AP Biology 2010-2011

MeasuringEvolution of Populations

Hardy Weinberg

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5 Agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

Migration Sexual Selection

Small populationNatural Selection

Differential Survival

ChemicalChanges to DNA

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Researchers know whether or not a population is evolving by tracking deviations from a baseline of genetic equilibrium.

If you notice, they violate all the mechanisms of microevolution.

Five conditions required for a stable gene pool:1. Mutations do not occur2. Population is infinitely large (genetic drift eliminated)3. No gene flow4. Random mating (no sexual selection)5. All individuals survive and reproduce equally (no NS)

If these 5 things happen, new alleles are not introduced to a population and therefore, the population isn’t evolving.

This doesn’t happen in reality but the rate these change has a direct affect on the rate of evolution.

III. Genetic Equilibrium

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Genotype Frequencies Vs. Allele FrequenciesStudying population genetics requires understanding alleles in a gene pool.Each individual has 2 alleles for each trait.This is the genotype, the phenotype is dependent upon this.The gene pool represents all available alleles in a population so genotypic and allelic frequencies always add up to 1.

Genotype Frequency(frequency of EE) + (frequency of Ee) + (frequency of ee) = 1

Allele Frequency: (frequency of E) + (frequency of e) = 1

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The Hardy-Weinberg Formula. Rules

Determining Genetic Equilibrium… the BASELINE The Hardy-Weinberg formula can be used to

determine if a population is in genetic equilibrium

p2 + 2pq +q2= 1.0 is essentially: (AA) + (Aa) + (aa) = 1.0

It should be noted that H-W examines only traits that have 2 alleles! We couldn’t use it for our turtle population b/c there

were 6 phenotypes.

The frequency of the dominant allele (A) plus the recessive allele (a) equals 1.0

p + q = 1.0

Homozygous dominant

Homozygous recessive

Heterozygote

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The Hardy-Weinberg Principle The Hardy-Weinberg

principle describes a population that is not evolving

If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving.

AP BiologyFig. 18-3a, p. 280

Finding out whether a population is evolving. The frequencies of wing-color alleles among all of the individuals in this hypothetical

population of morpho butterflies are not changing; thus, the population is not evolving.

AP BiologyFig. 18-3b, p. 280

490 AA butterflies dark-blue wings

490 AA butterflies dark-blue wings

490 AA butterflies dark-blue wings

420 Aa butterflies medium-blue wings

420 Aa butterflies medium-blue wings

420 Aa butterflies medium-blue wings

90 aa butterflies white wings

90 aa butterflies white wings

90 aa butterflies white wings

Starting Population Next Generation Next Generation

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Class Example 1 Let’s see if we can figure out allele frequencies for a population of pigs using H-W.

The goal is to find: q2, p2, p, q, 2pq,

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Example: White (pink) coats are dominant over black. Can you identify who is dominant and recessive. Yes. Can you determine who has what alleles?

Only for the homozygous recessive!

You have to calculate the others….

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Hardy-Weinberg Sample Pink is dominant over black. Calculate q2: Count the individuals that are

homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q2.

q2 = 25%

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Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele.

q = .5

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Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the dominant allele?

p = 1 – qp = 1 - .5

p = .5

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Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat:

q = .5p = .52pq = 2(.5)(.5)2pq = .5

This represents the carriers of the recessive alleles

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This shows the allele frequency in generation 1.

One would study the allele frequencies over several generations to see if the population is evolving.

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What is the Point of H-W? Why we use the H-W equation is to

determine if a population is evolving. By calculating the equilibrium in the

beginning of a population we know the distribution of alleles.

If we examine a population later in time we can see if there is a change.

According to H-W, if there is no change there is no evolution.

If there is change evolution has occurred.

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Independent Practice Complete Question #1 on the handout. You may ask questions – share out answers. The goal is to find: q2, p2, p, q, 2pq You start by determining the % of

homozygous recessives in a population (this is q2)

It is also important why you are doing this… it is to determine the genetic equilibrium baseline for the population.

Complete the rest of the worksheet for classwork/homework.

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Example Of Strong Selection Pressure Tay Sachs

Primarily in Ashkenazi Jews & Cajuns Recessive disease = aa lysosomal storage disease

lack of one functional digestive enzyme in lysosome

build up undigested fat in brain cells

children die before they are 5 years old

So where do new cases of Tay-Sachs come from?

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Example of heterozygote advantage Sickle cell anemia

inherit a mutation in gene coding for one of the subunits in hemoglobin oxygen-carrying blood protein normal allele = Hb

mutant allele = Hs

recessive trait = HsHs

low oxygen levels causes RBC to sickle clogging small blood vessels damage to organs

often lethal

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Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

1

2

3

liver

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Sickle cell frequency High frequency of heterozygotes

1 in 5 in Central Africans = HbHs

unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs

usually die before reproductive age

Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?

Suggests some selective advantage of being heterozygous… HbHs

Suggests some selective advantage of being heterozygous… HbHs

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Heterozygote Advantage In tropical Africa, where malaria is common:

homozygous dominant (normal) reduced survival or reproduction from malaria: HbHb

homozygous recessive reduced survival & reproduction from sickle cell anemia: HsHs

heterozygote carriers survival & reproductive advantage: HbHs

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele &

distribution of malaria

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Concept Check What is the usefulness of the H-W Law?

Homework Complete the H-W Online Tutorial by

your next period. http://www.montereyinstitute.org/

courses/AP%20Biology%20I/course%20files/multimedia/lesson23/lessonp.html?showTopic=2

AP Biology 2010-2011

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