The Evolution of Population: The Mechanisms of Microevolution
I. Evolution (What actually changes?)
A. Gene frequency: the amount of a particular allele as found in the gene pool
B. Gene Pool:
sum total of all the alleles in a population
Gene Pool A
Gene Pool C
Gene Pool B
Microevolution: The change in gene frequency in a population over time
Gene Pool A 100 Organisms
Phenotype
Dark
Genotype
DD
Gene Pool B 100 Organisms
Phenotype
#
Dark
25
Genotype
DD
#
10
Time
Medium
Dd
50
Medium
Light
dd
25
Light
GF of D = .5
GF of d = .5
Dd
30
dd
60
GF of D = .25 GF of d = .75
allele
Gene Frequency =
total alleles
(25 x 2) + 50
Gene Frequency D =
200
Gene Frequency d =
(25 x 2) + 50
200
(10 x 2) + 30
= .25
= .5 Gene Frequency D =
200
(60 x 2) + 30
= .5 Gene Frequency d =
Slide 4
= .75
200
The Hardy – Weinberg Theory
II. The Summary
A. States that gene frequencies will not change in a population only due to
sexual reproduction (Skip to III. Math Theory)
B. Gene frequencies will not change (stay in Hardy-Weinberg equilibrium)
unless one or more of the following is taking place:
1. Genetic drift (and/or small population size)
2. Migration of genes from other populations
3. Mutation
4. Selective mating
5. Natural selection
Implication:
The Hardy-Weinberg mathematically proves that microevolution will not take
place unless one or more of the above is occurring
IV. The Equations
Equation #2
Equation #1
Assumption: Genes in gene
pool interact independently
Assumption: Genes found as pairs
and interact within organism
2
p+ q = 1
2
p + 2pq + q = 1
2
p = gene frequency of dominant allele
p = % homozygous dominant
q = gene frequency of recessive allele
q = % homozygous recessive
1 = total genes in gene pool (100%)
2pq = % heterozygous
2
1 = 100% of individuals
Taste Lab and Application Worksheet
V. Concepts Relating to the 5 Hardy-Weinberg Conditions
1. Genetic drift (and/or small population size)
2. Migration of genes from other populations
A. Large Populations vs Small Populations
3. Mutation
(Genetic Drift)
5. Natural selection
4. Selective mating
1. Small populations are more susceptible to “genetic drift”
(random events that change gene frequencies due to sample size)
a) Bottleneck effect
A random “bottlenecking event” reduces the population
number that result in new gene frequencies
b) Founder effect
When a new habitat is colonized, the genotypes of the original
colonist will influence the gene frequencies as the population
grows
B. Migration (Gene Flow)
1. Tends to reduce differences between populations
2. Extensive gene flow will cause 2 populations
to interact as 1 population
C. Mutations
1. Can immediately affect gene pool by substituting one
allele for another
2. Rare; usually harmful
3. Becomes the source of new variations
D. Natural Selection interactions on genetic variations (Phenotypes)
1. Types of genetic variations
a) Polymorphism (morphs)
Two or more phenotypes found in a population. Allows for
natural selection to “pick” the most fit
b) Geographic Variation
Regional differences in gene frequencies in isolated
populations. Example: Island mice
c) Clines
Graded variations within a population along a geographic axis.
Environmental gradient may lead to genetic variations
Example: Yarrow
d) Balanced Polymorphism
Natural selection stabilizes gene frequencies of 2 or more phenotypes
1) Heterozygous Advantage
Natural selection selects against the homozygous dominant
and homozygous recessive. Example Sickle cell anemia
2) Frequency Dependent Selection
The Survival and reproduction of a particular
phenotype declines as the phenotype becomes more
common. Example: parasite/host relationships
3) Neutral Variation
Variations of no apparent selective advantage.
Most variations are probably neutral
2. What is Meant by “Fitness”
a) Darwinian Fitness
The contribution an individual makes to the gene pool of
the next generation as compared to other individuals
A “super” phenotype in a sterile organism has no fitness value
Evolutionary impact of a gene is only measured by the
continued success of offspring
b) Relative Fitness
A quantitative value that compares the Darwinian fitness of
phenotypes found in a population Example Frogs (page 1)
3. Patterns (Modes) of Natural Selection
# of
mice
Phenotypes
Lightest
Directional Selection
Selection favors one extreme
Darkest
Diversifying Selection
Selection favors both extremes
Stabilizing Selection
Selection favors heterozygous
“Heterozygous advantage”
E. Non-Random Mating
1. The disadvantage of sexual reproduction
a. Asexual reproduction produces more offspring more efficiently (less energy)
then sexual reproduction. Is sexual reproduction an unfit phenotype?
Males do not directly produce offspring. Is maleness an
unfit phenotype?
2. Sexual Dimorphism
Phenotypic differences between males and females resulting from
non-random mating. Example: peacocks and peahens
a. Intrasexual Selection
“within the same sex”
Males compete with their own sex for mates. Males
defeat other males for possession of females.
b. Intersexual Selection
“mate choice”
One sex (female) chooses over individuals of the other sex
Examples
Although grebes compete for mates why are the males and females so similar?
III. The Math Theory
1. What possible genotypes can results with
two random frogs producing offspring from this
gene pool?
Gene Pool A
D
d
d
d
D
D
d
D
d
D
d
Frog #1 Chance of picking D = .5
D
D
D
D
d
d
Chance Of picking d = .5
d
D
d
Frog #2 Chance of picking D = .5
Chance Of picking d = .5
Gene Frequency D = .5
Gene Frequency d = .5
Probability of genotypes of Possible Offspring
.5
As sexual reproduction takes
place over time, will the gene
frequencies ever change?
NO!
.5
.5
But… Gene Frequencies do change…. WHY?
.5
DD
Dd
.25
.25
Dd
dd
.25
.25
Slide 3
Genetic Drift due to Small Population Size
Slide 6
An Example of a Bottleneck Event
“earthquake hits flower island”
Gene Frequency
p=1
Gene Frequency
Gene
p = .7Frequency
q = .3
p =.83
q = .17
q=0
Slide 5
Flower Island “The Sequel”
Founder Effect
Flower Island
Guano Island
Colonization
Gene Frequency
P = .7
q = .3
Gene Frequency
P = .5
q = .5
Slide 6
Morphological Differences in Yarrow at Different Altitudes
Slide 7
Density Dependent Selection
Slide 7
Slide 8
Polymorphic Expressions of the Common Garter Snake
Slide 6
Normal cells
Sickle cells
The distribution of
the sickle cell gene
and the distribution
of malaria parasite
Slide 7
1. Which is the most fit gene?
“d” is the most fit
2. Which is the most “fit” phenotype?
Light colored frog
Gene Pool B 100 Organisms
Phenotype
Dark
Medium
3. What is the “relative fitness” of the light
Light
colored phenotype?
Most abundant phenotype is always
set at a relative fitness of “1”
Genotype
DD
#
10
Dd
30
dd
60
GF of d = .75 GF of D = .25
4. What is the relative fitness of the
medium color phenotype?
The medium colored frogs produce
½ the amount of surviving offspring
( 30/60) so its relative fitness is .5
5. What is the relative fitness of the dark phenotype?
The dark phenotype produces 1/6 the amount of surviving
offspring (10/60). Its relative fitness is .17
Slide 9
Directional Selection
Thicker beaks in the dry years
are more common due to the
abundance of hard seeds and
the lack of soft seeds
Slide 9
Black Bellied Seed Crackers
Smaller beaks feed
on soft seeds best
Medium beaked birds have a
hard time feeding on either
hard or soft seed
Slide 9
Larger beaks feed on
hard seeds best
The “Unfit” Nature of Sexual Reproduction
Male
Male
Male
Many offspring;
more fitness
Fewer offspring;
less fitness
Slide 10
Slide 10
Intrasexual selection or Intersexual selection?
Intrasexual
Intersexual
Intersexual
Intrasexual
Slide 10