Population Genetics

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PopulatiPopulation on

GeneticsGenetics

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The Gene PoolThe 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 PoolThe 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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PopulationsPopulations

•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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SpeciationSpeciation

•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 Modern Evolutionary Evolutionary ThoughtThought

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Modern Synthesis TheoryModern Synthesis Theory

•Combines Combines Darwinian selection Darwinian selection and and Mendelian Mendelian inheritanceinheritance

•Population genetics Population genetics - study of genetic - study of genetic variation within a variation within a populationpopulation

•Emphasis on Emphasis on quantitative quantitative characters (height, characters (height, size …)size …)

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Modern Synthesis TheoryModern Synthesis Theory

•1940s – 1940s – comprehensive comprehensive theory of evolution theory of evolution (Modern Synthesis (Modern Synthesis Theory)Theory)

•Introduced by Fisher Introduced by Fisher & Wright& Wright

•Until thenUntil then, many did , many did not accept that not accept that Darwin’s theory of Darwin’s theory of natural selection natural selection could drive evolutioncould drive evolution

S. Wright

A. Fisher

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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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MicroevolutionMicroevolution

•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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Hardy-Hardy-Weinberg Weinberg PrinciplePrinciple

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The Hardy-Weinberg PrincipleThe Hardy-Weinberg Principle

•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 The Hardy-Weinberg PrinciplePrinciple

•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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5 Assumptions of the H-W 5 Assumptions of the H-W PrinciplePrinciple

1.Large population size - small populations have fluctuations in allele frequencies (e.g., fire, storm).

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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5 Assumptions of the H-W 5 Assumptions of the H-W PrinciplePrinciple

3.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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Traits Selected for Random MatingTraits Selected for Random Mating

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The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change)

The Hardy-Weinberg Equation:

1.0 = p2 + 2pq + q2

Where:p2 = frequency of AA genotype2pq = frequency of Aa q2 = frequency of aa genotype

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Determining the Allele Frequency using Hardy-Weinberg:

1.0 = p + q Where:p = frequency of A alleleq = frequency of a allele

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Allele Frequencies Define Gene Allele Frequencies Define Gene PoolsPools

As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females):

320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

500 flowering plants

480 red flowers 20 white flowers

320 RR 160 Rr 20 rr

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MicroevolutioMicroevolution of Speciesn of Species

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Causes of MicroevolutionCauses of Microevolution

•Genetic Drift- the change in the gene pool of a small population due to chance

•Natural Selection - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance)- Cause ADAPTATION of Populations

•Gene Flow-is genetic exchange due to the migration of fertile individuals or gametes between populations

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Causes of MicroevolutionCauses of Microevolution

•Mutation- a change in an organism’s DNA

- Mutations can be transmitted in gametes to offspring

•Non-random mating- Mates are chosen on the basis of the

best traits

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Genetic DriftGenetic Drift

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Factors that Cause Genetic Factors that Cause Genetic DriftDrift

•Bottleneck Effect- a drastic reduction in population (volcanoes,

earthquakes, landslides …)

- Reduced genetic variation

- Smaller population may not be able to adapt to new selection pressures

•Founder Effect- occurs when a new colony is started by a few

members of the original population

- Reduced genetic variation

- May lead to speciation

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Loss of Genetic VariationLoss of Genetic Variation

•Cheetahs have little genetic variation in their gene pool

•This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age

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Founder’s EffectFounder’s Effect

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Modes of Modes of Natural Natural SelectionSelection

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Modes of Natural SelectionModes of Natural Selection

•Directional Selection- Favors individuals at one end of the

phenotypic range

- Most common during times of environmental change or when moving to new habitats

•Disruptive selection

- Favors extreme over intermediate phenotypes

- Occurs when environmental change favors an extreme phenotype

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DirectionalSelection

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Disruptive Selection

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Modes of Natural SelectionModes of Natural Selection

•Stabilizing Selection- Favors intermediate over extreme

phenotypes

- Reduces variation and maintains the cureent average

- Example: Human birth weight

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Variations in Variations in PopulationsPopulations

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Geographic VariationsGeographic Variations

•Variation in a species due to climate or another geographical condition

•Populations live in different locations

•Example: Finches of Galapagos Islands & South America

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Heterozygote AdvantageHeterozygote Advantage

•Favors heterozygotes (Aa)

•Maintains both alleles (A,a) instead of removing less successful alleles from a population

•Sickle cell anemia> Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age.> Heterozygotes are less susceptible to malaria

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Sickle Cell and MalariaSickle Cell and Malaria

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Other Sources of VariationOther Sources of Variation

•Mutations- In stable environments, mutations often

result in little or no benefit to an organism, or are often harmful

-Mutations are more beneficial (rare) in changing environments (Example: HIV resistance to antiviral drugs)

•Genetic Recombination- source of most genetic differences

between individuals in a population

•Co-evolution-Often occurs between parasite & host and

flowers & their pollinators

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CoevolutionCoevolution

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Population Genetics

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