Natural Selection
Understanding The Mechanism for
Evolution
Natural Selection and Evolution
• Genetic Variability arises from
– mutation
– sexual reproduction
• Natural Selection acts on individuals,
favoring those with adaptations to the
current environment
• Evolution occurs in populations as the
individuals with greatest fitness leave
the most offspring
Hardy-Weinberg Equilibrium
• A condition where allele frequencies and
genotypic frequencies remain constant
from generation to generation
• Changes from equilibrium values are
used to determine if natural selection is
occurring
Calculating Allele Frequencies
• In a population of 100 pea plants, there
are 200 alleles for seed color. If 60 of
those alleles are y for green color, what
is the frequency of the y allele?
• Total number of alleles = 200
• Total number of y alleles = 60
• y/total= 60/200 = 0.30
Conditions of Hardy-Weinberg
Equilibrium
Large population size
Random mating
No migration
No mutation
No selection
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
Large
Population
Size
Genetic Drift: Changes in allele
frequency due to small population
sizes
1. Founder effect
2. Population Bottleneck
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
Random
Mating
Non-random mating: Alters
genotypic frequencies
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Migration Migration: Can add new alleles,
remove alleles or change allele
frequency
Migration Contributes
to Gene Flow
• Gene Flow
–Spreads advantageous alleles
throughout the species
–Helps maintain all the organisms
over a large area as one species
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Mutation
Mutation: Alters allele frequency,
causes formation of new
genotypes
Conditions of Hardy-Weinberg Equilibrium
Condition
Non-equilibrium Condition
No Selection Natural Selection: Increases
frequency of genotypes with
higher fitness
Types of Natural Selection
Large Size Average Size Extreme Size
Favored
Favored
Favored
Time
Before
Selection
Mean
changed
over time
After
Selection
Variation
reduced
over time
Directional Stabilizing
Selection
Selection
Variation
becomes
polarized
Disruptive
Selection
Types of Selection
• Stabilizing: eliminates extremes
Stabilizing Selection & Balanced Polymorphism
SS
Homozygous
Normal
SS’
Heterozygous
S’S’
Homozygous
Defective
Dies of malaria
Lives and
reproduces
Dies of
sickle-cell anemia
SS
SS’
S’S’
Dies of malaria
Lives and
reproduces
Dies of
sickle-cell anemia
SS
SS’
S’S’
Types of Selection
• Disruptive: increases both extremes
Types of Selection
• Directional: increases one extreme
Applying Your Knowledge
1. Stabilizing Selection
2. Disruptive Selection
3. Directional Selection
Which type of selection has occurred if
• The background is sandy with dark
rocks and snails are found with
either dark or light shell colors?
• After spraying with malathion, more
fruit flies are found to be resistant to
this insecticide?
Environmental Influences
• Abiotic Factors
– climate
– availability of resources
Environmental Influences
• Biotic Factors
– competition
Environmental Influences
• Biotic Factors
– predator-prey relationships
Camouflage hides prey
Environmental Influences
• Biotic Factors
– symbiosis: close interaction between
species
Parasitism
One helped , other harmed
Environmental Influences
• Biotic Factors
– symbiosis
Mutualism
Both helped
Environmental Influences
• Biotic Factors
– symbiosis
Commensalism One helped, other neither
helped nor harmed
Environmental Influences
• Biotic Factors
– sexual selection
Environmental Influences
• Biotic Factors
– altruism and kin selection
Applying Your Knowledge
1. Mutualism
2. Commensalism
3. Parasitism
Which type of symbiosis is operative if
• A plant supplies a bee with nectar
and the bee takes pollen to other
plants of the same species?
• Athlete’s foot fungus starts growing
between your toes?