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AP Bio- Evolution 4: Measuring Evolution

4 of 7 of my Evolution Domain.. 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.
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David Knuffke

on 25 June 2014

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Transcript of AP Bio- Evolution 4: Measuring Evolution

Review:
Gene
– determines a trait (ex. eye color)
Allele
– A variant of a gene (ex. brown eyes vs. blue eyes)

All sexually reproducing organisms have
2 alleles
for any trait.

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)

New Terminology:
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
Measuring Evolution
p + q = 1
p + 2pq + q = 1
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
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 that
preserves allele frequencies.
Serves as a model for comparison (
null hypothesis
).
Natural populations are never in H-W equilibrium.
Useful model to measure how forces are acting on a population.
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
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
Symptoms:
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 = Hs/Hs
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
"
Hypothetical: what conditions would cause allele frequencies to NOT change?
H-W formulas:
Gene Pool:
p + q = 1
Individuals
p + 2pq + 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
2
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2
2
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Big Questions
Make Sure You Can
Solving HW Problems
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
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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.

Change in phenotype (copper tolerance)
in a population of grass plants living
on and near
a copper mine.
Full transcript