Download Evolution - Valhalla High School

Evolution
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, and 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.