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Figure 21.CO: Water buffalo grazing
© Robert Adrian Hillman/ShutterStock, Inc.
Variation and Selection
• Modern Synthesis (Darwin + Mendel)
– Darwin: evolution is heritable adaptation to
environment
• No knowledge of heredity
– Mendel: genes are passed down as units from one
generation to the next
• Genetics thought static, no understanding of the origin of
variation or impact on evolution
– Modern synthesis: evolution is the change in allele
frequency within a population over time
Variation and Selection
• Modern Synthesis (Darwin + Mendel)
– Mutation introduces variation
– Subject to selection if expressed as phenotype and
affects reproductive success
– Resulting in adaptive changes in phenotype
– Eventually resulting in organismal diversity
• Questions Provoked by the Modern Synthesis
– What are the allele frequencies in populations?
– What causes allele frequencies to change?
– Can alternate alleles be neutral with respect to
selection?
The effect of selective breeding on a domestic plant
Figure 21.01a: Teosinte
Figure 21.01b: Maize
Populations and Allele
Frequencies
• Deme
– Local group of an interbreeding population
• A subpopulation
– Geographic features can affect size of group
• Impassable river, landslip
• Allele frequency
– Proportion of the different alleles of the genes in a
population
• Gene pool
– Sum total of alleles in the reproductive gametes of a
population
Hardy-Weinberg
Equilibrium =
p2 + 2pq + q2 = 1
The frequency of a
recessive phenotype in
a population is q2. Then
p = 1 – q, and the
expected frequencies of
all genotypes can be
calculated
Figure 21.03: Genotypic frequencies generated under random mating for
two alleles
Assumptions of Hardy-Weinberg
Equilibrium
• Random mating - no mate choice, inbreeding
– Large population size (reduces inbreeding)
• No mutation (no new variation)
• No migration ( no transfer between populations)
• No selection (no single allele has any advantage)
• No genetic drift (no random change in frequency)
Departures from H-W equilibrium indicate that
one or more of these factors has affected
genotype frequency
Figure 21.04: Genotypic frequencies at Hardy-Weinberg equilibrium
(Adapted from Wallace.)
Figure 21.05: Genotypic frequencies generated under conditions of random
mating when there are three alleles
Alleles of different genes will
reach equilibrium (random
association) in a population
due to independent assortment
of the alleles of the two genes.
Linkage disequilibrium occurs
when two genes are linked and
do not assort independently.
Equilibrium is reached more
slowly as genes are more
tightly linked.
Figure 21.06: The proportion of linkage disequilibrium that remains in
various generations
(Adapted from Strickberger.)
Association Mapping and the
HapMap Project
• Gene mapping
– Linkage analysis of individuals with known pedigree
– Identifies genes affecting diseases or syndromes
• Association mapping
– Linkage analysis at the population level
• More complete analysis of allele past history
• Finer scale mapping of the allele
• Detection of rare alleles, or with small affects
• HAPMAP
– Consortium from six countries
– Mapping variation of humans from different regions
Inbreeding
• Related individuals of • Consequences of
similar genotype breed inbreeding
with each other
– Nonrandom mating
• Causes of inbreeding
– Self-fertilization
• most plants capable of
this cross fertilize using
pollinators
– Limited population size
interferes with HardyWeinberg equilibrium
• Does not affect allele
frequency
– Decreases heterozygosity
– Causes inbreeding
depression
• Rare harmful recessives
become homozygous at
higher frequency
Inbreeding coefficients at generations 1 to 15 for four different systems of
inbreeding
Degrees of heterozygosity
(Adapted from Strickberger.)
Mutation Rates, Genetic Drift
• As A mutates to a, a will
increase in frequency.
• Typical observed mutation
rates of 5 x 10-5
• Forward mutation rate
exceeds reverse mutation
rate
• New recessive mutations
are masked by dominant
alleles, not subject to
selection
– change in frequency only by
genetic drift
• Drift = random changes in
allele frequencies that lack
consistent direction
• Genetic drift
– Random sampling of the
gene pool of each
generation
– Leads to random variation
in allele frequency within
population, but in no
predictable direction
• Rate of fixation, 1/2N, N =
population size
Selection and Fitness:
Selection measured as s, the extent to which fitness is reduced
When s = 1, phenotype is lethal
Number of Generations Required
to Change Allele Frequencies
Change in
allele
frequency
Number of Generations Required under Different
Selection Coefficients – a Dominant Allele
From
To
s=1
lethal
0.99
0.80
0.50
0.20
0.10
0.01
0.001
0.75
4
7
17
35
350
3496
0.75
0.50
1
2
5
11
110
1099
0.50
0.25
1
2
5
11
110
1099
0.25
0.10
1
2
5
11
110
1099
0.10
0.01
3
5
12
24
240
2398
0.01
0.001
3
5
12
24
231
2312
0.001
0.0001
3
5
12
23
230
2303
1
Number of Generations Required
to Change Allele Frequencies
Change in allele
frequency
Number of Generations Required under Different
Selection Coefficients – a Recessive Allele
From
To
s=1
lethal
0.99
0.90
0.90
0.75
0.75
0.50
0.50
0.25
0.25
0.80
0.50
0.20
0.10
0.001
3
5
13
25
250
2
3
7
13
132
2
3
9
18
176
2
4
6
15
31
310
0.10
6
9
14
35
71
710
0.10
0.01
90
115
185
462
924
9240
0.01
0.001
900
1128
1805
4512
9023
90231
0.001
0.0001
9000
11515
18005
45011
90023
900230
1
Recessive alleles present at low
frequencies are likely to be
numerous in most populations
• Thousands of generations may be
necessary to eliminate a moderately
disadvantageous allele
• Environmental change likely to occur
before allele eliminated
• Recessive alleles will accumulate at low
frequencies in population
Figure 22.07: Relationship between birth weight and survival in females
(Adapted from Karn and Penrose.)
Figure 22.08: Modes and effects of selection
Some traits show
intermediate inheritance
that is controlled by more
than one gene:
This is called quantitative
inheritance and is
controlled by polygenes
Polygenes in progeny of a dihybrid self
Figure 3-15 step 6
Outcome of a cross with
three quantitative genes:
Grain color in wheat
Modern genetic usage:
ABC = A1A2A3
abc = a1a2a3
(From Genetics Third Edition by Monroe W. Strickberger.
Copyright © 1985 by Monroe W. Strickberger. Reprinted by
permission of Prentice Hall, Inc., Upper Saddle River, NJ.)
Modern Genetic
Designations:
a1 /a1; a2 /a2; a3 /a3
A1 /A1; A2 /A2; A3 /A3
Migration
• Gene flow
– A source other than mutation for “new” alleles, genetic
variation
– Overcomes effects of inbreeding
• Gene flow prevents genetic divergence of
populations
– New mutations in one population eventually are
passed to another
• When gene flow is blocked populations
eventually diverge genetically due to the
randomness of mutation
Bottlenecks and the Founder Effect
• Small, isolated population
– Exposed to genetic drift, increased
homozygosity from inbreeding, and
changes in adaptive landscape
• Radical change in
selection pressure
– Little interspecific competition
– Few predators, if any
– Very different environment
• Evolution: unpredictable
directions
Speciation and
adaptive radiation
result from:
genetic drift due to
founder effects
inhibited gene flow
from geographic
isolation
novel selective
environments
Figure 22.14: Colonization pattern
showing founder events
(Adapted from Carson.)
The Adaptive Landscape
• Adaptive Peak
– Situations of highest fitness for a specific
environment
• Population adaptive peaks can vary from individual
genomic peaks
– Altruism, parasitism
• Adaptive Landscapes
– Not all peaks are equal
– Fitness values, peaks and valleys
Evolution may take any
of several potential
paths
An adaptive
landscape with
different selective
peaks, or
potential optima
Multiple
potential paths
may lead to
the same peak
Multiple
potential
paths may
originate
from a single
origin