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The Theory of Evolution by
Natural Selection
Objectives
3.4.1- Explain how fossil, biochemical, and
anatomical evidence support the theory of
evolution.
3.4.2- Explain how natural selection
influences the changes in species over
time.
3.4.3- Explain how various disease agents
(bacteria, viruses, chemicals) can
influence natural selection.
The Theory of Evolution by Natural
Selection
Changes in species occur over time,
because organisms with favorable
variations in traits survive, reproduce, and
pass those variations on to the next
generation.
The basis of the theory is that all
organisms share a common ancestor.
Life’s history is not linear, but looks more
like a tree.
Studying Life’s History
Fossil evidence is clear, tangible evidence
that the types of living organisms on Earth
have changed over the course of time.
Fossils reveal organisms that no longer
exist, fossils that are similar to present day
organisms, and fossils that look the same
as some present day organisms.
New fossils are constantly being found.
Studying Evolution in Real Time
Unicellular organisms such as bacteria
multiply so fast that in just a few years
1000s of generations can be studied.
Bacteria evolve to tolerate diverse
environments and feed off of unlikely food
sources.
Changes and Variations
Mutations in DNA coding generate genetic
variation.
Natural selection increases the number of
beneficial mutations in organisms.
Species fit or suited for their environments
do not experience as much evolution as
species that are not.
New species can form over time.
Factors Affecting Evolution
Long-term and short-term changes in
geography.
Catastrophic events
Human influence
Common Ancestry
 Shared anatomical structures may reflect
common ancestry although some cases of
shared anatomy reflect similarities in habitat
or life strategies.
 Vestigial structures are structures that are
smaller, underdeveloped, or have no
purpose in an organism that did have a
purpose in ancestors.
The Endosymbiosis Theory
Eukaryotes evolved through a symbiotic
relationship between ancient prokaryotes.
The primary evidence supporting this
theory is that mitochondria and
chloroplasts have their own DNA which is
similar to the DNA found in prokaryotes.
Biochemical Similarities
The genetic code is universal amongst all
living organisms.
Different species share sequences of DNA
in common- some coding for functional
proteins and some just there.
The more similar two organisms are in
their biochemistry the more closely they
are related on the evolutionary tree of life.
Examples of Evolution
1. Darwin’s Finches
There are 13 types of finches found in the
Galapagos Islands. The beak shape is
one of the major differences which reflect
the types of food found on the islands. As
environmental conditions change such as
amount of rainfall, the number of finches
with certain beak shapes change
accordingly.
2. Rat Snakes
The United States is covered with rat
snakes of various colors. The colors tend
to reflect the local environments that the
snakes are found.
3. Three-toed Skinks
In Australia, skinks of the same species
are giving birth to young in two different
ways depending on their location. Skinks
near the coast are laying eggs while
skinks in mountainous regions are giving
live birth.
4. Lactose Tolerance in Humans
Some humans can digest lactose found in
milk on into adulthood thanks to a
mutation that occurred when cattle
became domesticated in Europe and
Africa centuries ago.
5. HIV
The virus HIV which can cause AIDS is
weakening in human populations that it
has been common in for long periods of
time. Although currently deadly, HIV may
weaken to a point that it is not deadly.
Strong immune systems and antiretroviral
therapy are causing mutations in the virus
so that it becomes milder.
6. Chemical Resistance
Numerous pests that humans have killed
with chemicals are developing a
resistance or immunity to the chemical
after just a few generations. The ones that
survive are naturally immune and create
offspring who are also naturally immune.
7. Antibiotic Resistance
Overuse and misuse of antibiotics is
causing many strains of bacteria to
become resistant or immune to antibiotics
which are drugs used to kill bacteria.