Download Natural Selection introduction

Evolution
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The change in
species by the
process of natural
selection
What is evolution
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Evolution: Evolution is
a process that results in
heritable changes in a
population spread over
many generations.
Evolution can also be
defined as any change in
the frequency of alleles
within a gene pool from
one generation to the
next.
How does evolution occur?
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For a long time, scientists have
observed that species of organism
changed over time.
Several ideas were put forward, but
none of them withstood the test of
time and were rejected.
Charles Darwin, an English
Naturalist, developed a theory called
natural selection to explain the
change in species over time.
What is natural selection?
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When Darwin saw the great
variety of different species in his
travels he thought the mechanism
for evolution was like the process
of artificial selection.
Artificial selection is the process
by which breeders of plants and
animals produce organisms
possessing desirable traits.
Natural selection cont.
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In natural selection it is not the breeder
that determines which members of the
population successfully breed, but the
environment.
Individuals that survive and are able to
breed pass their genetic information to
the next generation. Those that are not
as successful in the environment often die
without leaving any offspring.
In this manner those traits that best allow
individuals in a population to survive will
increase in frequency while those that do
not will become more and more
uncommon.
Isn’t it just a “theory”?
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Yes, it is. Unfortunately opponents to the
teaching of evolution prey on the average
American’s misconception of the term “theory”
to further their own agendas.
In reality the term “theory” refers to a concept
that has been tested and confirmed in many
different ways and can be used by scientists to
make predictions about the world.
Remember…Gravity is only a theory, too.
Interactions and evolution
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The driving force behind evolution
is the interaction between individual
organisms and their environment.
Certain conditions are vital to the
process of evolution.
These conditions are
overproduction, the struggle for
survival, variation within the
population and selection by the
environment.
1. Overproduction
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Overproduction refers to the fact that in
each generation, a species has the
potential to produce more offspring than
can possibly survive.
Organisms such as bacteria, insects and
rats all reproduce in great numbers. If all
were to survive they would soon over run
the earth.
This of course, does not happen, and in
fact, most natural populations remain
relatively stable year after year.
Most offspring do not survive to
adulthood.
2. The struggle for survival
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Overproduction leads to competition within a species.
In many cases, chance determines which members of a species
survive.
Chance alone in not the only factor which determines an
individual’s survival.
Changing environmental conditions, disease, parasites and
predators all remove individuals from the competition.
Those members of the population that do survive then must
complete for a finite number of resources, such as food, shelter
and water.
3. Variation
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New traits which can lead to
evolution come from normal
variation within species.
All the individuals in a species
are not exactly alike.
These differences arise from
genetic variation—the unique
combination of traits each
organism inherits from its
parents.
Variation cont.
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Some variations give individuals an advantage over
others in their struggle for survival.
Any trait that helps an organism survive and reproduce
under a given set of environmental conditions is said to
have adaptive value.
For example, a deer that can run just a little bit faster
than another will have a greater chance of escaping a
predator. This would be especially advantageous in
areas where there is a high density of predators.
4. Selection by the Environment
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Traits with an adaptive value in a specific
environment give individuals in that
environment a competitive advantage.
If the beneficial trait is passed on to the
offspring, they, too, are more likely to
survive and reproduce.
Over time the proportion of the
population possessing this trait will
increase.
This change in the characteristics present
in the population over time is called
evolution.
Selection by the environment cont.
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Although some evolution may occur
without much change in the
environment, it is usually the
adaptation of a species to changes in
its environment that brings about
evolution.
Therefore, a changing environment
is often the driving force for
evolutionary change.
Selection by the environment has
the same meaning as “Survival of
the fittest”.