Weak forces in Evolution

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Weak forces in Evolution

• Dan Graur

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Changes in allele frequencies are
important. Changes in genotype frequencies are
not so important.
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2.
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Mating
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Deviation from randomness By genetic
similarity Assortative mating Disassortative
mating By genetic relatedness Inbreeding Out
breeding
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Skin color
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disassortative
assortative
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Assortative mating is very strong in humans even
with respect to non-genetic traits.
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Degrees of inbreeding found in nature
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Cousins are preferred mates in many human
cultures.
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P Ptolemy C Cleopatra
Inbreeding is usually bad
P ix
P xii
C v
C vii
an exception?
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The fish Rivulus marmoratus exhibits the most
extreme form of inbreeding Selfing
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Random mating ? equilibrium allele frequencies
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Assortative mating ? excess homozygotes
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Disassortative mating ? excess heterozygotes
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Even if extreme deviations from random mating
occur in all generations, allele frequencies
remain constant.
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Mating pattern is not an important
evolutionary force
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Clinical effects of inbreeding
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Clinical effects of inbreeding
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Migration
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Migration will cause changes in the allele
frequencies of each of the two subpopulations.
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However, because of gene flow, the two
subpopulations are, in fact, one population, in
which allele frequencies do no change.
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Genotype frequencies will change in a similar
fashion to that in disassortative mating.
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Migration is not an important evolutionary
force
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Mutation
Mutation A transmissible change in the genetic
material
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Mutations are the ultimate source of variation.
Only mutations can create genetic novelty.
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Mutations arise all the time. Per definition, the
initial frequency of a mutation in a diploid
population is 1/2N.
N population size
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The human population on Jan 19, 2009 was
estimated to be 6.7 billion people. The number of
alleles at an autosomal locus is, therefore, 13.4
billion. A mutation arising today in the human
population will have an approximate initial
frequency of 7 10-11.
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The human population on Jan 19, 2009 was
estimated to be 6.7 billion people. The number of
alleles at an autosomal locus is, therefore, 13.4
billion. A mutation arising today in the human
population will have an approximate initial
frequency of 7 10-11
resulting in a change in allele frequencies from
0 to 7 10-11.
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Mutation is the ultimate source of variability,
but it is not an important evolutionary force.
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For a mutation to become significant, it must
increase its frequency, so that it becomes fixed
in the population.
frequency of allele 1.0
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Two factors can lead to the fixation of a new
mutation Selection Random genetic drift