1. Evolution of Populations
EQ: How does natural selection affect
populations?

2. 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.

3. 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.

4. 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

5. 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.

6. 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

7. 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.

8. 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

9. 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