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11/24/14
BIOSC 041
The Evolution of Populations
Reference: Chapter 23
Microevolution & Macroevolution
v  Microevolution
§ 
§ 
Small evolutionary changes within a species or population
of a species
Associated with adaptation
v  Macroevolution
§ 
§ 
Larger evolutionary changes brought about by the
accumulation of microevolutionary changes
E.g. one species evolving into another (speciation)
Genetic Variation Between Populations
v  Most
species exhibit geographic variation,
differences between gene pools of separate
populations
v  For example, Madeira is home to several isolated
populations of mice
§ 
§ 
All populations share roughly the same environment
Chromosomal variation between populations is due
to drift, not natural selection
Overview: The Population – the Smallest
Unit of Evolution
v  One
misconception is that organisms evolve during
their lifetimes
v  Natural selection acts on individuals, but only
populations evolve
v  Population- a group of individuals of the same
species living in the same area at the same time
Genetic Variation Within a Population
v  Both
discrete and quantitative characters
contribute to variation within a population
v  Discrete characters can be classified on an eitheror basis
v  Quantitative characters vary along a continuum
within a population
Clines
v  Some
examples of genetic variation occur as a
cline, which is a graded change in a trait along a
geographic axis
v  For example, mummichog fish vary in a coldadaptive allele along a temperature gradient- this
variation results from natural selection
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Sources of Genetic Variation
Ldh-Bb allele frequency
1.0
0.8
v  New
genes and alleles can arise by mutation or
gene duplication
v  Sexual reproduction
0.6
0.4
0.2
0
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Maine
Cold (6°C)
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Latitude (ºN)
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32
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Georgia
Warm (21ºC)
Genetic Variation in Populations
Sources of Genetic Variation
v  Individual
v  Mutations
variation abounds in populations
– 
Not all of this variation is heritable
– 
Only the genetic component of variation is
relevant to natural selection
Darwinian Fitness
– 
The contribution one individual makes to the gene
pool (offspring) relative to the contributions of
other individuals
– 
Selection favors certain genotypes by selecting for
advantageous phenotypes
§ 
§ 
and sexual recombination
Mutations are changes in the DNA of an organism
Sexual recombination shuffles alleles during
meiosis
Microevolution
v  Occurs
when the frequencies of alleles change
within a population
v  A gene pool consists of all the alleles for all genes
in a population
v  A gene (locus) is fixed if all individuals in a
population are homozygous for the same allele (e.g.
all “PP”)
v  Alleles that are not fixed occur in relative
proportions, or frequencies, within the population
and can change in response to many factors
§ 
§ 
§ 
Natural selection
Genetic drift
Gene flow
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Allele Frequencies
v  For
diploid organisms, the total number of alleles at a
locus is the total number of individuals times 2
v  The total number of dominant alleles at a locus is 2
alleles for each homozygous dominant individual plus 1
allele for each heterozygous individual
v  The total number of recessive alleles at a locus is 2
alleles for each homozygous recessive individual plus 1
allele for each heterozygous individual
v  It all adds up to 1…
The Allele Frequency
v  By
convention, if there are 2 alleles at a locus, p
(dominant) and q (recessive) are used to represent
their frequencies
v  The frequency of all alleles for a given trait in a
population will add up to 1
p+q=1
§ 
Calculating Allele Frequencies
Calculating Allele Frequencies
v  For
v  To
example, a population of wildflowers that is
incompletely dominant for color:
§ 
§ 
§ 
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
v  Calculate
§ 
§ 
§ 
calculate the frequency of each allele:
§ 
§ 
v  The
§ 
the number of copies of each allele:
p = freq CR = 800 / (800 + 200) = 0.8
q = freq CW = 200 / (800 + 200) = 0.2
sum of alleles is always 1
0.8 + 0.2 = 1
CR = (320 × 2) + 160 = 800
CW = (20 × 2) + 160 = 200
Total alleles for color in the population = 1,000
The Hardy-Weinberg Principle
Conditions for Hardy-Weinberg Equilibrium
v  The
v  The
Hardy-Weinberg principle describes a population
that is not evolving
§ 
§ 
v  If
Frequencies of alleles and genotypes in a population
remain constant from generation to generation
Gametes contribute to the next generation randomly and
allele frequencies will not change
a population does not meet the criteria of the
Hardy-Weinberg principle, it can be concluded that
the population is evolving
Hardy-Weinberg principle describes a
hypothetical population that is not evolving
v  In real populations, allele and genotype frequencies
do change over time
v  The five conditions for non-evolving populations are
rarely met in nature:
1. 
2. 
3. 
4. 
5. 
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow (immigration of new individuals)
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Constant Allele Frequencies
equilibrium describes the constant
frequency of alleles in such a gene pool
v  Consider, for example, the same population of 500
wildflowers and 1,000 alleles where
The Hardy-Weinberg Equilibrium
v  Hardy-Weinberg
§ 
§ 
CR
p = freq
= 0.8
q = freq CW = 0.2
Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
80% CR (p = 0.8)
20% CW (q = 0.2)
Constant Allele Frequencies
Sperm
CW (20%)
CR (80%)
v  The
§ 
§ 
§ 
frequency of genotypes can be calculated
CRCR = p2 = (0.8)2 = 0.64 (64% homozygous dominant)
CRCW = 2pq = 2(0.8)(0.2) = 0.32 (32% heterozygous)
CWCW = q2 = (0.2)2 = 0.04 (4% homozygous recessive)
CR
(80%)
64% (p2)
C RC R
Eggs
CW
(20%)
v  The
frequency of genotypes can be confirmed using
a Punnett square
16% (pq)
C RC W
16% (qp)
C RC W
64%
C RC R,
32%
CRCW,
and 4%
4% (q2)
CWCW
CWCW
Gametes of this generation:
R
64% CR
+ 16% C R W
= 80% CR = 0.8 = p
(from CRCR plants)
(from C C plants)
W
4% CW
+ 16% C R W
= 20% CW = 0.2 = q
(from CWCW plants)
(from C C plants)
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
Hardy-Weinberg Equation
Important Note
v  Each
v  Natural
member of the population must have one of
three genotypes. Thus,
populations can evolve at some loci, while
being in Hardy-Weinberg equilibrium at other loci
p2 + 2pq + q2 = 1
p2 is the frequency of homozygous dominant genotypes
q2 is the frequency of homozygous recessive genotypes
2pq is the frequency of heterozygotes
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Applying the Hardy-Weinberg Principle
PKU Allele Frequency
v  We
v  The
can assume the locus that causes phenylketonuria
(PKU) is in Hardy-Weinberg equilibrium given that:
occurrence of PKU (homozygous recessive
disorder) is 1 per 10,000 births
1. 
The PKU gene mutation rate is low
§ 
q2 = 0.0001
2. 
Mate selection is random with respect to whether or not
an individual is a carrier for the PKU allele
§ 
q = 0.01
3. 
Natural selection can only act on rare homozygous
individuals who do not follow dietary restrictions
4. 
The population is large
5. 
Migration has no effect as many other populations have
similar allele frequencies
What about genes with more than two
alleles?
v  The
v  The
v  (p
v  p2
+q+
which expands to...
+ 2pq + q2 + 2pr + 2qr + r2 = 1.0
frequency of carriers is
§ 
2pq = 2 × 0.99 × 0.01 = 0.0198
§ 
or approximately 2% of the U.S. population
Mechanisms of microevolution
Genetic drift: fixation of random changes in gene pool
Migration: movement of genes between gene pools
Mutations: cumulative changes in DNA
Natural Selection
§ 
Selection pressure from the environment
§ 
Sexual selection – nonrandom mating
1. 
r)2
frequency of dominant alleles is
p = 1 – q = 1 – 0.01 = 0.99
§ 
2. 
3. 
4. 
Mechanism 1. Genetic Drift
v 
v 
v 
v 
A change in the gene pool of a small population due to
chance
a.  Bottlenecks
b.  Founder effects
The smaller a sample, the greater the chance of deviation
from a predicted result
Genetic drift describes how allele frequencies fluctuate
unpredictably from one generation to the next in small
populations
Genetic drift tends to reduce genetic variation through loss
of alleles
C RC R
C RC R
C RC W
CWCW
C RC R
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plants
leave
offspring
CWCW
C RC R
CWCW
C RC R
C RC W
C RC W
C RC R
C RC R
C RC W
C RC W
C RC W
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
C RC W
2
plants
leave
offspring
C RC R
C RC R
C RC R
C RC R
C RC R
C RC R
C RC R
C RC W
Generation 2
p = 0.5
q = 0.5
C RC R
C RC R
C RC R
C RC R
Generation 3
p = 1.0
q = 0.0
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Genetic Drift a. The Bottleneck Effect
v 
The bottleneck
effect results
from a drastic
reduction in
population size
due to a change
in the
environment
The Bottleneck Effect
v  The
resulting gene pool may no longer be reflective
of the original population’s gene pool
v  If the population remains small, it may be further
affected by genetic drift
The Bottleneck Effect
v 
Example: northern elephant seal
–  Hunted almost to extinction in 1800s,
elephant seals were reduced to only 20
individuals by the 1890s
–  A hunting ban allowed the population
to increase to 30,000
–  Biochemical analysis shows presentday northern elephant seals are almost
genetically identical
The gene pool of a
population contains
equal numbers of
red, blue, yellow, and
green alleles
A bottleneck event
drastically reduces the
size of the population
Simulation of a population bottleneck
By chance, the gene
pool of the reduced
population contains
mostly blue and a
few yellow alleles
After the population
grows and returns to
its original size, blue
alleles predominate;
red and green alleles
have disappeared
–  Despite their population size, their
lack of genetic variation leaves them
little flexibility to evolve if
environmental circumstances change
Elephant seals
Genetic Drift b. The Founder Effect
Example: The Founder Effect
v  A
v  Ellis-van-Creveld
few individuals start a new population
syndrome in the Amish
Autosomal recessive trait
v  Amish founder had the syndrome in 18th century
v  Persists today because of reproductive isolation
§ 
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Effects of Genetic Drift: A Summary
Mechanism 2. Migration = Gene Flow
v  Genetic
– 
Genetic exchange with
another population
– 
Tends to reduce genetic
differences between
populations
v  Genetic
drift is significant in small populations
drift causes allele frequencies to change at
random
v  Genetic
drift can lead to a loss of genetic variation
within populations
v  Genetic drift can cause harmful alleles to become
fixed
Gene Flow
Gene Flow can Decrease Fitness
v  Gene
v  Gene
flow consists of the movement of alleles
among populations
v  Alleles can be transferred through the movement of
fertile individuals or gametes (for example, pollen)
v  Gene flow tends to reduce variation among
populations over time
flow can decrease the fitness of a population
for example, a bird population on the
Dutch island of Vlieland
v  Consider,
§ 
§ 
§ 
Mating causes gene flow between central and
eastern populations
Immigration from the mainland introduces alleles
that decrease fitness
Natural selection selects for alleles that increase
fitness
Gene Flow can Increase Fitness
Mechanism 3. Mutations
v  Gene
v  Changes
v  The
v  Isolated
§ 
§ 
§ 
flow can increase the fitness of a population
spread of alleles for resistance to insecticides
Insecticides have been used to target mosquitoes
that carry West Nile virus and malaria
Alleles have evolved in some populations that
confer insecticide resistance to these mosquitoes
The flow of insecticide resistance alleles into a
population can cause an increase in fitness
in an organism’s DNA
mutations do not have much effect
on a large population
v  Over time, cumulative mutations can have
significant effects on a population
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Mechanism 4. Natural Selection
v 
Evolution occurs when natural selection changes the
frequency of one or more alleles in a population
Phenotypes interact with the environment
v 
Three general outcomes of selection
v 
§ 
a. 
b. 
c. 
The most successful produce the most offspring
Directional: selects for extreme phenotype
Disruptive: selects for two or more phenotypes
Stabilizing: selects for narrow “average” phenotype
Directional Selection
• 
For single-gene traits,
leads to fixation of an
allele
• 
For quantitative traits,
acts to eliminate one
extreme from an array of
phenotypes
a. Heterozygote Advantage
v  In
3 Outcomes of Natural Selection
some cases, heterozygotes have a higher fitness
than either homozygote
v  Natural selection will tend to maintain two or more
alleles at that locus
Balancing Selection
v  Natural
selection maintains stable frequencies of
two or more alleles for a particular trait in a
population (single-gene traits)
v  Balancing selection includes
a) 
b) 
Heterozygote advantage
Frequency-dependent selection
Sickle-cell disease & Heterozygote Advantage
v  Sickle-cell
disease
–  Affects about one
out of every 500
African Americans
–  Why is the allele still
in the population?
–  Heterozygotes have
a higher resistance
to diarrhea- inducing
diseases like cholera
and malaria
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b. Frequency-Dependent Selection
v  In
frequency-dependent selection, the fitness of a
phenotype declines if it becomes too common in
the population
v  Selection can favor whichever phenotype is less
common in a population
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
§ 
“Advantage when rare”
v  For
example, frequency-dependent selection
selects for approximately equal numbers of “rightmouthed” and “left-mouthed” scale-eating fish
7.5–10.0%
10.0–12.5%
>12.5%
“Left-mouthed”
P. microlepis
v  Applies
Frequency of
“left-mouthed” individuals
1.0
“Right-mouthed”
P. microlepis
to quantitative
traits
v 
Equivalent to balancing
selection for a single-gene
trait)
v  Acts
to eliminate both
extremes from an array
of phenotypes
0.5
0
Stabilizing Selection
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Disruptive Selection
v  Applies
Directional, disruptive, or stabilizing?
to quantitative
traits
v  Can
lead to a balance
between two or more
contrasting morphs in
a population
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Directional, disruptive, or stabilizing?
Neutral Variation
v  Neutral
variation is genetic variation that does not
confer a selective advantage or disadvantage
v  Various mechanisms help to preserve neutral
variation in a population
§ 
§ 
Diploidy maintains genetic variation in the form of hidden
recessive alleles
Heterozygotes can carry recessive alleles that are hidden
from the effects of selection
The Special Case of Sexual Selection
v  Sexual
selection is natural
selection for mating success
v  Individuals select mates
§  Peacock’s tails
§  Big-brained humans (?)
v  Can
result in sexual
dimorphism, marked
differences between the
sexes in secondary sexual
characteristics
Sexual Selection
Sexual Selection
v  Intrasexual
v  How
selection is competition among
individuals of one sex (often males) for mates of the
opposite sex
v  Intersexual selection, often called mate choice,
occurs when individuals of one sex (usually females)
are choosy in selecting their mates
v  Male showiness due to mate choice can increase a
male’s chances of attracting a female, up to the
point of decreasing his chances of survival
do female preferences evolve?
good genes hypothesis suggests that if a trait is
related to male health, both the male trait and
female preference for that trait should increase in
frequency
v  The
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The Key Role of Natural Selection in
Adaptive Evolution
Adaptive Evolution
v  Natural
v  Striking
adaptations have
arisen by natural
selection
§ 
§ 
Octopus can change
color rapidly for
camouflage
Jaws of snakes allow
them to swallow prey
larger than their heads
Bones shown in
green are movable
Ligament
selection increases the frequencies of
alleles that enhance survival and reproduction
v  Adaptive evolution occurs as the match between an
organism and its environment increases
v  Because the environment can change, adaptive
evolution is a continuous process
v  Genetic drift and gene flow do not consistently lead
to adaptive evolution as they can increase or
decrease fitness, the match between an organism
and its environment
Why Natural Selection Cannot Fashion
Perfect Organisms
1. 
2. 
3. 
4. 
Selection can act only on existing phenotypes
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment
interact
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