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I. The Modern Synthetic Theory of Evolution A. Initial Structure – 1940

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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

1. Hardy Weinberg

a. Definitions:

b. Basic computations:

1. Determining the Gene and Genotypic Array:

1. Hardy Weinberg

a. Definitions:

b. Basic computations:

1. Determining the Gene and Genotypic Array:

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.

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

1. Hardy Weinberg

a. Definitions:

b. Basic computations:

c. Hardy-Weinberg Equilibrium:

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 FREQ’s

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

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

1. Hardy Weinberg

2. Effects of Different Agents

- non-random mating

1. Positive Assortative Mating

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.

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

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 100’s in the 1800s, now in the 100,000’s

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

1. Hardy Weinberg

2. Effects of Different Agents

- Selection: Differential reproductive success

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