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Measuring
Evolution of Populations
AP Biology
2010-2011
5 Agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Chemical
Changes to DNA
Migration
Sexual Selection
AP Biology
Genetic Drift
Selection
Small population
Natural Selection
Differential Survival
Hardy-Weinberg equilibrium
 Hypothetical situation


serves as null hypothesis
non-evolving population
REMOVE all agents of evolutionary change
1. no genetic drift (very large population size )
2. no gene flow (no migration in or out)
3. no mutation (no chemical change to DNA)
4. random mating (no sexual selection)
5. no natural selection (equal survival)
AP Biology
The Hardy-Weinberg Formula
 The Hardy-Weinberg formula can be used
to determine if a population is in genetic
equilibrium
p2(AA) + 2pq (Aa) +q2(aa) = 1.0
 The frequency of the dominant allele (A)
plus the recessive allele (a) equals 1.0
p + q = 1.0
AP Biology
A Population in Equilibrium
AP Biology
If 98 out of 200 individuals in a
population express the recessive
phenotype, what percent of the
population would you predict would be
heterozygotes?
Q2 = 98/200 = .49
q = square root .49 = .7
p = .3 (p + q = 1 so p + .7 = 1)
2pq = 2 x .7 x .3 = .42 = 42%
AP Biology
Directional Selection
 Directional selection
Changing
environmental
conditions can shift
allele frequencies in
a consistent
direction
 Forms of traits at
one end of a range
of phenotypic
variation become
more common

AP Biology
Predation and Rock-Pocket Mice
 In rock-pocket mice, two
alleles of a single gene
control coat color
 Night-flying owls are the
selective pressure that
directionally shifts the allele
frequency
AP Biology
Stabilizing Selection:
Body Weight of Sociable Weavers

Stabilizing selection
 Natural selection that
favors an intermediate
phenotype and
eliminates extreme
forms
AP Biology
Disruptive Selection

Disruptive selection
 Natural selection that
favors extreme forms of a
trait and eliminates the
intermediate forms
AP Biology
Example of strong selection pressure
 Tay Sachs
primarily in Ashkenazi Jews & Cajuns
 recessive disease = aa
 lysosomal storage disease

 lack of one functional digestive enzyme in
lysosome
 build up undigested fat in
brain cells
 children die before they
are 5 years old
So where do new cases of
come from?
APTay-Sachs
Biology
Sexual Selection
 Sexual selection acts on an organism's
ability to obtain or successfully copulate
with a mate
 Sexual selection is often powerful enough
to produce features that are harmful to the
individual's survival.
 Peacock Video - 11 minutes
AP Biology
Example of heterozygote advantage
 Sickle cell anemia

inherit a mutation in gene coding for one
of the subunits in hemoglobin





oxygen-carrying blood protein
normal allele = Hb
mutant allele = Hs
recessive trait = HsHs
low oxygen levels causes
RBC to sickle
 clogging small blood vessels
 damage to organs

AP Biology
often lethal
Sickle cell frequency
 High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
 unusual for allele with severe
detrimental effects in homozygotes

 1 in 100 = HsHs
 usually die before reproductive age
Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of
bHs
being
heterozygous…
H
AP Biology
Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
Malaria
1
liver
2
AP Biology
3
Heterozygote Advantage
 In tropical Africa, where malaria is common:

homozygous dominant (normal)
 reduced survival or reproduction from malaria: HbHb

homozygous recessive
 reduced survival & reproduction from sickle cell anemia: HsHs

heterozygote carriers
 survival & reproductive advantage: HbHs
Hypothesis:
In malaria-infected
cells, the O2 level is
lowered enough to
cause sickling which
kills the cell & destroys
the
parasite.
AP Biology
Frequency of sickle cell allele
& distribution of malaria