gametes
{GYY, GYy, Gyy},
{PY, Py}
Y PY
y Py
Y
PY
y
Py
YY
Yy
(PY)2
PYPy
Yy
yy
PYPy
(Py)2
{GYY, GYy, Gyy},
{PY, Py}
Random Mating =
Random Union of
Gametes
Human Blood Groups:
Number of Individuals: MM
1787
Genotype Frequencies: 0.292
Number of Alleles:
Allele Frequencies:
M
6611
0.539
MN
3037
0.495
NN Total
1305 6,129
0.213 1.00
N
5647
0.461
Total
12,258
1.00
Predicted Genotype
(0.539)2 2(0.539)(0.461) (0.461)2
Frequencies from HWE 0.291
0.497
0.212
Actual Observed
0.292
0.495
0.213
Human Blood Groups are in Hardy-Weinberg Equilibrium
Predicted Genotype
(0.539)2 2(0.539)(0.461) (0.461)2
Frequencies from HWE 0.291
0.497
0.212
Actual Observed
0.292
0.495
0.213
Conclude: Humans are mating at random with respect to
blood groups!
Conclude: No evidence of natural selection or any other
evolutionary forces acting on these alleles.
Conclude: We are mating like pelagic spawners as far as
this gene is concerned!
Conclude: Nonrandom mating can be occurring for some
traits and genes but not others in the same
population.