Evolution of
Populations
Question?
• Is the unit of evolution the
individual or the population?
• Answer = while evolution effects
individuals, it can only be tracked
through time by looking at
populations
So what do we study?
• We need to study populations, not
individuals
• We need a method to track the
changes in populations over time
• This is the area of Biology called
‘population genetics’
Population Genetics
• The study of genetic variation in
populations.
• Represents the reconciliation of
Mendelism and Darwinism.
• Modern Synthesis uses population
genetics as the means to track and
study evolution
• Looks at the genetic basis of
variation and natural selection
Population
• A localized group of individuals of
the same species.
Species
• A group of similar organisms.
• A group of populations that could
interbreed.
Gene Pool
• The total aggregate of genes in a
population.
• If evolution is occurring, then
changes must occur in the gene pool
of the population over time.
Microevolution
• Changes in the relative frequencies
of alleles in the gene pool.
Overview
• A common misconception is that
organisms evolve, in the Darwinian
sense, during their lifetimes
• Natural selection acts on individuals,
but only populations evolve
• Individuals are selected;
populations evolve!
The Smallest Unit of Evolution
• Genetic variations in populations
contribute to evolution
• Microevolution is a change in allele
frequencies in a population over
generations
Is this finch evolving by
natural selection?
Concept: Mutation and sexual
reproduction produce the genetic
variation that makes evolution
possible
• Two processes, mutation and sexual
reproduction, produce the variation
in gene pools that contributes to
differences among individuals
Genetic Variation
• Variation in individual genotype
leads to variation in individual
phenotype
• Not all phenotypic variation is
heritable
• Natural selection can only act on
variation with a genetic component
Nonheritable variation?
Nonheritable
variation?
Variation Within a Population
• Both discrete and quantitative
characters contribute to variation
within a population
• Discrete characters can be
classified on an either-or basis
• Quantitative characters vary along
a continuum within a population
• Population geneticists measure
polymorphisms in a population by
determining the amount of
heterozygosity at the gene and
molecular levels
• Average heterozygosity measures
the average percent of loci that are
heterozygous in a population
• Nucleotide variability is measured
by comparing the DNA sequences
of pairs of individuals
Variation Between Populations
• Most species exhibit geographic
variation, differences between
gene pools of separate populations
or population subgroups
Geographic variation in
isolated mouse
populations on Madeira
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
1.0
0.8
0.6
Maine
Cold (6°C)
0.4
Georgia
Warm
(21°C)
0.2
0
46
44
42
40
38
36
Latitude
(°N)
34
32
30
A cline determined by temperature
Mutation
• Mutations are changes in the
nucleotide sequence of DNA
• Mutations cause new genes and
alleles to arise
• Only mutations in cells that produce
gametes can be passed to offspring
Point mutations
• A point mutation is a change in one
base in a gene
• The effects of point mutations can
vary:
–Mutations in noncoding regions of
DNA are often harmless
–Mutations in a gene might not
affect protein production
because of redundancy in the
genetic code
Point mutations
• The effects of point mutations can
vary:
–Mutations that result in a change
in protein production are often
harmful
–Mutations that result in a change
in protein production can
sometimes increase the fit
between organism and
environment
Mutations That Alter Gene
Number or Sequence
• Chromosomal mutations that delete,
disrupt, or rearrange many loci are
typically harmful
• Duplication of large chromosome
segments is usually harmful
Mutations That Alter Gene
Number or Sequence
• Duplication of small pieces of DNA
is sometimes less harmful and
increases the genome size
• Duplicated genes can take on new
functions by further mutation
Hardy-Weinburg
Hardy-Weinberg Theorem
• Developed in 1908.
• Mathematical model of gene pool
changes over time.
• The frequency of an allele in a
population can be calculated
–For diploid organisms, the total
number of alleles at a locus is the
total number of individuals x 2
–The total number of dominant
alleles at a locus is 2 alleles for
each homozygous dominant
individual plus 1 allele for each
heterozygous individual; the same
logic applies for recessive alleles
• By convention, if there are 2 alleles
at a locus, p and q are used to
represent their frequencies
• The frequency of all alleles in a
population will add up to 1
–For example, p + q = 1
Basic Equation
•p + q = 1
• p = % dominant allele
• q = % recessive allele
Expanded Equation
• p+q=1
• (p + q)2 = (1)2
• p2 + 2pq + q2 = 1
Genotypes
• p2 = Homozygous Dominants
2pq = Heterozygous
q2 = Homozygous Recessives
• The Hardy-Weinberg principle
describes a population that is not
evolving
• If a population does not meet the
criteria of the Hardy-Weinberg
principle, it can be concluded that
the population is evolving
• The Hardy-Weinberg principle
states that frequencies of alleles
and genotypes in a population
remain constant from generation to
generation
• In a given population where
gametes contribute to the next
generation randomly, allele
frequencies will not change
• Mendelian inheritance preserves
genetic variation in a population
Fig. 23-6
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
80%
chance
20%
chance
Each sperm:
80%
chance
Selecting alleles at random from
a gene pool
20%
chance
• Hardy-Weinberg equilibrium
describes the constant frequency of
alleles in such a gene pool
• If p and q represent the relative
frequencies of the only two possible
alleles in a population at a particular
locus, then
p2 + 2pq + q2 = 1
• -where p2 and q2 represent the
frequencies of the homozygous
genotypes and 2pq represents the
frequency of the heterozygous
genotype
Conditions for Hardy-Weinberg
Equilibrium
• The Hardy-Weinberg theorem
describes a hypothetical population
• In real populations, allele and
genotype frequencies do change
over time
• Natural populations can evolve at
some loci, while being in HardyWeinberg equilibrium at other loci
• The five conditions for nonevolving
populations are rarely met in
nature:
–No mutations
–Random mating
–No natural selection
–Extremely large population size
–No gene flow
Example Calculation
• Let’s look at a population where:
– A = red flowers
– a = white flowers
Starting Population
•
•
•
•
N = 500
Red = 480 (320 AA+ 160 Aa)
White = 20
Total alleles = 2 x 500 = 1000
Dominant Allele
• A = (320 x 2) + (160 x 1)
= 800
= 800/1000
A = 80%
Recessive Allele
• a = (160 x 1) + (20 x 2)
= 200/1000
= .20
a = 20%
A and a in HW equation
• Cross: Aa X Aa
• Result = AA + 2Aa + aa
• Remember: A = p, a = q
Substitute the values for A and a
• p2 + 2pq + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32 + .04 = 1
Dominant Allele
• A = p2 + pq
= .64 + .16
= .80
= 80%
Recessive Allele
• a = pq + q2
= .16 + .04
= .20
= 20%
Importance of Hardy-Weinberg
• Yardstick to measure rates of
evolution.
• Predicts that gene frequencies
should NOT change over time as
long as the HW assumptions hold
(no evolution should occur).
• Way to calculate gene frequencies
through time.
Example
• What is the frequency of the PKU
allele?
• PKU is expressed only if the individual is
homozygous recessive (aa).
Applying the Hardy-Weinberg
Principle
• We can assume the locus that
causes phenylketonuria (PKU) is in
Hardy-Weinberg equilibrium given
that:
–The PKU gene mutation rate is low
–Mate selection is random with
respect to whether or not an
individual is a carrier for the PKU
allele
–Natural selection can only act on
rare homozygous individuals who
do not follow dietary restrictions
–The population is large
–Migration has no effect as many
other populations have similar
allele frequencies
• The occurrence of PKU is 1 per
10,000 births
q2 = 0.0001
q = 0.01
• The frequency of normal alleles is
p = 1 – q = 1 – 0.01 = 0.99
• The frequency of carriers is
2pq = 2 x 0.99 x 0.01 = 0.0198
–or approximately 2% of the U.S.
population
PKU Frequency
• PKU is found at the rate of 1/10,000
births.
• PKU = aa = q2
q2 = .0001
q = .01
Dominant Allele
• p+q=1
p = 1- q
p = 1- .01
p = .99
Expanded Equation
• p2 + 2pq + q2 = 1
(.99)2 + 2(.99x.01) + (.01)2 = 1
.9801 + .0198 + .0001 = 1
Final Results
• Normals (AA) = 98.01%
• Carriers (Aa) = 1.98%
• PKU (aa) = .01%
Result
• Gene pool is in a state of equilibrium and
has not changed because of sexual
reproduction.
• No Evolution has occurred.
AP Problems Using HardyWeinberg
•
•
•
•
Solve for q2 (% of total).
Solve for q (equation).
Solve for p (1- q).
H-W is always on the national AP Bio
exam (but no calculators are allowed).
Remember Hardy-Weinberg
Assumptions
1. Large Population
2. Isolation
3. No Net Mutations
4. Random Mating
5. No Natural Selection
If H-W assumptions hold true:
• The gene frequencies will not
change over time.
• Evolution will not occur.
• But, how likely will natural
populations hold to the H-W
assumptions?
Microevolution
• Caused by violations of the 5 H-W
assumptions.
Causes of Microevolution
1. Genetic Drift
2. Gene Flow
3. Mutations
4. Nonrandom Mating
5. Natural Selection
Genetic Drift
• Changes in the gene pool of a small
population by chance.
• Types:
–1. Bottleneck Effect
–2. Founder's Effect
• The smaller a sample, the greater
the chance of deviation from a
predicted result
Genetic Drift
• Genetic drift describes how allele
frequencies fluctuate unpredictably
from one generation to the next
• Genetic drift tends to reduce
genetic variation through losses of
alleles
Fig. 23-8-1
Genetic Drift
CR CR
CR CR
CR CW
CR CR
CW CW
CR CW
CR CR
CR CW
CR CR
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Fig. 23-8-2
Genetic Drift
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
Fig. 23-8-3
Genetic Drift
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CR
CR CW
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
Genetic Drift By Chance
Bottleneck Effect
Loss of most
of the
population by
disasters.
Bottleneck Effect
• The bottleneck effect is a sudden
reduction in population size due to a
disaster or a change in the
environment
• Surviving population may have a
different gene pool than the original
population, it may no longer be
reflective of the original population’s
gene pool
• If the population remains small, it may
be further affected by genetic drift
• Understanding the bottleneck effect
can increase understanding of how
human activity affects other species
Case Study: Impact of Genetic
Drift on the Greater Prairie
Chicken
• Loss of prairie habitat caused a
severe reduction in the population of
greater prairie chickens in Illinois
• The surviving birds had low levels of
genetic variation, and only 50% of
their eggs hatched
Fig. 23-10a
Prebottleneck
(Illinois, 1820)
(a)
Range
of greater
prairie
chicken
Postbottleneck
(Illinois, 1993)
Fig. 23-10b
Location
Population
size
Number
Percentage
of alleles of eggs
per locus hatched
Illinois
1000–25,000
5.2
93
<50
3.7
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Minnesota, 1998
(no bottleneck)
4,000
5.3
85
1930–1960s
1993
(b)
Prairie Chicken
• Researchers used DNA from museum
specimens to compare genetic variation in
the population before and after the
bottleneck
• The results showed a loss of alleles at
several loci
• Researchers introduced greater prairie
chickens from population in other states
and were successful in introducing new
alleles and increasing the egg hatch rate to
90%
Importance
• Reduction of population size may reduce
gene pool for evolution to work with.
• Ex: Cheetahs
Founder's Effect
• The founder effect occurs when a
few individuals become isolated
from a larger population
• Allele frequencies in the small
founder population can be different
from those in the larger parent
population
Founder's Effect
• Genetic drift in a new colony that
separates from a parent population.
• Examples:
• Old-Order Amish
• A butterfly that gets
blown to an island and
lays her eggs
Result of Bottleneck and
Founder’s effects
• Genetic variation reduced.
• Some alleles increase in frequency
while others are lost (as compared
to the parent population).
• Very common in islands and other
groups that don't interbreed
Effects of Genetic Drift: A
Summary
1. Genetic drift is significant in small
populations
2. Genetic drift causes allele
frequencies to change at random
3. Genetic drift can lead to a loss of
genetic variation within populations
4. Genetic drift can cause harmful
alleles to become fixed
Gene Flow
• Gene flow consists of the movement
of alleles among populations (in or out
of a population)
–Immigration
–Emigration
• Alleles can be transferred through
the movement of fertile individuals
or gametes (for example, pollen)
Gene Flow
• Gene flow tends to reduce
differences between populations over
time
• Gene flow is more likely than mutation
to alter allele frequencies directly
Fig. 23-11
• Gene flow can decrease the fitness
of a population
• In Bent grass, alleles for copper
tolerance are beneficial in
populations near copper mines, but
harmful to populations in other soils
• Windblown pollen moves these alleles
between populations
• The movement of unfavorable alleles
into a population results in a
decrease in fit between organism and
environment
Fig. 23-12a
7
NON0
MINE
60
SOIL
MINE
SOIL
50
NONMINE
SOIL
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
Distance from mine edge
(meters)
80
10
0
120
140
160
Fig. 23-12b
Bent grass
• Gene flow can increase the fitness of
a population
• 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
Result
• Changes in gene frequencies within a
population.
• Immigration often brings new alleles
into populations increasing genetic
diversity.
Mutations
• Inherited changes in a gene.
Result of mutations
• May change gene frequencies (small
population).
• Source of new alleles for selection.
• Often lost by genetic drift.
Nonrandom Mating
• Failure to choose mates at random
from the population.
• Inbreeding within the same
“neighborhood”.
• Assortative mating (like with like).
• Results in increases in the number
of homozygous loci.
• Does not in itself alter the overall
gene frequencies in the population.
Sexual Mate selection
• Sexual selection is natural selection
for mating success
• May not be adaptive to the
environment, but increases
reproduction success of the
individual.
Sexual Mate selection
• This is a VERY important selection
type for species.
• It can result in sexual dimorphism,
marked differences between the
sexes in secondary sexual
characteristics
Fig. 23-15
Sexual dimorphism
• Intrasexual selection is competition
among individuals of one sex (often
males) for mates of the opposite sex
• Intersexual selection, often called
mate choice, occurs when individuals
of one sex (usually females) are
choosy in selecting their mates
• Male showiness due to mate choice
can increase a male’s chances of
attracting a female, while decreasing
his chances of survival
• How do female preferences evolve?
• The good genes hypothesis suggests
that if a trait is related to male
health, both the male trait and
female preference for that trait
should be selected for
Fig. 23-16a
EXPERIMENT
Female gray
tree frog
SC male
gray
tree frog
LC male gray
tree frog
SC sperm
Eggs
LC sperm
Offspring of Offspring of
LC father
SC father
Fitness of these half-sibling offspring compared
Fig. 23-16b
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Result
• Sexual dimorphism.
• Secondary sexual features for
attracting mates.
Comment
• Females may drive sexual selection
and dimorphism since they often
"choose" the mate.
Natural Selection
• Differential success in survival and
reproduction.
• Differential success in reproduction
results in certain alleles being passed
to the next generation in greater
proportions
Natural Selection
• Only natural selection consistently
results in adaptive evolution
• Natural selection brings about adaptive
evolution by acting on an organism’s
phenotype
• Result - Shifts in gene frequencies.
Comment
• As the Environment changes, so does
Natural Selection and Gene Frequencies.
Result
• If the environment is "patchy", the
population may have many different
local populations.
The Key Role of Natural
Selection in Adaptive Evolution
• Natural selection increases the
frequencies of alleles that enhance
survival and reproduction
• Adaptive evolution occurs as the
match between an organism and its
environment increases
Fig. 23-14a
Color-changing ability in cuttlefish
Fig. 23-14b
Movable bones
Movable jaw
bones in
snakes
• Because the environment can change,
adaptive evolution is a continuous
process
• Genetic drift and gene flow do not
consistently lead to adaptive
evolution as they can increase or
decrease the match between an
organism and its environment
Genetic Basis of Variation
1. Discrete Characters – Mendelian
traits with clear phenotypes.
2. Quantitative Characters –
Multigene traits with overlapping
phenotypes.
Polymorphism
• The existence of several
contrasting forms of the species in
a population.
• Usually inherited as Discrete
Characteristics.
Examples of Polymorphism
Garter Snakes
Gaillardia
Human Example
• ABO Blood Groups
• Morphs = A, B, AB, O
Other examples
Quantitative Characters
• Allow continuous variation in the
population.
• Result –
–Geographical Variation
–Clines: a change along a
geographical axis
Yarrow and Altitude
Sources of Genetic Variation
• Mutations.
• Recombination though sexual
reproduction.
–Crossing-over
–Random fertilization
The Preservation of Genetic
Variation
• Various mechanisms help to preserve
genetic variation in a population
1. Diploidy - preserves recessives as
heterozygotes.
2. Balanced Polymorphisms preservation of diversity by natural
selection.
Diploidy
• Diploidy maintains genetic variation
in the form of hidden recessive
alleles
Heterozygote Advantage
• Heterozygote Advantage - When the
heterozygote or hybrid survives
better (have a higher fitness) than
the homozygotes. Also called Hybrid
vigor.
• Natural selection will tend to
maintain two or more alleles at that
locus
Heterozygote Advantage
• Can't bred "true“ and the diversity of the
population is maintained.
• Example of hybrid vigor – Sickle Cell
Anemia
• The sickle-cell allele causes mutations in
hemoglobin but also confers malaria
resistance
Fig. 23-17
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%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent
Selection
• In frequency-dependent selection,
the fitness of a phenotype declines
if it becomes too common in the
population
• Selection can favor whichever
phenotype is less common in a
population
Fig. 23-18a
“Right-mouthed”
“Left-mouthed”
Fig. 23-18
“Right-mouthed”
1.0
“Left-mouthed”
0.
5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Comment
• Population geneticists believe that
ALL genes that persist in a
population must have had a
selective advantage at one time.
• Ex – Sickle Cell and Malaria, TaySachs and Tuberculosis
Fitness - Darwinian
• The relative contribution an
individual makes to the gene pool of
the next generation.
Relative Fitness
• Relative fitness is the contribution
an individual makes to the gene pool
of the next generation, relative to
the contributions of other individuals
• Selection favors certain genotypes
by acting on the phenotypes of
certain organisms
Relative Fitness
• Contribution of one genotype to the
next generation compared to other
genotypes.
• The phrases “struggle for existence”
and “survival of the fittest” are
misleading as they imply direct
competition among individuals
• Reproductive success is generally
more subtle and depends on many
factors
Rate of Selection
• Differs between dominant and
recessive alleles.
• Selection pressure by the
environment.
Modes of Selection
• Three modes of selection:
–Directional selection favors
individuals at one end of the
phenotypic range
–Disruptive selection favors
individuals at both extremes of
the phenotypic range
–Stabilizing selection favors
intermediate variants and acts
against extreme phenotypes
Stabilizing
• Selection toward the average and
against the extremes.
• Ex: birth weight in humans
Directional Selection
• Selection toward one extreme.
• Ex: running speeds in race animals.
• Ex. Galapagos Finch beak size and
food source.
Diversifying
• Selection toward both extremes
and against the norm.
• Ex: bill size in birds
Comment
• Diversifying Selection - can split a
species into several new species if
it continues for a long enough
period of time and the populations
don’t interbreed.
Fig. 23-13
Original population
Original
population
Evolved
population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive
selection
(c) Stabilizing
selection
Balancing Selection
• Balancing selection occurs when
natural selection maintains stable
frequencies of two or more
phenotypic forms in a population
Neutral Variation
• Neutral variation is genetic
variation that appears to confer no
selective advantage or disadvantage
• For example,
–Variation in noncoding regions of
DNA
–Variation in proteins that have
little effect on protein function or
reproductive fitness
Why Natural Selection Cannot
Fashion Perfect Organisms
1. Selection can act only on existing
variations
2. Evolution is limited by historical
constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the
environment interact
Fig. 23-19
Question
• Does evolution result in perfect
organisms?
Answer - No
1. Historical Constraints
2. Compromises
3. Non-adaptive Evolution (chance)
4. Available variations – most come
from using a current gene in a
new way.
Summary
• Know the difference between a
species and a population.
• Know that the unit of evolution is
the population and not the
individual.
Summary
• Know the H-W equations and how to
use them in calculations.
• Know the H-W assumptions and
what happens if each is violated.
Summary
• Identify various means to introduce
genetic variation into populations.
• Know the various types of natural
selection.
You should now be able to:
1. Explain why the majority of point
mutations are harmless
2. Explain how sexual recombination
generates genetic variability
3. Define the terms population,
species, gene pool, relative fitness,
and neutral variation
4. List the five conditions of HardyWeinberg equilibrium
5. Apply the Hardy-Weinberg equation
to a population genetics problem
6. Explain why natural selection is the
only mechanism that consistently
produces adaptive change
7. Explain the role of population size in
genetic drift
8. Distinguish among the following sets
of terms: directional, disruptive,
and stabilizing selection; intrasexual
and intersexual selection
9. List four reasons why natural
selection cannot produce perfect
organisms
End of Chapter 23!