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Transcript of Measuring Evolution
assume 2 alleles = B, b
frequency of dominant allele (B) =
frequency of recessive allele (b) =
frequencies must add to 1 (100%), so:
p + q = 1
frequency of homozygous dominant: p x p =
frequency of homozygous recessive: q x q =
frequency of heterozygotes: (p x q) + (q x p) =
frequencies of all individuals must add to 1 (100%), so:
p + 2pq + q = 1
Individuals are selected
Differential survival, differential reproductive success
The genetic makeup of a population changes over time
Fitness increases: Favorable traits (greater fitness) become more common
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?
: random changes to DNA (Why?)
: mixing of genes ("
"). New arrangements in every offspring
Offspring inherit traits from parents
5 Sources of Evolution
– determines a trait (ex. eye color)
– A variant of a gene (ex. brown eyes vs. blue eyes)
All sexually reproducing organisms have
for any trait.
– An allele that will show a trait, regardless of the other other allele (ex. brown eyes)
– An allele that will only show a trait if both alleles are recessive (ex. blue eyes)
– any individual who has 2 copies of the same allele. Can be homozygous dominant or homozygous recessive.
- any individual who has one copy of a dominant allele and one copy of a recessive allele. WILL SHOW THE DOMINANT TRAIT!
- a localized group of interbreeding individuals
- the collection of alleles in the population
remember difference between alleles & genes!
= change in allele frequencies in a population
Describes a Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model for comparison (
natural populations are never in H-W equilibrium
useful model to measure if forces are acting on a population
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)
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
Effect of chance events; founder effect, bottlenecks
Loss of alleles from gene pool: reduces variation, reduces adaptability
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
(oxygen-carrying blood protein)
recessive allele =
- makes a defective protein
dominant allele =
- 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
Hypothetical: what conditions would cause allele frequencies to NOT change?
p + q = 1
p + 2pq + q = 1
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.
aka "Enter Math"
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.
-No migration (gene flow)
-Large population size (no drift)
*If one of these occurs then the allele frequencies are changing