Evolution by Means of Natural Selection:
Mechanisms of Natural Selection
Chapter 11
Mutations in genes is the source of all new variations
1.Helpful mutations lead to
adaptation by increasing
fitness & decreasing the
death rate
• Frequency of helpful
mutation (gene) will
increase in the
population as more
individuals survive &
leave offspring
How is the fruit fly on
the bottom different
from the fruits above?
Helpful Mutations:
Camouflage: having a
shape or color that blends
in with the environment
Find the canyon tree frog!
• Difficult for predators to
see the prey
Find the green leaf mantid!
Helpful Mutations:
Mimicry: Taking Advantage of Another’s Coloration
Mimicry: when a harmless individual looks like
a similar, harmful (poisonous) individual
• Predators learn to avoid both
Viceroy: looks like
poisonous monarch
Monarch is
poisonous
Helpful Adaptations: Examples
Adaptations
of Birds
Feet
The woodpecker’s sharp
beak allows it to drill into
trees to catch insects.
Helpful Adaptations: Examples
The giraffe’s long neck helps it
reach higher in the tree for food
where other animals can’t.
The alternating black & white
stripes help keep zebras cool.
Black stripes absorb heat & white
stripes reflect heat.
Adaptations: A Result of Compromise
An adaptation may
work really great
for some things
like reaching food,
but they may pose
a problem for other
things such as
drinking water.
Remember:
Individuals Cannot
Change to Fit the
Environment!
Adaptations That
Increase Fitness
are Inherited
Variations!
Mutations in genes is the source of all new variations
2. Harmful mutations will
decrease fitness of the
individual in that environment &
cause a higher mortality rate
• Frequency of the mutation
(gene) decreases = might
or might not disappear from
the gene pool
• Gene Pool - Consists of all
genes, including all the
different alleles, that are
present in a population.
How is the fruit fly on
the bottom different
from the fruits above?
Other sources of variation
Natural selection is not the only source of
evolutionary change.
In small populations, an allele can become
more or less common simply by chance.
Other sources of variation
1. Genetic Drift - Random change in allele
frequencies that occurs in small
populations.
• Happens by chance - such as random
mating or a natural disaster (fire,
landslide or lightning strike).
• Doesn’t work to produce adaptations
like natural selection does.
Other sources of variation
2. Gene Flow - The transfer of alleles or
genes from one population to another; also
known as gene migration.
Affected by:
1. Mobility- ability to move
2. Barriers- physical and behavioral
What mechanisms lead to speciation?
SPECIES - A GROUP OF SIMILAR LOOKING
ORGANISMS THAT CAN BREED TOGETHER TO
PRODUCE FERTILE OFFSPRING. (FERTILEABLE TO REPRODUCE).
Mechanisms that lead to speciation
Reproductive Isolation Leads to Speciation
Speciation: Members of an original species can no
longer breed together to produce fertile offspring.
They are considered two separate species.
Three species of Flycatcher.
Reproductive Isolation keeps these as
three separate species. Why?
Females only respond to mating call
(song) of males that are like them
Behavioral Isolation Example
Mechanisms that lead to speciation
What mechanisms lead to reproductive isolation?
Grand Canyon is NOT a physical
barrier for all species. What kind
of organisms would be able to
cross the canyon?
1.Geographic isolation: barrier that physically
separates members of a species into two or more
groups
a. EX: Mountains, large body of water (lake or
ocean), volcanic eruption, canyon, road
b.A barrier may physically separate some
species, but not others
Mechanisms that lead to speciation
Reproductive isolation can
occur by:
2. Behavioral Isolation: Two
populations are capable of
interbreeding, but have
differences in courtship rituals
or other reproductive
strategies that involve
behavior
Mechanisms that lead to speciation
3. Temporal Isolation - Form of
reproductive isolation in which
two populations reproduce at
different times.
– Example:
– Three similar species of orchid
all live in the same rain forest.
– Each species releases pollen
only on a single day.
– Because the three species
release pollen on different days,
they cannot pollinate one
another.
Three types of Natural Selection:
A. Directional Selection
1. Directional selection: members at
one end of distribution curve have a
higher fitness than those in the
middle or at the other end of the
curve.
• Can lead to one population
evolving into a new species.
Food becomes scarce.
Notice the shift in beak size
Directional Selection
The woodpeckers
with the longer
beaks are more fit
for this environment,
so there are more of
them than
woodpeckers with
shorter beaks.
Selection
for longer
beaks
B. Stabilizing Selection
1. Stabilizing selection: members in the
center of the distribution curve have
a higher fitness than those at each
end.
• Favors average individuals.
•
Reduces variation in a population;
evolution is not likely to happen.
•
Ex: babies that are between 6 & 8
pounds at birth have a better chance
of surviving 1st year of life.
Stabilizing Selection
In the example below, averaged size spiders survive more
often, because birds eat the very large and small ones.
C. Disruptive Selection
Number of Birds
in Population
1. Disruptive selection: when members at
BOTH ends of the distribution curve
have a higher fitness than those in the
middle
• In some cases, there are no
intermediate forms.
• This can lead to the evolution of two
new species.
Beak Size
Population splits
into two
subgroups
specializing in
different seeds.
Disruptive Selection
In the example below, the light and dark limpets
(Clams) are camouflaged, whereas the
intermediate ones are not, and are eaten.
Selection
for dark
limpets
Matching Practice (Types of
Selection):
1. Does not lead to speciation or
evolution.
2. Cause one species to evolve into two
different species.
3. Causes one species to evolve into a
different species.
4. Type of selection that favors one
extreme variation.
5. Type of selection that favors the
average variation.
6. Type of selection that favors the two
extreme variations.
A. Directional
B. Stabilizing
C. Disruptive
Gradualism, Punctuated Equilibrium and
Genetic Equilibrium
• How long does it take for evolution to
occur?
• Does evolution always occur?
Gradualism vs. Punctuated Equilibrium
How quickly does evolution
occur?
– Gradualism – Long time
with gradual change.
– Punctuated Equilibrium Long, stable periods
interrupted by brief
periods of more rapid
change.
Evolution vs. Genetic Equilibrium
To clarify how evolutionary change operates, scientists
often find it helpful to determine what happens when no
change takes place.
So biologists ask: Are there any
conditions under which evolution will not
occur?
– The answers to those questions are
provided by the Hardy-Weinberg
principle.
Hardy-Weinberg Principle
Five conditions are required to maintain genetic equilibrium
from generation to generation:
1. There must be random mating.
2. The population must be very large.
» Genetic drift has less effect on
large populations than on small
ones.
Hardy-Weinberg Principle
3. There can be no movement into or
out of the population.
» Individuals may bring new
alleles into a population.
4. There can be no mutations.
5. There can be no natural selection.
» No phenotype can have a
selective advantage over
another.
Two Patterns of Evolution1. Divergent Evolution (adaptive radiation)- type
of evolution in which one species evolves into two
separate species because it is adapting to different
environmental situations
2. Convergent Evolution- type of evolution in
which two unrelated species evolve similar
characteristics because they are both adapting to
the same environmental situations
Raven and Johnston,
Biology
Divergent Evolution
or Adaptive Radiation
Raven and Johnston,
Biology
Divergent Evolution
Raven and Johnston, Biology
Convergent
Evolution