Chapter 23:
Microevolution
{ The Evolution of Populations
Genetic Variation
• Why these flowers aren’t all identical
• Why we aren’t all identical
• Genes vary within and between
populations

Mutations
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Altered gene number and position
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Rapid Reproduction = more mistakes
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Sexual Reproduction – different genes from
each parent
Causes of Variation
The Hardy-Weinberg Principle
It’s Pretty Important

Explains a gene pool that is not
evolving
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p2 +2pq+q2

Shows if population is undergoing evolution
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NO MUTATIONS
RANDOM MATING
NO NATURAL SELECTION
EXTREMELY LARGE POPULATION SIZE
NO GENE FLOW
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How did this relate to the experiment?
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Conditions for H-W
Equilibrium
Using the H-W Equation

Question 3 on page 474

It was your homework so you better
have done it already
Natural Selection,
Genetic Drift, and
Gene Flow
Altering allele frequencies one
generation at a time
Natural Selection

Variation in heritable
traits

Best traits help you
survive to reproduce
Genetic Drift
When random events cause allele frequencies to
fluctuate
 Significant in small populations
 Can cause allele frequencies to change at
random
 Can lead to a loss of genetic variation
 Can cause harmful alleles to become fixed

Examples of Genetic Drift
Founder Effect –
small group breaks off from
original population
 British Colonists on
Tristan Da Cunha
 Amish - Polydactyl
Examples of Genetic Drift
Bottleneck Effect –
size of population is
dramatically reduced


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Human actions can create
Elephant Seal
Purebred Animals
Gene Flow

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The flow of genes into or out of a
population
Reduces genetic differences between
populations
Can change survival abilities
Important in humans
Natural Selection –
It’s What Adaptive
Evolution is All About
Relative Fitness –
fitness relative to
others in terms of gene
pool contribution
Disruptive, Directional, and
Stabilizing Selection
Sexual Selection



Sexual Dimorphism – males and females
differ
Intrasexual Selection – competing against
other members of same gender for mate
Intersexual Selection – mate choice
Preserving Genetic Variation
How does Natural Selection keep the variation
going?

Diploidy – recessive alleles

Balancing Selection – 2 or more favorable forms

Heterozygote Advantage – sickle-cell in Africa

Frequency-Dependent Selection – fitness =
frequency
Why Natural Selection
Can’t Create Perfection
Selection can act only on existing
variations
 Evolution is limited by historical
constraints
 Adaptations are often compromises
 Chance, natural selection, and the
environment interact

Sorry guys you
aren’t actually
perfect