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Evolution,
Natural Selection
& Selective
Pressures
Page 1: Concept Map
Evolution!
Definition:
Natural
Selection
Definition:
Genetic
Variation:
Selective
pressure:
Many
Generations:
Page 2: Categories of
Natural Selection
Directional
selection:
Stabilizing
selection:
Disruptive
selection:
What is it?
What is it?
What is it?
Example graph
with the change
Example graph
with the change
Example graph
with the change
What is Evolution?
Evolution = Change in allele
frequency in a population
over time (generations)
Ex: If any inherited trait is
increasing (or decreasing) in
frequency, the population is
evolving
How does Evolution Occur?
• Evolution occurs through a
process called natural
selection
• Natural selection occurs when
certain individuals are better
able to survive and reproduce
due to their phenotypes
(traits)
Theory of
Evolution by Natural
Selection
• Charles Darwin was the first person
to propose the idea of evolution via
natural selection in 1859
• Many people before him had
hypothesized about evolution, but
he was the first person to propose
a valid mechanism for how it works
What is Needed for Nat.
Selection to Occur?
1. A population containing genetic
variation
2. Selective pressures in the
environment that cause certain
individuals to have greater fitness
(survival and reproduction)
3. MANY reproductive generations
(time)
What is a Selective
Pressure?
• Selective pressures are any
factors in the environment
that affect the survival &/or
reproduction of individuals in
a population
• Can be biotic (living) or
abiotic (non-living)
What is a Selective
Pressure?
• Examples of biotic
selective pressures:
– Predators,
competitors
(organisms that
compete for the
same resources),
pathogens….
Predation is
a biotic
selective
pressure
What is a Selective
Pressure?
• Examples of abiotic
selective pressures:
– Temperature,
sunlight, humidity,
rainfall, snow,
wind, soil minerals,
salinity, fire…
Fire is an
abiotic
selective
pressure
Monogenic Traits
• Some traits are monogenic –
coded for by only one gene
• Ex: Widow’s peak, blood type,
earlobe attachment, many
genetic disorders
• Monogenic traits appear as
distinct categories, e.g. blood
type A, B, AB or O
Monogenic Traits
• Phenotype frequencies
for monogenic traits
are shown on simple
bar graphs
• If evolution is
occurring, the
phenotype frequencies
will change over
generations
3 phenotypes:
brown OR black
OR red
G e n e r a tio n 1 0
Frequency of
phenotype
In itia l P o p u la tio n
G e n e r a tio n 2 0
G e n e r a tio n 3 0
What do you notice in this population
of lizards with three skin color
phenotypes?
Generation 10
Frequency of
phenotype
Initial Population
•
Evolution is
occurring!
• What selective
pressure could cause
this selection for
black lizards over
red and brown?
Generation 20
Generation 30
Polygenic Traits
• Most traits are polygenic – coded for
by more than one gene
• Polygenic traits show a wide range of
phenotypes
• Polygenic traits are continuous,and
NOT separate categories
• Ex: Height, eye color, skin color
Polygenic Traits
The fact that there is
such a wide range of
lengths indicates this
trait is polygenic
35
30
Frequency
25
20
15
10
5
0
20mm 30mm 40mm
Length
Polygenic traits
generally have
frequencies that follow
a bell-shaped curve
S e a G u ll P o p u la t io n - G e n e r a t io n 1
20
F req u en cy
• What do you
notice about
the wingspan
phenotypes
after 250
generations of
sea gulls?
15
10
5
0
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
W in g s p a n ( c m )
S e a G u ll P o p u la t io n - G e n e r a t io n 1
20
15
F req u en cy
• There has
been selection
for longer
wingspans
10
5
0
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
W in g s p a n ( c m )
Sea Gull Population - Generation 250
30
25
Frequency
What selective
pressure could
cause this?
20
15
10
5
0
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97
Wingspan (cm)
Three Categories Of
Natural Selection
• Directional (common)
• Stabilizing (common)
• Disruptive (uncommon)
Directional Selection
•
Hawk vision ratings
in population
before selection
Vision ratings after
many generations –
selection for better
vision has occurred
Stabilizing Selection
• When phenotypes in the middle
give the organism an advantage
• Ex: Human birth weight
• Low birth weight = infant
mortality
• High birth weight = delivery
complications
Stabilizing Selection
Peak weight does NOT change over time since
weights at the ends are selected against
Disruptive Selection
• Selection in favor of phenotypes on
both ends of the curve
• Ex: Short and long beaks when two
sources of food are present
• Short-beaked birds can eat the
seeds more easily, long beaked
birds can eat the insects more
easily
Disruptive Selection
Over many generations, birds with short
and long beaks become more common, while
those in the middle die out