Evolution Lecture 13
Chapter 9
Topics for today
- Detecting evolutionary change at the
genetic level
- Effect of evolutionary processes on
genetic variation
EvoBeaker: Sickle
cell alleles Exercise 2
Typo on page 8,
question 6. Should refer to question 5b
and 5c instead of 4b and 4c
In the absence of evolutionary processes, genotype
frequencies will remain constant over generations
1.
Prove
conceptually to ourselves that this is true
2.
Explore
underlying assumptions
3.
Statistically
test whether this is true for any given population
In the absence of evolutionary processes, gene
frequencies will remain constant over generations
Assumptions?
Everyone
has an equal chance of reproducing
Everyone
has an equal chance of surviving
Amount
of reproduction of any given genotype is proportional to its frequency
Number
of alleles (forms of the gene) doesnt change
Why would any of these be false?
1.
Nonrandom
mating
2.
Mutation
3.
Gene
flow
4.
Genetic
drift (Chapter 10)
5.
Natural
selection (Chapter 11)
How can we test for these violations our assumptions?
1.
If
observed and expected genotypic frequencies do match, the population is in
Hardy-Weinberg equilibrium
2.
Evolutionary
changes are not occurring
3.
If
observed and expected genotypic frequencies do not match, evolutionary change
of some kind is happening
4.
How bad
is the deviation? Is it statistically significant?
5.
Chi-square
test of significance
Genotype frequencies that are in HW equilibrium
Heterozygotes at highest frequency when p = q = 0.5
Most genes have more than 2 alleles
1.
HW equations
can be derived for any number of alleles
2.
p2
+ 2pq + q2 + r2 +2rp + 2rq + s2 + 2sp +2sq +
2sr + t2 + 2pt + 2qt + 2rt + 2st
Fig. 9.13
Do new mutations cause deviation from HW?
- Only for a single generation
- One generation of random mating restores
HW equilibrium
- Small effects of a single mutation
probably not detected
HW is a tool to generate hypotheses
Assumptions:
1.
Nonrandom
mating
2.
Mutation
3.
Gene
flow
4.
Genetic
drift
5.
Natural
selection
An example
Calculate
Observed
genotypic numbers
Observed
genotypic frequencies
Allele
frequencies
Expected
genotype frequencies if in HW
Expected
genotypic numbers
Do they
match?
Major
difference?
How does inbreeding effect
genetic diversity?
Heterozygous class reduced by ½ with each generation of inbreeding
How do you measure the extent of inbreeding?
F-statistics compare the observed heterozygosity to what is expected based on HW
F = (Ho
H)
Ho
F ranges from -1 to 1
0 = no inbreeding as expected under HW
1 = full inbreeding everyone is homozygous
-1 = full outbreeding everyone is heterozygous
What effect does inbreeding have?
-Exposes recessive alleles because they are more often in the homozygous state
-Italian population 1903-1973
-Should you kiss your more distant cousins?
-New study of data from 1800-1965 in
How can you relieve inbreeding depression?
-Small isolated population of snakes was declining
-Added 20 males from a different population in 1992
-Removed in 1996
-Observed population recovery
Can you avoid inbreeding depression?
1. Dont mate with your relatives
2. Keep natural population size large
3. Inbreeding avoided in captive breeding programs
a. Golden lion tamarin is an endangered monkey
b. Complex pedigree designs
c. 140 zoos, 500 individuals
Evolution Lecture 13
Chapter 9
Topics for today
- Detecting evolutionary change at the genetic level
- Effect of evolutionary processes on genetic variation
EvoBeaker: Sickle cell alleles Exercise 2
Typo on page 8, question 6. Should refer to question 5b and 5c instead of 4b and 4c