Transcript What is Natural Selection?

Population Genetics
What is Population Genetics?

The genetic study of the process of
natural selection.
(The study of the change of allele
frequencies, genotype frequencies,
and phenotype frequencies)
What is Natural Selection?

Natural selection causes changes in a
population if
(1) There is variation in fitness (selection)
(2) That variation can be passed from one
generation to the next (inheritance)
Hence the “survival of the fittest” which leads to
changes within populations.
This is the central insight of Darwin.
Populations evolve genetically to survive.
We know this happens. Look at the
following clip.
Newt video clip
http://www.pbs.org/wgbh/evolution/library/
01/3/quicktime/l_013_07.html

Who is Hardy and Weinberg?
Hardy and Weinberg constructed a model
of a population that does NOT change.
Five factors necessary to remain
in equilibrium





No mutations – no new alleles enter population.
No gene flow (i.e. no migration of individuals
into, or out of, the population).
Random mating (i.e. individuals must pair by
chance)
Population must be large so that no genetic drift
(random chance) can cause the allele
frequencies to change.
No selection so certain alleles are not selected
for, or against.
Why Hardy Weinberg?
Due to the fact that this balance cannot work,
scientists can use it to detect changes from
generation to generation.
Thus allowing a simplified method of
determining that evolution is occurring.
How?
If we mate two individuals that are
heterozygous
Girl Bb
Boy Bb
25% BB
 50% Bb
 25% bb (not like their parents, express the
recessive phenotype).

This is what Mendel found when he
crossed monohybrids
But the frequency of two alleles in an
entire population of organisms is
unlikely to be exactly the same.
The Hardy-Weinberg equation allowed
geneticists to do the same thing
Mendel did for individual families for
entire populations.
Allele Frequency
Let’s talk about p and q
 p = the frequency of the dominant allele

q = the frequency of the recessive allele
For a population in genetic equilibrium:
p + q = 1.0 (The sum of the frequencies
of both alleles is 100%.)





For example:
Imagine a
'swimming' pool
of genes as
shown in Figure
1.
Count the number
of A and a inside
the pool (not the
ones on the outside
of the pool).
A= 12
a = 18
Then take (A + a) or
This shows the total population in the
pool is what?
12 +18 = 30
To determine the frequencies of A and a.
 Dominant Gene (A)/total population to
equal (p)
Step 1: Frequency of A or f(A) = 12/30 =
0.4 so
p= 0.4
 Recessive gene (a)/total population to
equal (q)
Step 2: Frequency of a or f(a) = 18/30 =
0.6
q = 0.6
Then, take what you found for p + q= 1
p= 0.4
q = 0.6
Step 3: Example: 0.4(fA) + 0.6(fa) =
1
Let’s try another one.
For example:
The population of squirrels near Harvard
University has a mix of brown and
black squirrels. The students surveyed
the number of Brown and black
squirrels. There are 200 Brown
squirrels and 400 black squirrels. Black
is dominant over grey squirrels.
Brown is recessive.
What is the number of black squirrels?
A: 400
What is the number of Brown squirrels?
A: 200
What is the total population?
p+q=1
A: 400 + 200 = 600
To determine the
frequencies of p and q.
Dominant Gene A/total population to equal (p)
Step 1: Frequency of A or
f(A) = 400/600
p=0.7
Recessive gene (a)/total population to equal (q)
Step 2: Frequency of a or
f(a) = 200/600 = 0.6
q = 0.3
Then, take what you found for p + q= 1
Step 3: 0.7(fA) + 0.3(fa) = 1
Work on problems #1 and 2 in
homework if done.
Let’s take both examples one step
further.
Genotype Frequency

The proportion of individuals in a
group with a particular genotype.
(Genotype can refer to one locus, two
loci, or the whole genome, depending
on the context
Starting with the pool
example
If the Frequency of p= 0.4
And the Frequency of q = 0.6
Then determine the genotypic frequencies of AA, Aa and aa.
How?
Using the following equations: p2 + 2pq + q2
(Same as AA + Aa + aa)
Step 1: Example:
p2 = (0.4 x 0.4) = .16
2pq = 2(0.4 x 0.6) = .48
q2 = (0.6 x 0.6) = .36
Which equals: .16 + .48 + .36 = 1
16% of the population are AA
48% are Aa
36% are aa
Let’s try with our squirrels.
If the frequency of p=0.7
The frequency of q = 0.3
Then determine the genotypic frequencies of BB, Bb
and bb.
How?
Using the following equations: p2 + 2pq + q2
(Same as BB + Bb + bb)
Step 1: Example: p2 = (0.7 x 0.7)
2pq = 2(0.7 x 0.3)
q2 = (0.3 x 0.3)
Which equals: .49 + .42 + .09 = 1
49% of the population are BB
42% are Bb
9% are bb
Simplify




p = frequency of the dominant gene
q = frequency of the recessive gene
p2 = frequency of the homozygous
dominant trait
q2 = frequency of the recessive trait
Now, suppose more 'swimmers' dive in as
shown in Figure 2. What will the gene and
genotypic frequencies be?
Apply the same thing we did for the first example to the
additional members of the population to the swimming
pool. Counting the a’s that were outside the pool now
entering the pool.
What is the frequency of A or p?
What is the frequency of a or q?
What is the genotypic frequencies?
Solution:
 f(A) = 12/34 = .35 = 35 %
 f(a) = 21/34 = .65 = 65%
Genotypic frequencies:
f(AA) = .12, f(Aa) = .46 and f (aa) = .41
Which equals: .12 + .46 + .41 = 1
Results:
AA: 12%
Aa: 46%
Aa: 41%

The results show that Hardy-Weinberg
Equilibrium was not maintained. The
migration of swimmers (genes) into the pool
(population) resulted in a change in the
population's gene frequencies

Try problems #1-3 in your Homework
packet.
Part II
Now that you know that Hardy-Weinberg
Equilibrium is not naturally
maintained.
We can lead into the idea of natural
selection and a result in the change in
the population's gene frequencies.
Natural Selection is due
to Mutations
How do Mutations happen?
U.V. Rays
Radiation
chemicals
Mistakes made during DNA replication (approximately 6 eve
time cells undergo mitosis)
What affects do mutations
have on organisms?





Lethal – deadly
Major: HOX genes cause rather unusual changes
such as an extra limb, antennae, etc.
Small
None – same amino acid or in DNA not used
Beneficial – leads to a positive change that allows
an organisms a better chance of survival.
Examples of Natural Selection
A. Sickle Cell Anemia
Protects those individuals who are
afflicted with this genetic disease
from Malaria.
B. Antibiotic Resistant
Bacteria
Bacteria reproduce fast so they
can produce several generations
in a very few hours and therefore
evolve in a relatively short time.
Most mutations lead to death but
some survive
Video on antibiotic resistance Part I
http://www.youtube.com/watch?v=2L82V6VPJkQ&fea
ture=related
Part II
http://www.youtube.com/watch?v=D0_FTJnhzXA&fea
ture=related
C. Peppered Moth
Prior to industrial revolution (1850),
most common phenotype was
light colored
After industrial revolution, dark
phenotype became more common
D. Viruses


Viruses also mutate
quickly just as
Bacteria do.
They cannot
reproduce without a
host
How do they Trick immune
system?

Viruses mutate to survive the immune
system by changing or evolving to strike
again
Example: Avian flu
Let’s look at the life cycle of a virus….
1. Attachment
3. Un-Coating
5. Release
2. Penetration
4. Assembly
PBS clip
Immunity to HIV?
http://www.pbs.org/wgbh/evolution/libr
ary/10/4/l_104_05.html
Immune System
I. Lymph vessels



Immune system pathway
return water to blood
Nodes = sites of immune system action,
invaders destroyed
Immunity

The system that gives the
body the ability to resist
disease.
I. Active - the body produces its own antibodies to defend
against a certain antigen.
II. Passive - is only for a short period of about one month
because a person is given the antibodies required to defend
against the antigen.
First Line of Defense







Skin
Mucus
Hair
Ear Wax
Tear Drops
Sweat
Stomach Acid
Second Line of Defense
Inflammatory Response – response to
tissue damage which increase white
blood cell production and fever
Third Line of Defense

Specific Immunity – immunity to
specific pathogens by recognizing,
attacking, destroying, and then
remembers each foreign substance and
pathogen that enters the body. It does
this by making specialized cells
and antibodies that makes the pathogens
useless.
How?
Attacks using WBC’s. Two types:
 B Cells – produce antibodies
 T Cells – killer cells
Then produces Antibodies
 Hold the pathogen so unable to
infect other cells until T-cell
destroys
Vaccination
 Give a piece of dead or weakened
virus so body fights off and forms
antibodies