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Population Genetics

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The Gene Pool

•Members of a species can interbreed & produce fertile offspring•Species have a shared gene pool•Gene pool – all of the alleles of all individuals in a population

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The Gene Pool

•Different species do NOT exchange genes by interbreeding•Different species that interbreed often produce sterile or less viable offspring e.g. Mule

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Populations

•A group of the same species living in an area•No two individuals are exactly alike (variations)•More Fit individuals survive & pass on their traits

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Speciation•Formation of new species•One species may split into 2 or more species•A species may evolve into a new species•Requires very long periods of time

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Modern Synthesis Theory• Today’s theory on evolution• Recognizes that GENES are responsible for the

inheritance of characteristics• Recognizes that POPULATIONS, not individuals,

evolve due to natural selection & genetic drift• Recognizes that SPECIATION usually is due to the

gradual accumulation of small genetic changes

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Microevolution

• Changes occur in gene pools due to mutation, natural selection, genetic drift, etc.

• Gene pool changes cause more VARIATION in individuals in the population

• This process is called MICROEVOLUTION• Example: Bacteria becoming unaffected by

antibiotics (resistant)

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Species & Populations• Population - a localized group of individuals of

the same species. • Species - a group of populations whose

individuals have the ability to breed and produce fertile offspring.

• Individuals near a population center are, on average, more closely related to one another than to members of other populations.

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Gene Pools• A population’s gene pool is the total of all

genes in the population at any one time. • If all members of a population are

homozygous for a particular allele, then the allele is fixed in the gene pool.

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The Hardy-Weinberg Theorem• Used to describe a non-evolving

population.• Shuffling of alleles by meiosis and

random fertilization have no effect on the overall gene pool.

• Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

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The Hardy-Weinberg Theorem• Deviation from Hardy-Weinberg

equilibrium usually results in evolution• Understanding a non-evolving population,

helps us to understand how evolution occurs

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Assumptions of the H-W Theorem1. Large population size

- small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). (Genetic Drift)

2. No migration- immigrants can change the frequency of an allele by bringing in new alleles to a population.

3. No net mutations- if alleles change from one to another, this will change the frequency of those alleles

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Assumptions of the H-W Theorem3. Random mating

- if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles.

4. No natural selection- if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

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Hardy-Weinberg Equilibrium The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time. The Hardy-Weinberg Equation: 1.0 = p2 + 2pq + q2

p2 = frequency of AA genotype or homozygous 1 st allelle2pq = frequency of Aa plus aA genotype or heterozygous q2 = frequency of aa genotype

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Hardy-Weinberg Equilibrium

Population of cats n=10016 white and 84 blackbb = whiteB_ = black

Can we figure out the allelic frequencies of individuals BB and Bb?

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p2 + 2pq + q2

and

p+q = 1 (always two alleles)

16 cats white = 16bb then (q2 = 0.16)This we know we can see and count!!!!!If p + q = 1 then we can calculate p from q2

Q = square root of q2 = q √.16 q=0.4p + q = 1 then p = .6 (.6 +.4 = 1)P2 = .36All we need now are those that are heterozygous

(2pq) (2 x .6 x .4)=0.48

.36 + .48 + .16

Hardy-Weinberg Principle

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Hardy-Weinberg Equilibrium

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21*Yes, I realize that this is not really a cheetah.

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2) Natural selection As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance).

The only agent that results in adaptation to environment.

3) Gene flow -is genetic exchange due to the migration of fertile individuals or gametes between populations.

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4) Mutation Mutation is a change in an organism’s DNA and is represented by changing alleles. Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool.

The original source of variation.

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Genetic Variation, the Substrate for Natural Selection Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type). Not all variation is heritable.

Environment also can alter an individual’s phenotype [e.g., the hydrangea we saw before, and…

…Map butterflies (color changes are due to seasonal difference in hormones)].