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Charles Darwin
Natural Selection
Charles Darwin (1809-1882)
• 5 year voyage around the world on the
HMS Beagle as the ships naturalist.
• Observations
- Great diversity of organisms
- Organisms well suited to environment
- Different types of organisms in different
parts of the world
- Similar organisms with different
EX: Galapagos Island Examples
Different shaped shells and necks that adapt them
to their island habitat
Different type of beak’s that adapt a bird to its food
Natural selection was Darwin’s explanation
of how evolution occurs. The mechanism
of how organisms chanced over time.
• Organisms adapt to their environment,
structural or functional changes occur from
one group of descendants to another
• Descended from a common ancestor
Darwin’s finches
Common Ancestor
A shared link between
living and extinct
Living organisms share a
common ancestor, the
more alike organisms
are the nearer the
common ancestor the
more different
organisms are the
further back in
geological time the
common ancestor
Descent with Modification
Each living species has descended, with changes
from other species over time
Artificial selection- Humans have been doing this
for a thousand years with domesticated species
His theory included the following:
Struggle for existence:
Survival of the fittest:
Advantageous characteristics past on to
• Gradual change:
• Read Cambridge pages 129, explain each
• Look at pg 130 at the example of natural
selection in cacti
EX: Beetles
Brown and green coloration
Many produced
Struggle for existence:
Avoiding been eaten
Survival of the fittest:
Brown beetles are better suited to
Advantageous characteristics past on to
Brown beetles survive, reproduce pass on
genes for brown color
Gradual change:
Over time population changes to mainly
brown beetles
Descent with Modification
• We’ve defined evolution as descent with
modification from a common ancestor
• Evolution only occurs when there is a
change in gene frequency within a
population over time.
• Beetle example: genes for the brown
color become more frequent
Natural selection- selects among whatever variations
exist in the population. The result is evolution.
Mutations are random in the DNA sequences that lead to
these variations
Evolution of Populations
• A Gene Pool consists
of all genes, including
all alleles that are
• Allele Frequency of an
allele is the number of
times that the allele
occurs in the gene pool
• The relative frequency
has nothing to do
whether the allele is
dominant or recessive.
Allele frequencies
measure how
common certain
alleles are in a gene
Source of Genetic Variation
• Mutations are any change in a sequence of
DNA occur because of mistakes in replication or
environmental factors. Can form a new allele
• Recombination occurs during production of
gametes (meiosis) during sexual reproduction, it
is enhanced by crossing over.
• Gene shuffling does not change the relative
frequency of alleles in a population.
Normal Distribution
• Highest frequency of phenotypes is
around the mean value
• Extreme phenotypes are not favored
• Creates a bell-shaped curve
• Observable change in
allele frequency over
time within a
• Small scale
• Natural selection
changes a trait in that
Directional Selection
• When individuals on
one end of the curve
have higher fitness
than individuals in the
middle or at the other
end directional
selection takes place
• Ex: bacteria
Stabilizing Selection
• When individuals
near the center of
the curve have
higher fitness than
individuals at either
end of the curve
stabilizing selection
takes place.
• Ex: Gall fly
Disruptive Selection
• When individuals
at the upper and
lower ends of the
curve have higher
fitness than
individuals near
the middle,
disruptive selection
• EX: Lazuli buntings
Gene Flow
• Gene flow is the process in which new alleles
are added to the gene pool through an outside
• Genes move from one population to another,
creating genetic diversity
• For example- Migration
Genetic Drift
• In small populations an allele can become more
or less common simply by chance. Overtime, a
series of chance occurrences of this type can
cause an allele to become common in a
population (Just lucky or unlucky)
• Commonly occurs when a small group of
individuals colonizes a habitat.
• Decrease in
Genetic diversity
Bottleneck Effect
• When genetic drift reduces a population
• A destructive event leaves only a few survivors
• Ex: Holocaust
Q. What happens to
the genetic diversity
in the population?
Founder Effect
• Small number of individuals colonize a new area
• Gene pool can be different from original
population. Ex: Amish
Sexual Selection
• Females are choosy about mates
• Certain traits that the males have will increase
mating success
• Ex: Peacocks showy tail, bigger the tail the
healthier the male
Reproductive Isolation
• Different populations can no longer mate
together successfully
• Can not physically mate
• Offspring that are produced do not survive
Ex: Fruit Flies, pheromones
• Speciation is the
process in which new
species are formed.
• Sometimes
populations are
disrupted to the point
where they can no
longer mate. This is
referred to as
reproductive isolation
so forming different
• Elimination of a species from earth
Punctuated Equilibrium
Adaptive radiation
Convergent evolution
Divergent evolution
Convergent Evolution
Unrelated species evolve similar traits due to their
similar environments
Ex: Wings of birds and insects
Caudal fin of sharks and dolphins
Hardy-Weinberg principle
• Scientists find it easier to understand
evolution by looking at a hypothetical
situation where evolution does not occur.
• The populations alleles remain constant
(at equilibrium) from generation to
• Although the situation is unattainable it still
provides scientists with accurate
information about what drives evolution.
5 Conditions of the Hardy-Weinberg Principle
• Random Mating - All members of the population
must have equal opportunity to produce offspring
• Large Population - there is less effect of genetic
drift on large populations
• No Movement into or out of the Population- there
can be no additional alleles in the population
• No Mutations - the DNA must remain consistent
• No Natural Selection - all genotypes must have
equal opportunity to reproduce.