Measuring Evolution of Populations Hardy Weinberg

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Measuring Evolution of Populations Hardy Weinberg. 5 Agents of evolutionary change. Mutation. Gene Flow. Non-random mating. Chemical Changes to DNA. Migration. Sexual Selection. Genetic Drift. Selection. Natural Selection Differential Survival. Small population. - PowerPoint PPT Presentation

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PowerPoint PresentationMutation
AP Biology
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:
Mutations do not occur
No gene flow
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
Studying 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
AP Biology
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
The Hardy-Weinberg principle describes a population that is not evolving
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Fig. 18-3a, p. 280
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Starting Population
Next Generation
Next Generation
Figure 18.3
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 Biology
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,
AP Biology
Can you identify who is dominant and recessive.
Yes.
Only for the homozygous recessive!
You have to calculate the others….
AP Biology
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%
AP Biology
Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele.
q = .5
AP Biology
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 – q
p = 1 - .5
p = .5
AP Biology
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 = .5
p = .5
2pq = 2(.5)(.5)
2pq = .5
AP Biology
This shows the allele frequency in generation 1.
One would study the allele frequencies over several generations to see if the population is evolving.
AP Biology
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.
AP Biology
Independent Practice
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.
AP Biology
Tay Sachs
Recessive disease = aa
lysosomal storage disease
build up undigested fat in
brain cells
Tay-Sachs come from?
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
Malaria
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
1
2
3
liver
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?
Suggests some selective advantage of being heterozygous… HbHs
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Sickle Cell:
In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite.
Cystic fibrosis:
Cystic fibrosis carriers are thought to be more resistant to cholera:
1:25, or 4% of Caucasians are carriers Cc
AP Biology
Heterozygote Advantage
homozygous dominant (normal)
homozygous recessive
heterozygote carriers
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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Homework
http://www.montereyinstitute.org/courses/AP%20Biology%20I/course%20files/multimedia/lesson23/lessonp.html?showTopic=2