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Lecture 3 - Concepts of Marine
Ecology and Evolution II
1) Review: Forces of evolution, DNA
structure and function, genes, alleles,
genotype, phenotype
2) Fitness: viability and fertility
3) Detecting evolution: HW Equilibrium Principle
-Calculating allele frequencies, predicting genotypes
4) Detecting selection: LAP evolution in mussels
What is Evolution?
DNA Structure and Function
DNA
Replication
DNA to Protein: Genotype to Phenotype
transcription
translation
Genes, alleles, genotypes, phenotypes
Forces of Evolution
1) Natural selection
2) Gene flow
3) Genetic drift
4) Mutation
5) Non-random mating
Darwin’s Theory of Natural Selection
Observation 1:
Observation 2:
Observation 3:
Fitness
1) Differential survival:
2) Differential reproductive success:
Example
A1A1
0.90
3.0
A1A2
0.85
3.0
A2A2
0.80
5.0
Absolute fitness W 2.7
Relative fitness w 0.68
2.55
0.64
4.0
1.0
Prob. of survival
# of zygotes
Deterministic process = outcome is determined by prior states; predictable outcome
Gene Flow (Migration)
Mixing of genotypes btn pops
Evolutionary consequences of migration
Pop. 1, high
Pop. 2, high A2
Random migrants (prop to freqs)
Genetic Drift
A consequence of random mating
Y = 0.4
R = 0.2
O = 0.4
N = 100
Y = 0.399
R = 0.188
O = 0.413
N=5
2Y, 2R, 1O
Y = 0.4
R = 0.4
O = 0.2
N=5
2Y, 3R, 0O
Y = 0.4
R = 0.6
O=0
Genetic Drift
Software: AlleleA1
http://faculty.washington.edu/herronjc/SoftwareFolder/AlleleA1.html
Genetic Drift
If pop size is small enough, for long enough, drift
will cause
Most Mutations
Examples of Adaptive Mutations
Malaria resistance: HbS and HbC
HbS: heterozygotes resistant
HbC: hets: 29% less likely, homoz: 93% less likely
Increased bone density (LRP5)
Significantly denser and stronger, less skeletal degeneracy
Reduced risk of arteriosclerosis (HDLs: Apo-AI-Milano)
One copy: HDLs significantly more effective at dissolving arterial
plaques
HIV resistance (CCR5d32)
One copy: AIDs does not develop
Two copies: completely resistant to HIV
How Often Are Mutations Beneficial?
1 in 150 mutations beneficial
1% fitness advantage
Detecting Evolutionary Change
G. H. Hardy and W. Weinberg (1908)
The HW Equilibrium Principle
If no “disturbing influences” act on a genetic
locus, allele freqs will remain constant
“Disturbing influences”
1) Natural selection
2) Gene flow
3) Genetic drift
4) Mutation
5) Non-random mating
Basic Skill: Calculation of allele frequencies
Sample of 100 individuals:
Genotype
AA
Aa
aa
# each allele
2(36) = 72
48
= 48 (of each)
2(16) = 32
N
36
48
16
f(A) = p = 72 + 48 = 120
total number of A alleles
= 120/ 200 = 0.6
f(a) = q = 32 + 48 = 80
= 80/ 200 = 0.4
NOTE: 0. 4 + 0.6 = 1
freq of A allele
total number of a alleles
freq of a allele
Predicting Genotype Frequencies
What’s the prob 2 gametes meet to form a zygote with a
particular genotype?
f(A) = p = 0.6 and f(a) = q = 0.4
Possible
Genotypes
AA
Aa
aa
Predicting Genotype Frequencies
What’s the prob 2 gametes meet to form a zygote with a
particular genotype?
f(A) = p = 0.6 and f(a) = q = 0.4
Converting to Genotype Numbers
What’s the prob 2 gametes meet to form a zygote with a
particular genotype?
f(A) = p = 0.6 and f(a) = q = 0.4
Expected
Genotype
Frequencies
AA = 0.36
Expected #s in a
sample of 100:
AA = 36
aa = 0.16
aa = 16
Aa = 0.48
Aa = 48
Hardy-Weinberg Equilibrium Principle
If no evolutionary forces, we can expect:
Generation 1
AA = 36
Aa = 48
aa = 16
p = 0.6
q = 0.4
HWE Expected (84):
AA = 42.86
Aa = 34.28
aa = 6.856
p2 = 0.5102
2pq = 0.4082
q2 = 0.0816
Observed (84):
AA = 36
Aa = 48 Selection
aa = 0
p = 0.7143
q = 0.2857
Generation 2
AA = 51.02
Aa = 40.82
aa = 8.16
p = 0.7143
q = 0.2857
HWE Expected (100):
AA = 51.12
Aa = 40.76
aa = 8.12
p2 = 0.5112
2pq = 0.4076
q2 = 0.0812
Observed (100):
AA = 51
Aa = 41 No Selection
aa = 8
p = 0.7150
q = 0.2850
Are the 5 HWE assumptions ever met?
Loci with alleles whose phenotypes have no + or
– fitness effects: neutral polymorphisms
e.g. blood cell-surface antigens
Race and Sanger (1975) – MN genotypes in
London
MM
MN
NN
Observed
363
634
282
Expected (HWE) 361.54 636.93 280.53
Is HWE for MN blood groups reasonable?
-London: large population (12M)
-Immigration: low relative to pop size
-Heritable mutations are too rare…
…to change frequencies
-No selective advantage for either allele
-Mating is random with respect to blood type
15 - 35 days
High dispersal potential
10s to 100s of km
75
85
90
100
Gulf of Maine
Long Island Sound
94
98
96
Leucine aminopeptidase (LAP) is
an exopeptidase that catalyzes the
hydrolysis of amino acid residues
from the amino terminus of
polypeptide chains. LAPs are widely
distributed, ubiquitous in nature,
and are of critical biological
importance because of their role in
protein degradation (Burley et al.
1990).
Smaller
HW deviation
Larger
94
98
96
Gulf of Maine
Forces of Evolution
1) Natural selection
2) Gene flow
3) Genetic drift
4) Mutation
5) Non-random mating