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Natural Selection
 The process by which
individuals that are better
adapted to their environment
are more likely to survive and
reproduce than other members
of the same species.
Some factors that affect
natural selection:
 1-overproduction
 2-competition
 3-variations
 4-selection
1. Overproduction
 Most species produce far more off spring
than can possibly survive
 Often so many are produced that there
are not enough resources for them all.
 Ex: Sea turtles
What would happen if all the
young sea turtles lived?
Sea turtles lay of 100 eggs—if they all
survived, the ocean would be full of
turtles.
Darwin knew better…
2. Competition
 Offspring must compete with each other
to survive.
 Competition does not necessarily mean
physical fighting.
 It is usually indirect
For example,
Some baby turtles
May not be able to find enough to
eat.
Variations
 Members of a species differ from each
other in many of their traits.
 Any difference in between individuals of
the same species is called variation.
For example…
Some turtles
are able to swim
faster than others
To escape from
predators.
 Remember you don’t have to
be faster than the predator,
you just have to be faster
than the other prey!
Selection
 Over a long period of time, natural
selection can lead to evolution.
 Helpful variations gradually accumulate
in a species, while unfavorable ones
disappear.
For example…
 A new faster predator moves into the
turtles’ habitat.
OK…it won’t be Speed
Racer, but…
Perhaps a faster
breed of shark…
The turtles that are able to
swim faster would be more
likely to escape from the
new predator.
 The faster turtles would be
more likely to survive and
reproduce. Over time, more
turtles in the species would
have the “fast-swimmer” trait.
Genes
 What role do genes have in
evolution?
 Variations can occur due to
mutations in genes or from the
shuffling of alleles during meiosis.
 Therefore, only traits that are
inherited are acted upon by
natural selection
How are new species formed?
 A new species might form when a
group of individuals remains
separated form the rest of its
species long enough to evolve
different traits.
 This is called isolation or
complete separation.
Continental drift
 Millions of years ago on the super
continent Pangaea, organisms
could travel from one part of the
continent to another.
 Pangaea gradually split apart
over millions of years in a
process called continental drift.
 As this separation happened,
species became isolated from
one another and began to
evolve independently.
 Adapted from Prentice Hall Life Science.