Download Lecture 15: Population Genetics - Linn

11/24/2015
Species Variation
Biology 102
Lecture 15: Population
Genetics
•
Natural variation occurs in every population of
every species
•
Due to different alleles for genes
•
Evolution is measured as changes in allele and
genotype frequency in a population
HardyHardy
-Weinberg Principle
•
If evolution is not at
HardyHardy
-Weinberg Principle
•
Without evolution, alleles in a population will
quickly reach equilibrium
•
Once the frequency of alleles in a population is
known, we can use the HardyHardy-Weinberg equation
work, allele and genotype
frequencies will stay the
same from one generation
to the next
•
p = frequency of the dominant allele in the population
•
q = frequency of the recessive allele in the population
Where does it come from?
•
Assume that 2 heterozygous individuals produce
offspring
p
q
p
pp
(p2)
pq
q
pq
qq
(q2)
What does it mean?
•
We can calculate what proportion of individuals
in the next generation will have a given genotype
and phenotype
•
You will do this in Lab 9
1
11/24/2015
Example
Example
• Two alleles exist for color in a certain type of
beetle. Red (R) is dominant to blue (r).
• In a specific population of beetles, 51% are
are red
and 49% are blue.
= RR and Rr
= rr only
p2 and pq
q2 only
q2 = 0.49
• Assuming the population is in HH-W equilibrium,
what are the frequencies of the red and blue
alleles in the gene pool?
q = 0.7
p+q=1
1 – 0.7 = p
p = 0.3
Example
Example
• Cystic fibrosis is an autosomal recessive disorder
that affects 1/2500 (0.0004) Caucasians.
• Assuming the population is in HH-W equilibrium,
what percentage of Caucasians are carriers?
= CC and Cc
= cc only
p2 and pq
q2 only
q2 = 0.0004
q = 0.02
p+q=1
1 – 0.02 = p
p = 0.98
Example
Check Your Work!
= CC and Cc
= cc only
p2 + 2pq + q2 = 1
p = 0.98
p2 and pq
q = 0.02
q2 only
(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?
Frequency of heterozygotes = 2pq
2(0.98
2(0.98)(0.02)
)(0.02) = 2pq
1
0.0392 or 3.92%
2
11/24/2015
Problems
•
The HardyHardy-Weinberg equation makes some
pretty big assumptions
Problems
•
Adjustments can be made for these assumptions,
but not for the biggest problem of all…
•
Organism is diploid
•
Only sexual reproduction is used
•
Only 2 alleles exist for a given gene
•
Complete dominance
•
Very large sample size
•
Not a sexsex-linked trait
•
No migration
•
No mutation
•
Random mating
•
No differential reproductive success
IT ASSUMES NO EVOLUTION!!
•
Assumes…
Problems
•
•
The HardyHardy-Weinberg equation is great for…
•
Getting a snapshot of alleles in time
•
Tracking evolutionary changes over time
Contibuting Factors
• Five agents of evolutionary change
• All bring about changes in allele frequency and
disrupt HardyHardy-Weinberg equilibrium
It is NOT good for predicting evolutionary
changes
• Genetic drift
• Gene flow
• Mutations
• Sexual selection
• Natural selection
Genetic Drift
• Changes in allel
allele
e frequency due to chance events
in a small population
• Example: Flipping a coin
3 out of 10
300 out of 1000
• Drift tends to reduce variation
• 2 main types
• Founder effect
• Bottlenecking event
Founder Effect
•
When a small group separates from a larger
population
•
Small group might…
•
Carry alleles that are rare in the original
population
•
Be missing alleles that are present in the
original population
3
11/24/2015
Founder Effect
•
Example:
Founder Effect
•
•
A population of red and yellow beetles exists
•
The wind randomly blows only red beetles to a
new island
Example:
•
Ellis
Ellis--Van Creveld syndrome is common among
old order Amish in Lancaster County,
Pennsylvania
•
All are descendents
of one couple that
founded the community
in 1744
Bottlenecking Event
•
A catastrophe (e.g. earthquake, tsunami, etc)
kills off most of a population
•
Handful of survivors may not represent the full
genetic diversity of the original population
•
Reduction in variation and adaptability
IMPORTANT!!
•
A bottlenecking event is random
•
A plague that kills off individuals lacking a
particular allele is natural selection
•
Natural selection kills individuals due to genetic
make
make--up
•
Bottlenecking kills indiscriminately
Bottlenecking Event
•
Example:
•
Endangered species
Genetic Drift
•
It’s important to remember that all sources of
genetic drift…
•
Genetic variation low
•
Occur in small populations
•
Suceptibility high
•
Are RANDOM
4
11/24/2015
Gene Flow
•
Transfer of alleles from one population to
another
•
Due to individuals moving into or out of a
population
Gene Flow
Examples:
•
Mutations
•
Permanent changes in the DNA sequence
•
Multiple sources (UV radiation, chemicals, etc)
•
We’ve talked about this before
•
A migrating bird changes flocks
•
Grolar bears (gene flow between species)
Mutations
•
If mutations occur in the gametes, changes can
be passed to the next generation
•
Frequency = 1/10,000 to 1/100,000,000
•
Almost always leads to early death or lowered
reproductive fitness
Mutations
Sexual Selection
•
But not always
•
Mating is not random
•
Examples:
•
Certain characteristics increase an organism’s
chances of reproduction
5
11/24/2015
Sexual Selection
•
In many species, males and females look very
different
•
Sexual dimorphism
Sexual Selection
•
Some of these differences give a competitive
advantage when competing with each other for
mates
•
Intrasexual
Intra
sexual selection
Sexual Selection
•
Some of these differences give a competitive
advantage when attempting to attract mates
•
Intersexual
Inter
sexual selection
6