The document discusses genetic variation and evolution in populations, focusing on key concepts like gene pools, allele frequency, and mechanisms of evolution such as genetic drift. It explains two models of genetic drift—bottleneck effect and founder effect—and introduces Hardy-Weinberg equilibrium as a theoretical framework for understanding allele frequency stability in non-evolving populations. Additionally, it addresses the relationship between the sickle cell allele and malaria, highlighting the selective advantage of heterozygotes in certain populations.

1. Measuring
Evolution of Populations

2. Genetic variations in populations
 Genetic variation and evolution are both studied in populations.
 Because members of a population interbreed, they share a common
group of genes called a gene pool.
 Gene pools consist of all the genes, including the different alleles for
each gene, that are present in a population
 Allele Frequency – the number of times an allele occurs in a gene
pool, compared to the total number of alleles in that pool for the same
gene
 EVOLUTION, IN GENETIC TERMS, INVOLVES A CHANGE IN THE
FREQUENCY OF ALLELES IN A POPULATION OVER TIME!!!

3. Genetic Drift
 In small populations, individuals that carry a particular allele may leave more
descendant than other individuals, just by chance.
 Over time, a series of chance occurrences can cause an allele to become more or less
common in a population.
 This kid of random change in allele frequency is called genetic drift.
 Tends to reduce genetic variation
 Tends to take place in smaller populations
 Two different models for genetic drift: bottleneck effect & founders effect
Figure 23.7
CR
CR
CR
CW
CR
CR
CW
CW
CR
CR
CR
CW
CR
CW
CR
CW
CR
CR
CR
CR
Only 5 of
10 plants
leave
offspring
CW
CW
CR
CR
CR
CW
CR
CR CW
CW
CR
CW
CW
CW
CR
CR
CR
CW CR
CW
Only 2 of
10 plants
leave
offspring
CR
CR
CR
CR
CR
CR
CR
CR
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CR
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CR
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CR
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CR
CR
CR
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR
) = 0.7
q (frequency of CW
) = 0.3

4. The Bottleneck Effect
 A sudden change in the environment (ex: natural
disaster) may drastically reduce the size of a
population
 Wipes out a random part of the population
 The gene pool may no longer be reflective of the
original population’s gene pool
Original
population
Bottlenecking
event
Surviving
population
Figure 23.8 A
(a)
Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.

5. The Founder Effect
 The founder effect
 Occurs when a few individuals become
isolated from a larger population
 Migrate to another area
 Can affect allele frequencies in a population

6. 5 Agents of evolutionary change
Mutation Gene Flow
Genetic Drift Selection
Non-random mating

7. Evolution of populations
 Evolution = change in allele frequencies
in a population
 hypothetical: what conditions would
cause allele frequencies to not change?
 non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)

8. Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population
 preserves allele frequencies
 Serves as a model (null hypothesis)
 natural populations rarely in H-W equilibrium
 useful model to measure if forces are acting
on a population
 measuring evolutionary change
W. Weinberg
physician
G.H. Hardy
mathematician

9. Hardy-Weinberg theorem
 Counting Alleles
 assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q
 frequencies must add to 1 (100%), so:
p + q = 1
bb
Bb
BB

10. Hardy-Weinberg theorem
 Counting Individuals
 frequency of homozygous dominant: p x p = p2
 frequency of homozygous recessive: q x q = q2
 frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2
+ 2pq + q2
= 1
bb
Bb
BB

11. H-W formulas
 Alleles: p + q = 1
 Individuals: p2
+ 2pq + q2
= 1
bb
Bb
BB
BB
B b
Bb bb

12. What are the genotype frequencies?
Using Hardy-Weinberg equation
q2
(bb): 16/100 = .16
q (b): √.16 = 0.4
0.4
p (B): 1 - 0.4 = 0.6
0.6
population:
100 cats
84 black, 16 white
How many of each
genotype?
bb
Bb
BB
p2
=.36 2pq=.48 q2
=.16
Must assume population is in H-W
equilibrium!

13. Using Hardy-Weinberg equation
bb
Bb
BB
p2
=.36 2pq=.48 q2
=.16
Assuming
H-W equilibrium
Sampled data
bb
Bb
BB
p2
=.74 2pq=.10 q2
=.16
How do you
explain the data?
p2
=.20 2pq=.64 q2
=.16
How do you
explain the data?
Null hypothesis

14. Application of H-W principle
 Sickle cell anemia
 inherit a mutation in gene coding for
hemoglobin
 oxygen-carrying blood protein
 recessive allele = Hs
Hs
 normal allele = Hb
 low oxygen levels causes
RBC to sickle
 breakdown of RBC
 clogging small blood vessels
 damage to organs
 often lethal

15. Sickle cell frequency
 High frequency of heterozygotes
 1 in 5 in Central Africans = Hb
Hs
 unusual for allele with severe
detrimental effects in homozygotes
 1 in 100 = Hs
Hs
 usually die before reproductive age
Why is the Hs
allele maintained at such high
levels in African populations?
Suggests some selective advantage of
being heterozygous…

16. Malaria Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
1
2
3

17. Heterozygote Advantage
 In tropical Africa, where malaria is common:
 homozygous dominant (normal) die of malaria: Hb
Hb
 homozygous recessive die of sickle cell anemia: Hs
Hs
 heterozygote carriers are relatively free of both: Hb
Hs
 survive more, more common in population
Hypothesis:
In malaria-infected
cells, the O2 level is
lowered enough to
cause sickling which
kills the cell &
destroys the parasite.
Frequency of sickle cell allele &
distribution of malaria