I. The Modern Synthetic Theory of Evolution

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I. The Modern Synthetic Theory of Evolution A.
Initial Structure 1940 Sources of
Variation Agents of Change Mutation Natural
Selection Recombination Drift - crossing
over Mutation - independent
assortment Migration Non-random
Mating
VARIATION
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B. Population Genetics 1. Hardy Weinberg a.
Definitions b. Basic computations 1.
Determining the Gene and Genotypic Array
AA Aa aa
Individuals 60 80 60 (200)



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B. Population Genetics 1. Hardy Weinberg a.
Definitions b. Basic computations 1.
Determining the Gene and Genotypic Array
AA Aa aa
Individuals 60 80 60 (200)
Genotypic Array 60/200 0.30 80/200 .40 60/200 0.30 1
''A' alleles 120 80 0 200/400 0.5
'a' alleles 0 80 120 200/400 0.5
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B. Population Genetics 1. Hardy Weinberg a.
Definitions b. Basic computations 1.
Determining the Gene and Genotypic Array 2.
Short Cut Method - Determining the Gene Array
from the Genotypic Array a. f(A) f(AA)
f(Aa)/2 .30 .4/2 .30 .2 .50 b.
f(a) f(aa) f(Aa)/2 .30 .4/2 .30 .2
.50 KEY The Gene Array CAN ALWAYS be computed
from the genotypic array the process just counts
alleles instead of genotypes. No assumptions are
made when you do this.
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B. Population Genetics 1. Hardy Weinberg a.
Definitions b. Basic computations c.
Hardy-Weinberg Equilibrium 1. If a population
acts in a completely probabilistic manner,
then - we envision an infinitely large
population with no migration, mutation, or
selection, and random mating. - we could
calculate genotypic arrays from gene arrays -
the gene and genotypic arrays would equilibrate
in one generation
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B. Population Genetics 1. Hardy Weinberg a.
Definitions b. Basic computations c.
Hardy-Weinberg Equilibrium
AA Aa aa
Initial genotypic freq. 0.4 0.4 0.2 1.0
Gene freq. f(A) p .4 .4/2 0.6 f(A) p .4 .4/2 0.6 f(a) q .2 .4/2 0.4 f(a) q .2 .4/2 0.4
Genotypes, F1 p2 .36 2pq .48 q2 .16 1.00
Gene Freq's f(A) p .36 .48/2 0.6 f(A) p .36 .48/2 0.6 f(a) q .16 .48/2 0.4 f(a) q .16 .48/2 0.4
Genotypes, F2 .36 .48 .16 1.00
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - mutation 1.
Consider a population with f(A) p .6
f(a) q .4 2. Suppose 'a' mutates to 'A' at a
realistic rate of µ 1 x 10-5 3. Well,
what fraction of alleles will change? 'a' will
decline by qm .4 x 0.00001 0.000004 'A'
will increase by the same amount. 4. So, the new
gene frequencies will be p1 p µq
.600004 q1 q - µq q(1-µ) .399996. VERY
LITTLE EFFECT on GENE FREQs
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - migration
p2 0.7 q2 0.3
p1 0.2 q1 0.8
suppose migrants immigrate at a rate such that
the new immigrants represent 10 of the new
population
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - migration
IMPORTANT EFFECT, BUT MAKES POPULATIONS SIMILAR
AND INHIBITS DIVERGENCE AND ADAPTATION TO LOCAL
CONDITIONS (EXCEPT IT MAY INTRODUCE NEW ADAPTIVE
ALLELES)
p2 0.7 q2 0.3
p1 0.2 q1 0.8
suppose migrants immigrate at a rate such that
the new immigrants represent 10 of the new
population
p(new) p1(1-m) p2(m) (0.2)(0.9)
(0.7)(0.1) 0.25
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - non-random
mating 1. Positive Assortative Mating
AA Aa aa
.2 .6 .2
offspring ALL AA 1/4AA1/2Aa1/4aa ALL aa
.2 .15 .3 .15 .2
F1 .35 .3 .35
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - non-random
mating 1. Positive Assortative Mating B.
Inbreeding - reduction of heterozygosity
across the entire genome, at a rate that
correlates with the degree of relatedness.
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - Genetic Drift 1.
The organisms that actually reproduce in a
population may not be representative of the
genetics structure of the population they may
vary just due to sampling error
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - Genetic Drift 2.
patterns
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - Genetic Drift 2.
patterns
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- Genetic Bottleneck If a population crashes
(perhaps as the result of a plague) there will be
both selection and drift. There will be
selection for those resistant to the disease (and
correlated selection for genes close to the genes
conferring resistance), but there will also be
drift at other loci simply by reducing the size
of the breeding population.
Cheetah have very low genetic diversity,
suggesting a severe bottleneck in the past. They
can even exchange skin grafts without rejection
European Bison, hunted to 12 individuals, now
number over 1000.
Fell to 100s in the 1800s, now in the 100,000s
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - Selection
Differential reproductive success
A. Measuring fitness differential
reproductive success 1. The mean number of
reproducing offspring (or females)/female 2.
Components of fitness - probability of
female surviving to reproductive age - number
of offspring the female produces -
probability that offspring survive to
reproductive age
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B. Population Genetics 1. Hardy Weinberg 2.
Effects of Different Agents - Selection
Differential reproductive success
p 0.4, q 0.6 AA Aa aa
Parental "zygotes" 0.16 0.48 0.36 1.00
prob. of survival (fitness) 0.8 0.8 0.2
Relative Fitness 1 1 0.25
Survival to Reproduction 0.16 0.48 0.09 0.73
Geno. Freq., breeders 0.22 0.66 0.12 1.00
Gene Freq's, gene pool p 0.55 q 0.45
Genotypes, F1 0.3025 0.495 0.2025 100