Evolution of Populations
Chapter 23
Basics of Population Genetics
 Linked Mendelian genetics with Darwinian evolution.
 Population -- Group of organisms which belong to the
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same species and live in the same area.
Species -- Groups of interbreeding natural
populations.
Allele – A version of a particular gene.
Gene pool – All genes in a population at any one time;
usually two or more alleles for a gene, each having a
relative frequency in the gene pool.
Gene flow -- Movement of alleles between
populations.
Hardy-Weinberg
 Hardy-Weinberg equilibrium compares how common
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certain alleles are in natural populations whose gene
pools may be changing.
For Hardy-Weinberg equilibrium to be maintained, five
conditions must be met:
1. Very large population size. (No genetic drift)
2. Isolation from other populations. (No migration)
3. No mutations. (Changes only due to recombination
during reproduction)
4. Random mating. (Non-random can promote
inbreeding)
5. No natural selection. (Equal survival and
reproductive success)
H-W cont.
 In real populations, several
factors can upset Hardy-Weinberg
equilibrium and cause
microevolutionary change.
 Microevolution – a shift in a
population's allele frequencies; can
be caused by genetic drift, gene flow,
mutation, nonrandom mating, and
natural selection.
Genetic Drift
 Genetic drift -- Changes in the gene pool of a
small population due to chance.
 A. Bottleneck Effect: size of a population
reduced drastically by a natural disaster which
kills organisms non-selectively; reduces overall
genetic variability in a population.
 South African cheetahs -- the large population
was severely reduced during the last ice age
and again by hunting to near extinction.
Genetic Drift cont.
 B. Founder Effect -- When a few individuals
colonize a new habitat, the smaller the
founding population, the less likely its gene
pool will be representative of the original
population's genetic makeup.
 Tristan da Cunha islands colonized by 15
people in 1814; frequency retinitis pigmentosa
is much higher on this island than in the
populations from which the colonists came.
 Amish also have various recessive genetic
disorders; including anemia, dwarfism,
polydactyly, etc.
The Blue People
“I’m My Own Grandpaw”
 Many, many years ago when I was twenty-three
I was married to a widow who was pretty as could be.
This widow had a grown-up daughter who had hair of
red.
My father fell in love with her and soon they, too, were
wed.
This made my dad my son-in-law and changed my
very life
For my daughter was my mother, 'cause she was my
father's wife.
To complicate the matter, even though it brought me
joy
I soon became the father of a bouncing baby boy.
 My little baby then became a brother-in-law to dad
And so became my uncle, though it made me very sad
For if he was my uncle, then that also made him
brother
To the widow's grown-up daughter, who, of course,
was my step-mother.
 My father's wife then had a son who kept them on the
run
And he became my grand-child, 'cause he was my
daughter's son.
My wife is now my mother's mother, and it makes me
blue
Because, although she is my wife, she's my
grandmother too.
 If my wife is my grandmother, then I am her grandchild
And every time I think of it, it nearly drives me wild.
This has got to be the strangest thing I ever saw. As
husband of my grandmother, I am my own grandpaw.
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Chorus
I'm my own grandpaw
I'm my own grandpaw
It sounds funny I know
but it really is so
Oh, I'm my own grandpaw
Using the Hardy-Weinberg
Theorem
 Genotypes: AA, Aa, aa
 p = frequency of A allele.
 q = frequency of a allele.
 p + q = 1 in a population.(q = 1-p; p = 1-q)
 p2 = frequency of AA genotype.
 q2 = frequency of aa genotype.
 2pq = frequency of Aa genotype.
 p2 + 2pq + q2 = 1 in a population.
H-W continued
 Frequency of A is 0.3. What is freq of a?
 q = 1 – 0.3 = 0.7
 Freq of AA genotype?
 p2 = 0.3 x 0.3 = 0.09
 Freq of aa genotype?
 q2 = 0.7 x 0.7 = 0.49
 Freq of Aa genotype?
 2pq = 2 x 0.3 x 0.7 = 0.42
 So, 9% of population are AA, 49% are aa, and
42% are Aa.
More H-W
 In a population,16% of people show a
recessive trait. Determine the frequencies
of the 3 genotypes.
 aa = q2 = 0.16; so q = 0.4
 p = 1 – 0.4 = 0.6
 AA = p2 = 0.6 x 0.6 = 0.36 (or 36%)
 Aa = 2pq = 2 x 0.6 x 0.4 = 0.48 (or 48%)
Practice makes perfect…
 1 of every 10,000 babies in the United States
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is born with phenylketonuria (PKU). The allele
for PKU is recessive, so babies with this
disorder are homozygous recessive. What
percentage of the U.S. population are carriers
for PKU?
q2 = 0.0001, so q = 0.01.
p = 1 - 0.01 = 0.99
Carriers (heterozygotes) are 2pq.
2pq = 2(0.99)(0.01) = 0.0198 (or about 2%)
Thus, about 2% of the U.S. population are
carriers for PKU.
Maintenance of Variation
 Genetic variation results from mutation and sexual
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reproduction.
Although natural selection tends to produce genetic
uniformity, variation is preserved by:
1. Diploidy – hides recessive alleles in
heterozygotes; this maintains a large pool of alleles
which may be beneficial if conditions change.
2. Heterozygote advantage -- maintains two or more
alleles if heterozygous individuals have a greater
reproductive success.
• Example: Carriers of one allele for sickle cell
anemia are resistant to malaria; an advantage in
tropical areas where malaria is prevalent.
Malaria and Sickle Cell Allele
Maintenance of Variation cont.
 3. Outbreeding – plants mechanisms
discourage self-pollination; most animals
reproduce sexually; hermaphrodite rarely
self-fertilize.
 Hybrid vigor -- Crossbreeding different
inbred varieties often produces hybrids
which are more vigorous than the parent
stocks.
Fitness
 Darwinian fitness: contribution an individual
makes to the gene pool of the next generation
(survival and fecundity).
 Organisms may produce more progeny
because they are more efficient feeders, attract
more pollinators, avoid predators, mature
earlier, or simply live longer.
 Example, if pink flower plants (AA and Aa)
produce 20% more offspring than white flower
plants (aa), then AA and Aa genotypes have a
higher relative fitness = 1; relative fitness of
white flowers would be 0.8.
Types of Selection
 1. Stabilizing
Selection
 Favors intermediate
traits; selects against
extreme phenotypes.
 Best suited to relatively
stable environments.
 Ex: average human birth
weight 6 – 8 lbs; Much
smaller and much higher
birth weight babies have
a greater infant mortality.
Types of Selection cont.
 2. Directional
Selection
 Favors one extreme.
 Most common during
environmental
change.
 Ex: fossils of black
bears show increased
size after periods of
glaciation; decrease
during warmer
Types of Selection cont.
 3. Disruptive
Selection
 Opposite extremes
are favored.
 Variable
environmental
conditions.
 Ex: in a prey
population where
body color varies,
middle shades may
be poor camouflage.
Types of Selection cont.
 Sexual Selection
 Sexual dimorphism – Different
secondary sexual characteristics
in males and females.
 Differences in size, plumage,
lion's manes, deer antlers, sports
cars, etc.
 In some species, males use their
secondary sexual characteristics
to compete for female mates.
 If a male’s reproductive success is
increased, he contributes more to
the gene pool of the next
generation.