Transcript Evolutionary Mechanisms

Evolutionary Mechanisms
Biological evolution: change in genetic
composition of a population over time
• How can the gene pool of a population be
characterized quantitatively?
• What happens to the gene pool of a sexually
reproducing population over generations?
• What mechanisms cause evolutionary
change?
– Model systems to study evolutionary
mechanisms
Quantifying genetic variation in sexually
reproducing populations
Only locus X is shown,
with three alleles
(X1, X2 , and X3 )
The gene pool is
the sum of all alleles
Genetic structure is the frequency of the different genotypes in the population.
Fig. 21.03
Allele frequencies
Cystic fibrosis is a recessive genetic disease. Among
Northern Europeans, the incidence of CF is 1 per 2500
live births.
Q1: What is the frequency of the CF allele in the
Northern European population?
Q2: What proportion of the population are carriers of the
CF allele?
Random Mating
In Generation II, the allele frequencies are:
p=
q=
For a population in equilibrium:
F(AA) =
F(Aa) =
F(aa) =
Q3 - equilibrium
In both populations shown below, p = 0.6 and q =
0.4; which population(s) are in Hardy-Weinberg
equilibrium?
Population A
36 red (CRCR),
48 roan (CRCr), and
16 white (CrCr).
a. Population A
b. Population B
Population B
32 red (CRCR),
56 roan (CRCr),
12 white (CrCr).
c. Both A and B
d. Neither A nor B
Hardy-Weinberg (H-W) Equilibrium
• Assumptions.
• If the H-W assumptions are met, then allele
frequencies will not change from one
generation to the next.
HIV infection
• Is there genetic variation among HIV virus
particles in an infected individual?
• Is there significant mortality in the virus
population of an infected individual?
• Does genetic variation make a difference in
survival and reproduction of HIV virus?
Numbers of people living with HIV/AIDS
www.avert.org; data from WHO/UNAIDS
HIV infection time course
Q4: 3TC resistant viruses
Patient
No. 1
a.
Patient No. 2
b.
Patient No. 3
c.
Arose by mutations induced
by 3TC
Arose from a small pool of
mutant viruses already
resistant to 3TC
Arose by gradual adaptation
of viruses to 3TC
Weeks
Figure 22.13 Evolution of Drug Resistance in HIV
Campbell & Reece 7th ed. p. 448
Why do anti-HIV drugs become
ineffective?
• Structure of HIV reverse
transcriptase & resistance
mutations
• Blue = AZT resistance
• Lt. Blue = ddI, ddC, 3TC
• Violet = both AZT + ddI
Huang et al., 1998, Science 282:1669
Origin of Genetic Variation: Mutation
• Point mutations
• Insertions/Deletions
• Inversions/Translocations
Q5: Fate of Mutations:
a.
b.
c.
d.
e.
Most new mutations are not beneficial to the organism.
In a diploid organism, mutations will initially be present
only in a heterozygous condition.
Mutations may be lost by chance, even if beneficial.
All of the above are true.
None of the above are true.
Q6: How many times did SIV make the jump to
human hosts to become HIV?
a. Once
b. Twice
c. 3 times
d. 4 times
e. 5 or more
Q7: What anti-HIV therapies are informed by the
theory of natural selection?
A.
Multiple-drug cocktails
B.
Drug treatment immediately after exposure
C.
Stopping drug treatment when resistance emerges
D.
All of the above.
E.
None of the above.
HIV infects T cells via CD4 and CCR5 cell
surface receptors
Frequency of CCR5-delta32 allele in
different human populations
• Northern Europe
• Central Asia
• Asia, Africa
10%
2%
0%
Why is the CCR5-delta 32 allele so frequent among
Northern Europeans? Propose at least two alternative
hypotheses.
What percentage of people in each region are expected to
be resistant to HIV infection?
Q8: A high proportion of the cats on Key West have extra
toes (polydactyly). The most likely explanation is:
a)
b)
c)
d)
e)
High rate of mutation
Founder effect
Bottleneck effect
Selection for extra toes
Extra toes is sexually
appealing to female cats