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AP Bio-Forbes-Evolution 4: Measuring Evolution

4 of 6 of my evolution unit. Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet. Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke.

Shani Forbes

on 17 February 2013

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Transcript of AP Bio-Forbes-Evolution 4: Measuring Evolution

Hardy-Weinberg theorem:
assume 2 alleles = B, b
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
frequencies must add to 1 (100%), so:

p + q = 1

frequency of homozygous dominant: p x p = p
frequency of homozygous recessive: q x q = q
frequency of heterozygotes: (p x q) + (q x p) = 2pq

frequencies of all individuals must add to 1 (100%), so:

p + 2pq + q = 1 Populations Evolve Individuals are selected
Differential survival, differential reproductive success

Populations evolve
The genetic makeup of a population changes over time
Fitness increases: Favorable traits (greater fitness) become more common Variation Variation is the raw material for natural selection.
There have to be differences within population. Some individuals must be more fit than others

Where does variation come from?
Mutation: random changes to DNA (Why?)
Sex: mixing of genes ("recombination"). New arrangements in every offspring

Offspring inherit traits from parents 5 Sources of Evolution Required Terminology Hardy-Weinberg Equilibrium Describes a Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model for comparison (null hypothesis)
natural populations are never in H-W equilibrium
useful model to measure if forces are acting on a population Measuring Evolution 1. Mutation
Mutation creates variation
new mutations are constantly appearing
Mutation changes DNA sequence, changes amino acid sequence, changes protein structure & function, changes traits, changes fitness (maybe) 2. Gene Flow
Movement of individuals & alleles in & out of populations.
seed & pollen distribution by wind & insects
migration of animals
reduces differences between populations
Gene flow in human populations is increasing today thanks to modern travel technology 3. Non-random mating
Sexual selection 4. Genetic drift
Effect of chance events; founder effect, bottlenecks
Loss of alleles from gene pool: reduces variation, reduces adaptability 5. Natural selection
Differential survival & reproduction due to changing environmental conditions
Combinations of alleles that provide “fitness” increase in the population
Adaptive evolutionary change
Natural selection can't fashion perfect organism. Why??? An Application of H-W principle: Sickle cell anemia
Due to a mutation in a gene coding for hemoglobin (oxygen-carrying blood protein)

recessive allele = Hs - makes a defective protein
dominant allele = Hb - makes a normal protein

A recessive disease: individuals must be Hs Hs to have sickle cell anemia
low oxygen levels causes RBC to sickle
breakdown of RBC
clogging small blood vessels
damage to organs
often lethal in childhood

Sickle cell frequency:
High frequency of heterozygotes ("sickle cell trait") in African population
1 in 5 in Central Africans = Hb Hs
Unusual for allele with severe detrimental effects in homozygotes
1 in 100 = HsHs
usually die before reproductive age

In tropical Africa, malaria is common.
Heterozygous (Hb Hs): confers resistance to malaria
Homozygous dominant (Hb Hb): die/reduced reproduction from malaria.
Homozygous recessive (Hs Hs): die/reduced reproduction from sickle cell anemia.
Heterozygote carriers survive & reproduce: Hs allele becomes common in population
"Heterozygote Advantage"

Balanced Polymorphism
two versions of a gene are maintained in a population
of organisms because individuals carrying
both versions are better able to survive than
those who have two copies of either version alone Hypothetical: what conditions would cause allele frequencies to NOT change? H-W formulas:
Gene Pool
p + q = 1
p + 2pq + q = 1 2 2 2 2 2 2 Big Questions Make Sure You Can Solving HW Problems Gene – determines a trait (ex. eye color)

Allele – A variant of a gene (ex. brown eyes vs. blue eyes)

Diploidy-All sexually reproducing organisms have 2 alleles for any trait. Advantage??

Dominant – An allele that will show a trait, regardless of the other other allele (ex. brown eyes)

Recessive – An allele that will only show a trait if both alleles are recessive (ex. blue eyes)

Homozygous – any individual who has 2 copies of the same allele. Can be homozygous dominant or homozygous recessive.

Heterozygous- any individual who has one copy of a dominant allele and one copy of a recessive allele. WILL SHOW THE DOMINANT TRAIT!

Population - a localized group of interbreeding individuals

Gene Pool - the collection of alleles in the population
remember difference between alleles & genes!

Evolution = change in allele frequencies in a population 1. Write both equations.
2. Identify any given information.
3. Don't screw up squaring frequencies!
Remember that the square root of a decimal is a LARGER number than that decimal.
ex: What is the square root of 81? What is the square root of 0.81?
4. First: figure out q. q is the magic key that lets you unlock all of the other variables in the equations.
5. Work your way around the problem until all terms are solved for.
6. Practice makes perfect!

A. If 64% of the individuals in a population exhibit the recessive appearance, what % of the gene pool is dominant (assume HW equilibrium)?

B. A population contains individuals, 16% of whom show the recessive trait What % of the population is pure dominant? What % of the gene pool is recessive? What percent of the population is hybrid (assume HW equilibrium)?

C. How can you identify if a population is in HW equilibrium? Refer to the HW equations in your answer. Any Questions? aka "Enter Math" 2 2 p + q = 1
p + 2pq + q = 1 How is variation generated and maintained in a population?

How do we know evolution is happening in populations?

What aspects of a population contribute to evolution?

How can evolution be qualitatively and quanititatively measured?

How does measuring evolution help us understand how populations are evolving? Explain how variation is produced and maintained in a population.

Define all new terms used in this presentation in your own words and give descriptive examples.

Explain how each source of evolution in a population affects variation and selection.

Use the HW theorem with facility (be able to move through all terms of both equations).

Apply the HW theorem to actual populations.
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