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Evolution of Populations
Microevolution
Variations in Gene Pools
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Microevolution: evolution on the smallest
scale – generation to generation change in
the frequencies of alleles in a population
Gene Pool: consists of all the genes that are
present in a population
Relative Frequency: The number of times an
allele occurs in a gene pool compared with
the number of times other alleles for the
same gene appear
Relative Frequencies of
Alleles
Sample Population
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
brown fur
allele for
black fur
Sources of Genetic Variation
1. Mutations: a change in the genetic
sequence. Some may produce changes
in the phenotype that result in better
fitness.
2. Gene Shuffling: which results from the
shuffling of gametes and sexual
reproduction; produces many different
combinations of genes.
Single Gene & Polygenic Traits
• A single gene trait is
controlled by 1 gene
with 2 alleles. Ex.
Widow’s peak.
• Polygenic Traits:
Each gene has two or
more alleles which
results in many
possible genotypes
and phenotypes.
Frequency of phenotype
Widow’s peak
No widow’s peak
Frequency
Phenotype Height
Natural Selection of Single-gene
Traits
• Can lead to change in
allele frequencies and
thus evolution.
Initia
l
Pop.
80%
Brow
n
10 %
red
Gen. Gen. Gen.
10
20
30
80%
Brow
n
0%
red
70%
Brow
n
0%
red
40%
brow
n
0%
red
10% 20% 30% 60%
blac blac blac blac
k
k
k
k
Genetic Drift
• The random change in allele frequency.
• In a small population, individuals that carry
a particular allele may leave more
descendants than other individuals, just by
chance. Over time, a series of chance
occurrences of this type can cause an
allele to become more common.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Founder’s Effect: Allele frequencies change
as a result of the migration of a small subgroup.
Evolution Versus Genetic
Equilibrium
• Hardy-Weinberg Principle (1908) states
that allele frequency in a population will
remain constant unless one or more
factors cause those frequencies to
change.
• Genetic Equilibrium: allele frequencies
remain the same.
Five Conditions of Hardy-Weinberg
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Random mating – no sexual selection
Large population – no genetic drift
No migrations – immigration or emigration
No mutations
No natural selection
• STOP
Natural Selection on Polygenic
traits
• Can affect the distribution of phenotypes in
three ways: direction, stabilizing, or
disruptive.
Graph of Directional
Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
The environment favors one extreme
Graph of Stabilizing
Selection
Stabilizing Selection
Key
The average
Organism is
More fit
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Graph of Disruptive Selection
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
Organisms with either of the two
Extremes are better fit.