Download Evolution of Populations

Evolution of
Populations
EQ: How does natural selection affect
populations?
Gene Flow
• Gene flow - also called migration—is any movement
of genes from one population to another.
EX: pollen being blown to a new destination or people
moving to new cities or countries.
Another force behind evolution…
•Genetic drift: In each generation, some
individuals may, by chance, leave behind a few
more descendants (and genes) than other
individuals. Results in rapid, random changes in
gene frequencies.
Speciation
• Natural Selection and chance
events can lead to changes in a
population, causing the formation of
new species.
• Ex. Darwin's finches
• Speciation video
Causes of Speciation
Geographic Isolation: What was once a
continuous population is divided into two or
more smaller populations that are no longer able
to interact.
Can occur when rivers change course,
mountains rise, continents drift, or
organisms migrate.
Causes of Speciation
Behavioral Isolation is a form of reproductive
isolation in which two populations have
differences in courtship rituals or other types
of behavior that prevents them from
interbreeding.
• For example: firefly signals, frog calls, bird dance/song
Affect of Natural Selection on Phenotypes
•Disruptive Selection – When individuals the
upper and lower ends of the curve have higher
fitness than individuals near the middle.
•Ex: Food shortage leaves medium sized
seeds less common and large and small
seeds more common.
Affect of Natural Selection on Phenotypes
•Directional Selection – When individuals on
one end of the curve have higher fitness than
individuals in the middle or at the other end.
•Ex: Food shortage leaves
only large seeds available
Affect of Natural Selection on Phenotypes
•Stabilizing Selection – When individuals near
the center of the curve have higher fitness than those
at the ends.
•Ex: Human babies