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Chapter 16
Evolution of Populations
Gene pool
– Sum of all the
different genes
present in a
population
Population
– All of the given
species in a
certain locale
Relative frequency: How frequent a
particular allele appears in the population is
called the relative frequency.
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
The number of phenotypes produced for
a given trait depends on how many genes
control the trait
• Single-gene trait (widow’s peak)
Polygenic traits (human height)
Typical distribution of polygenic traits
Bell curve: The medium phenotype has the
highest relative frequency.
Types of selection (p. 398-99)
• Directional
• eliminates one extreme
What can change the bell curve,
signaling population evolution?
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
• Stabilizing
• eliminates extremes at both ends
What can change the bell curve,
signaling population evolution?
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
• Disruptive
• eliminates intermediate type
What can change the bell curve,
signaling population evolution?
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic drift: random changes in allele
frequency within small populations
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
An Amish child with Ellis-van Creveld Syndrome (6 fingers
and short limbs); direct descendants of a single couple
who founded Lancaster County, Pennsylvania in 1744
What can maintain relative
frequencies and prevent evolution?
•
•
•
•
•
Random mating
Large population
No movement in or out of populations
No mutations
No natural selection
Can all 5 conditions ever be met?
This is called the Hardy-Weinberg Principle.
Hardy-Weinburg said that IF…
• Population is large
• Matings are random
• No mutation occurs
• No migration in or out of population
• No selection occurs
THEN
• No evolution occurs
Hardy-Weinburg theorem
• Provides a baseline model in which gene
frequencies do not change and evolution
does not occur.
• If A and a are alleles & each diploid
individual has 2, then p = frequency of A
and q = frequency of a
• So the frequency of AA, Aa, aa is…
p2 + 2pq + q2 = 1 (100% of the population)
Valid only if…
•
•
•
•
•
•
Population is large
Matings are random
No mutation occurs
No migration in or out of population
No selection occurs
By testing the H-W theorem, evolutionists
can investigate the allele frequency
changes in natural populations
Teddy Graham lab:
p2 + 2pq + q2 = 1 (100% of the population)
We can use math to help us
analyze the Hardy-Weinburg
theorem
• If you know the number of individuals that
are homozygous recessive, then you can
find what q2 is.
• convert that to a % by dividing the
recessive alleles by the total.
• then find the q number by taking the
square root of q2
• Remember: p2 + 2pq + q2 = 1 (100% of
the population)
LETS DO A SAMPLE PROBLEM
• Example: if 10 out of 20 individuals display
the homozygous recessive trait then
• 10/20 = .5 = q2
• Square root of q2= q
• 1-q=p
• p x p = p2
• 2 x p x q = 2pq
Follow these steps
If 10 out of 20
individuals display the
homozygous recessive
trait then
SAMPLE DATA
CALCULATION
10/20 = .5 = q2
Square root of q2= q
Square root of .5 =.7071
1-q=p
1-.7071=.2929
p x p = p2
.2929x.2929=.0857
2 x p x q = 2pq
2x.2929x.7071=.4142
Check the math
.0857+.4142+.5=.9999
• Look at the data you have collected and
describe what is happening to the allele
frequencies in the population of teddy
grahams
• What would you expect to happen if you
continued the selection process for
additional generations?
• How would the frequencies change if you
were to now select (eat) the sad bears?
• Why doesn’t the recessive allele disappear
from the population?
• How is the recessive allele protected from
complete elimination?
What selective pressures promote
speciation?
• Behavioral
isolation:
must have
similar
courtship
patterns
• This occurs when
two populations are
capable of
interbreeding but
have differences in
courtship rituals or
other reproductive
strategies that
involve behavior.
What selective pressures promote
speciation?
• Geographic isolation: physical separation, like
rivers, mountains, or bodies of water.
What selective pressures promote
speciation?
• Temporal isolation:
different breeding
seasons
• Both are spiny
reptiles, but the upper
(bunch grass spiny)
breeds only in the
spring. The Yarrow’s
spiny only breeds in
the fall.
Speciation
• Species
• Capable of
reproducing with each
other
• Changes in gene pool
produce changes in
species
• Changes result in
speciation =
evolution of new
species
Darwin’s finches
• Speciation from
geographic
isolation and
founder’s effect
leading to
breeding
incompatability