TYPES OF NATURAL
SELECTION
DEFINE IN YOUR NOTES
Directional Selection
Stabilizing Selection
Disruptive Selection
DIRECTIONAL SELECTION
A form of natural selection;
occurs when individuals at one
end of the distribution curve have
higher fitness than individuals in
the middle or at the other end of
the curve.
One EXTREME is favored.
DIRECTIONAL SELECTION
Example: Giraffes
Selection for a long neck
STABILIZING SELECTION
A form of natural selection;
occurs when individuals near
the center of a distribution
curve have higher fitness than
individuals at either end.
EXTREMES are NOT selected AVERAGE is
better.
STABILIZING SELECTION
STABILIZING SELECTION
Example: Human infant birth weight—it is
a disadvantage to be really small or
really big, and it is best to be somewhere
in between.
DISRUPTIVE SELECTION
A form of natural selection;
occurs when two individuals at
the upper and lower ends of a
distribution curve have higher
fitness than individuals near
the middle.
BOTH EXTREMES are selected for.
DISRUPTIVE SELECTION
DARWINS FINCHES
WORD ASSOCIATIONS
Directional = 1 extreme
Stabilizing = Average
Disruptive = 2 extremes
ANALOGOUS STRUCTURES
Structure's that evolve because of
the same environmental pressures.
For example, Birds and Bats. They do
not have a recent common ancestor,
but have managed to both evolve
wings.
THE FOUNDER EFFECT
The loss of genetic variation
when a new colony is formed
by a very small number of
individuals.
(This is why incest is not good)
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ISOLATING MECHANISMS
When a new species evolves it’s
because a population has become
reproductively separated.
REPRODUCTIVE ISOLATION
THERE’S 3 T YPES
When members of 2 populations cannot
interbreed and make fertile offspring.
This is why we will
never have a real life
CatDog.
Different species
CANNOT combine and
reproduce.
BEHAVIORAL ISOLATION
 This is when the same species are CAPABLE of
reproducing but DON’T because they have different mating
rituals.
Like a lion and a cheetah.
BOTH are felines (cats)
BUT they DON’T mate with each other.
GEOGRAPHIC ISOLATION
When species are separated by a
geographic barrier, like an ocean, river
or mountains.
Like a Brown Bear and a Polar Bear
They are BOTH bears
BUT they are geographically separated so they
can’t mate.
TEMPORAL ISOLATION
When species reproduce at
different times.
For example, these 2 different types of frogs
BOTH are frogs
BUT one mates in the spring, while the other
mates in the fall
ISOLATING MECHANISMS
Each of these could lead to the
development of a new species.
https://www.youtube.com/watch?v=2oKlKmrbLoU