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
Altered gene number and position
Rapid Reproduction = more mistakes
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
p2 +2pq+q2
Shows if population is undergoing evolution
NO MUTATIONS
RANDOM MATING
NO NATURAL SELECTION
EXTREMELY LARGE POPULATION SIZE
NO GENE FLOW
How did this relate to the experiment?
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
Human actions can create
Elephant Seal
Purebred Animals
Gene Flow
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
0
