POPULATION GENETICS

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Outcomes

4. Discuss the application of population genetics to the study of evolution.

4.1 Describe the concepts of the deme and the gene pool.

4.2 Consider the Hardy-Weinberg principle.

4.3 Describe the factors which influence genetic drift.

4.4 Consider the relevance of the gene pool and the idea of mutations to the concept of evolution which will be studied later in unit 5.

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 EVOLUTIONARY

THOUGHT

Modern Synthesis Theory

•Combines Darwinian selection and Mendelian inheritance

•Population genetics - study of genetic variation within a population

•Emphasis on quantitative characters (height, size …)

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Modern Synthesis Theory

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

• Introduced by Fisher & Wright

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

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S. Wright

A. Fisher

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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HARDY-WEINBERG PRINCIPLE

The 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. 13

The Hardy-Weinberg Principle

•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 Principle

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 Principle

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 Mating

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

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•The Hardy-Weinberg Equation: 

•               1.0 = p2 + 2pq + q2

• Where:•p2 = frequency of AA genotype•2pq = frequency of Aa •q2 = frequency of aa genotype

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The Hardy-Weinberg Principle

•Determining the Allele Frequency using Hardy-Weinberg: 

           

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

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Allele Frequencies Define Gene Pools

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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MICROEVOLUTION OF SPECIES

Causes 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 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 DRIFT

Factors that Cause Genetic Drift•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 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 Effect

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MODES OF NATURAL

SELECTION

Modes 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

Disruptive Selection

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Modes 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 POPULATIONS

Geographic 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 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 Malaria

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Other 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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