Download PowerPoint

THE GENETIC BASIS OF EVOLUTIONARY CHANGE
1. Simple, discrete traits
2. Continuous traits
Evolution: A change in gene frequency
GG
BB
GB
1 locus, 2 alleles
Co-dominant
Heterozygote detectable
Phenotypes
GG
3 (.25)
BB
3 (.25)
GB
6 (.50)
12 (1.0)
What are the frequencies
of the three phenotypes
in this population?
Phenotypes
Genotypes
GG
3
3 (.25)
BB
3
3 (.25)
GB
6
6 (.25)
12
12 (1.0)
What are the frequencies
of the three genotypes
in this population?
Phenotypes
Genotypes
GG
3
3
BB
3
3
GB
6
6
12
12
What are the frequencies
of the two alleles or genes
in this population?
How many G’s? 12 (50%)
How many B’s? 12 (50%)
Hardy-Weinberg
p = frequency of one of the alleles
(the dominant one, if there is dominance)
q = frequency of the other allele
(recessive, if there is recessiveness)
p + q = 1.0
p (G) = 0.5
q (B) = 0.5
0.5 + 0.5
=
1.0
The relationship between gene frequencies
and genotype frequencies
p + q
=
1.0
GG + GB + BB = 1.0
p2 + 2pq + q2 = 1.0
.25 + .50 + .25 = 1.0
Red heads is caused by the recessive gene r
What are the phenotype frequencies of
people with red and blue heads?
Red: .25
Blue: .75
Genotype frequencies?
RR
Rr
rr: 25
How do we find the
frequencies for RR and Rr?
The relationship between gene frequencies
and genotype frequencies
p + q
=
1.0
GG + GB + BB = 1.0
p2 + 2pq + q2 = 1.0
.25 + .50 + .25 = 1.0
If you know what q2, or rr, is, you can solve the others
Red heads is caused by the recessive gene r
What are the phenotype frequencies of
people with red and blue heads?
Red: .25
Blue: .75
Genotype frequencies?
RR: .25
Rr: .50
rr: .25
r =
= .5
.25= .5
R = 1-r
p2 + 2pq + q2 = 1.0
Gene and genotype frequencies will be the same
every generation if the population is
1. Large
2. Randomly mating
3. Not receiving migrants
4. Not undergoing selection
IN “HARDY-WEINERG EQUILIBRIUM”
Factors that can change gene frequencies:
1. Selection
2. Drift
3. Mutation
4. Migration
Factors that can change gene frequencies:
1. Selection: Differential reproductive success
or reproductive FITNESS
Red leaves fewer offspring relative to Blue
time
Blue produces more offpring relative to Red
time
Factors that can change gene frequencies:
1. Selection
2. Drift: In small populations, “sampling error”
chance fluctuations in reproduction
DRIFT
time
Founder effect
Time and space
Factors that can change gene frequencies:
1. Selection
2. Drift
3. Mutation : recurrent and directional
Recurrent mutation of R --------> r
time
Factors that can change gene frequencies:
1. Selection
2. Drift
3. Mutation
4. Migration: introduction of genes from outside
the population.
time
THE GENETIC BASIS OF EVOLUTIONARY CHANGE
1. Simple, discrete traits
2. Continuous traits
Example of disruptive selection:
Sexual dimorphism
Example of Directional selection:
cranial capacity
Example of balancing selection:
Birth weight