Weak forces in
Evolution
Dan Graur
1
1.
2.
2
Mating
3
Deviation from randomness:
By genetic similarity:
Assortative mating
Disassortative mating
By genetic relatedness:
Inbreeding
Outbreeding
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5
disassortative
assortative
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Human assortative mating: Rule #1
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Strong assortative mating by skin color in the US
Percentage of non-hispanic whites: 80%
Percentage of blacks: 13%
Expected percentage of interracial marriages: 10.4%
Observed percentage of interracial marriages: 0.9%
Ratio of white female/black male = 2:1
black female/white male
8
Assortative mating is very
strong in humans even
with respect to non-genetic
traits.
9
Degrees of inbreeding found in nature:
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11
Inbreeding is
usually forbidden…
P = Ptolemy
C = Cleopatra
P ix
Cv
P xii
C vii
An exception!
12
The fish Rivulus marmoratus
exhibits the most extreme form
of inbreeding: Selfing
13
Random
mating

equilibrium
allele
frequencies
14
Assortative
mating

excess
homozygotes
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Disassortative
mating

excess
heterozygotes
16
Even if
extreme
deviations
from random
mating
occur in all
generations,
allele
frequencies
remain
constant.
17
Mating pattern
is not an important
evolutionary force
18
Clinical effects
of inbreeding
P.T. Barnum + Tom Thumb
19
Clinical effects
of inbreeding
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Migration
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23
Migration will cause changes in the
allele frequencies of each of the two
subpopulations.
24
However, because of gene flow, the
two subpopulations are, in fact, one
population, in which allele frequencies
do no change.
25
Genotype frequencies will change in
a similar fashion to that in
disassortative mating.
26
Migration
is not an important
evolutionary force
27
Mutation
Mutation:
A transmissible
change in the genetic
material
28
Mutations are the ultimate
source of variation. Only
mutations can create
genetic novelty.
29
Mutations arise all the
time. Per definition, the
initial frequency of a
mutation in a diploid
*
population is 1/2N.
*N
= population size
30
The human population on October 31,
2011 was estimated to be 7 billion
people. The number of alleles at an
autosomal locus is, therefore, 14 billion.
A mutation arising today in the human
population will have an initial frequency
-11
of about 7 × 10 …
…resulting in a change in allele
frequencies from 0 to 0.00000000007.
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32
Mutation is the
ultimate source of
variability, but it is
not an important
evolutionary force.
33
For a mutation to become
significant, it must increase
its frequency, so that it
*
becomes fixed in the
population.
*frequency
of allele = 1.0
34
Two factors can lead to the
fixation of a new mutation:
Selection
Random genetic drift
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