Genes and Their Evolution: Population Genetics Chapter 4.
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Genes and Their Genes and Their Evolution: Population Evolution: Population
GeneticsGeneticsChapter 4Chapter 4
Population A population is a group of individuals of the
same species who share a geographic area and usually mate within the group
The total genetic variation of that population is the gene pool
The number of times different genes or alleles occur is the frequency
And evolution is change in allele frequency over generations
Species We only look at reproductive populations of
organisms. This is important because to be called a species, organisms must be able to mate and have fertile offspring Are these different species?
Lion and tiger
Horse and donkey
Dog and wolf
Species When reproductive isolation occurs, this
means that two populations are kept from mating
If enough time passes, these two groups will become two different species This is allopatric speciation
Example: two groups of beetles get separated by a river. Over time, enough differences arise that they become different species and would not be able to mate again
Population Genetics This is the study of changes in genetic material
More specifically, the change in allele frequency allele= different versions of genes
Frequency= how often they occur
Microevolution: small-scale; happens in a short period of time
Macroevolution: large-scale; occurs over many generations; speciation
Different Views of Evolution
Darwin thought evolution was small changes accumulating over long periods of time This is phyletic gradualism
Gould and Eldredge said evolution could have long periods of no or minor change, interrupted by sudden change, such as speciation or extinction This is punctuated equilibrium
The Two Views
More Types of Evolution
We know evolution shows common ancestry
When two related species share phenotypic traits because of common ancestry, this is parallel evolution All primates have eyes close together and 5 fingers
When distantly related species develop similar adaptations to similar environments, this is convergent evolution Crocodyles and cats have tails because they walk
on 4 legs
Chickens, bees, and bats all have wings to fly
Parallel Evolution
Convergent Evolution
Population Genetics We focus on the idea of change over time,
especially in the frequency of alleles
Example: we are looking at a trait, which we will call R The two alleles are R and r (dominant and
recessive)
Generation 1 has 50% R and 50% r
When we come back and look at Generation 2, the frequency has changed to 40% R and 60% r
This shows evolution
Population Genetics We need evidence that evolution is occurring,
and we do this by looking at the frequencies of alleles in populations
If they do not change, there is no evolution
If they do change, there is evolution
Intro to Hardy-Weinberg
If the frequency never changed, the population would be in equilibrium
So, there is an equation to test for equilibrium
If the numbers don’t change = equilibrium = no evolution
If they do change = no equilibrium = evolution
Hardy-Weinberg Godfrey Hardy and Wilhelm Weinberg developed
a way to test for equilibrium in allele frequency
In order for equilibrium to exist, you must have:
No mutations
No natural selection
Completely random mating
An infinitely large population
Each organism having the exact same number of offspring
Would this ever occur in nature?
Hardy-Weinberg Don’t let the equation intimidate you!
Focus on what each part stands for and follow the steps
There will only be a couple of questions of this on the midterm
Hardy-Weinberg Here is the equation:
p2 + 2pq + q2 = 1.00 (100% of population)
p2 = all individuals who are homozygous dominant
q2 = all individuals who are homozygous recessive
2pq = all individuals who are heterozygous
Also important: p + q = 1.00
p = the dominant allele
q = the recessive allele
Example A population of snails has a trait for either
spotted or striped shells. Striped shells are dominant. Out of 100 snails, 16 have spotted shells.
Find the frequencies of SS, Ss, and ss
The next year we return and find that 25 out of 100 have spotted shells. Why?
Hardy-Weinberg Please use the document titled “HW
Explanation” on BlackBoard to see details and how to solve a problem
Sources of Evolution So, evolution is change over time, but how do
these changes arise?
4 sources: Mutation
Natural Selection
Gene Flow
Genetic Drift
Mutation Small errors in DNA, especially during
replication
Most go unnoticed. Some can be harmful and some beneficial
They occur at random
They are the only source of new genetic variation in a population
Mutations Point mutation: a single base is changed
ATCGGTC ATCGGTA
Frameshift mutation: caused by a deletion or insertion of genetic information; causes codons to be read incorrectly
Parts of chromosomes can be mutated, or entire chromosomes can be mutated
Kleinfelter’s Syndrom: males have extra sex chromosome: XXY
Trisomy 21: person has extra 21st chromosome. AKA Down Syndrome
Mutation Example A point mutation changes the hemoglobin blood cell in
some people
This causes sickle-cell anemia. Their blood cells are deformed and cannot carry enough oxygen
It is often fatal
Why do so many sub-Saharan Africans have this mutation?
Because it protects against malaria, the #1 killer in Africa
If a person is heterozygous for the sickle-cell trait, he/she will have enough normal blood to carry oxygen and enough affected blood to kill malaria parasites that enter body
Watch: http://www.youtube.com/watch?v=1fN7rOwDyMQ
Malaria and Sickle-Cell
The malaria/sickle-cell relationship is a balanced polymorphism
The heterozygous trait balances one negative trait with another, giving the person a better chance of surviving
This is selected for, and is also an example of natural selection
Pp. 317-320
Natural Selection Survival of those best adapted to current
environmental pressures
Based on the idea of fitness: number of offspring produced in a lifetime
Has nothing to do with strength, speed, or intelligence
Is just how good are you at surviving and making babies
Natural Selection Read the box on p. 104
Who has better fitness?
Male, Harvard degree, $500,000 salary, no kids Male, high school drop
out, unemployed, 7 kids
Types of Natural Selection
Directional: selection shifts in one direction Example: large
beak sizes in finches when droughts leave only hard food to eat
Types of Natural Selection
Stabilizing: selection favors the average and is against the extremes Example: birth
weight. Babies in the normal range survive more than premature ones or obese ones
Types of Natural Selection
Disruptive: selection favors the extremes, and against the average. Leads to speciation Example: beetles
are reproductively isolated until 2 new species are created.
Example of Natural Selection
In Great Britain, most moths were light colored to blend in to the environment
With the Industrial Revolution in the 1800s, the smog and soot produced changed the environment to be darker
Did the light moths have the advantage still?
No, they were eaten and darker moths survived. This changed allele frequencies
Practical Example #2 Practical Example #2 (from lecture 2)(from lecture 2)
Can you see viruses or bacteria?Can you see viruses or bacteria?
They are alive and, like everything else, they They are alive and, like everything else, they evolveevolve
Can evolve in a matter of hoursCan evolve in a matter of hours
This is why not taking antibiotics/medication This is why not taking antibiotics/medication correctly leads to drug resistancecorrectly leads to drug resistance
Yellow = weak virusesPurple = medium virusesRed = strong viruses
When people do not take medication correctly
Yellow = weak virusesPurple = medium virusesRed = strong viruses
When people do not take medication correctly
Genetic Drift This is an over-representation or an under-
representation of traits because of a small sample size
Example: In a class of 25 people, I find that 20 have Type B blood, 3 have Type O and 2 have Type AB
Does this accurately reflect the frequency of blood types in the entire human population?
What about Type A?
Genetic Drift This shows why large populations are
healthier…there is more variation
Endogamous groups only breed within their population
Exogamous groups breed with members outside their population
Which is better for variation and health?
2 Types of Genetic Drift
1. Founder Effect: a small group breaks off from the original population and forms its own group
Will that small group accurately reflect all the variation of the original population?
Huntington’s Chorea and Tay Sachs: genetic defects that are exaggerated due to founder effect and genetic drift
2. Population Bottleneck: when a population is reduced drastically, there is not enough variation to keep it going
Can cause extinction
This is what happens to endangered species
Genetic Drift: Founder Effect
A small group of original population creates new population
Some traits will be over-represented
Some traits will be lost
Genetic Drift: Bottleneck
Severe reduction in population
Loss of variation
Gene Flow Movement of genes and mixture of them
through breeding
Not only migration: have to mate as well, in order to add variation So it is migration and nonrandom mating
If there is no gene flow between 2 populations, they could evolve into 2 different species
With global travel and more open-mindedness in cultural ideals, our human gene pool has had a large increase in gene flow and variation
Gene Flow Variation is the key to success!
Why is inbreeding so bad?
It limits variation in the gene pool and can increase harmful mutations
Discussion Read pp.113-115, including box on p. 114
Why is biodiversity important?
Why should we worry about our actions as humans?
Why should we participate in conservation?
Review Questions What is the difference between
microevolution and macroevolution?
How does the Hardy-Weinberg equation show that evolution occurs?
What are the sources of evolution? What are some examples?
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