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Theory of Evolution
Chapter 16 & 17
• change over time
Acquired trait hypothesis
• Jean-Baptiste
Lamarck proposed
a theory that
organisms pass on
traits they acquire
in their lifetime.
Thomas Malthus
• In 1798, English economist Thomas
Malthus noted that humans were being
born faster than people were dying,
causing overcrowding.
• Malthus believed:
– if the human population grew unchecked,
there wouldn’t be enough living space and
food for everyone
– the forces working against population growth
included war, famine & disease.
Natural Selection
• Charles Darwin, a
naturalist, proposed a
mechanism for
explaining how
evolution occurs
• During a voyage on the
HMS Beagle, Darwin
collected a vast
amount of data,
including the famous
study of the finches on
the Galapagos islands.
• Aided by some ideas from Malthus,
Lamarck, and his own studies, Darwin
began suggesting the mechanism of
natural selection.
Evolution by Natural Selection
1. The Struggle for Existence:
If more individuals are produced than can
survive, members of a population must
compete to survive.
2. Variation & Adaptation:
In a particular environment, some individuals
of a population or species are better suited to
3. Survival of the Fittest
Over time, the traits that make
certain individuals able to
survive and reproduce tend to
spread in that population
Evidence of Evolution
1. Biogeography
2. Fossils
– There is clear proof from fossils that living
species evolved from older organisms
– homologous structures
Structures that are shared by related species and
that have been inherited from a common ancestor
Analogous Structures – share common function
but not structure
vestigial structures - structures that have no use
or are of little importance
3. DNA
• Differences in DNA sequences are less
between species that are more closely
• Very important DNA sequences are highly
conserved between species.
• After over 25 years of
research, Darwin
published his ideas in
his book The Origin of
• Darwin’s ideas are
the basis for the latest
insight on the
evolution of species
• the changing of a
species that results
in its being better
suited to its
• enough change
within a species
eventually leads to
a new species
• development of a new species
• Isolation can lead to species
• Isolation is the condition in
which 2 populations of the
same species cannot breed
with one another; therefore they
do not share gene pools and
variations can develop
separately in each population
that prevent them from
interbreeding later.
• Ex: the Kaibab (north rim) and
Abert (south rim) squirrels of
the Grand Canyon
• speciation occurs
Punctuated Equilibrium
• speciation occurs
rapidly between
periods of little or no
5 Principle Evolutionary Forces
1. Mutation-the source of all variation, won’t
significantly change allele frequencies
except over long periods of time.
2. Gene Flow-migration allows for genes to
move into and out of a population
3. Nonrandom Mating-mates with others
nearby or with similar phenotype;
inbreeding a type of nonrandom mating.
4. Genetic Drift-a random event changes the
frequency of an allele
5. Natural Selection-frequency of an allele
will increase or decrease depending on
allele’s effects on survival and
Trait Distribution
• Normal distribution(bell curve) average
value at the summit
• Directional selectionentire curve shifts in
one direction
• Stabilizing selectionindividuals become
more and more alike
• Over long periods of time, events can
occur that result in the species going
extinct (disappear permanently)
Overview of natural selection:
• All species have genetic variation
• The environment presents many different
challenges to an individual’s ability to
• Organisms tend to produce more offspring
than a space can support, so individuals
compete to survive
• Individuals that are better able to cope
with the challenges of survival tend to
leave more offspring than those who do
• Traits of the individuals best suited to their
environment tend to increase in the
population over time