Evolution as Genetic Change Subtitle

Control of Natural Selection Natural Selection can affect the distributions of phenotypes in any of three ways: Directional selection Stabilizing selection Disruptive selection

Directional Selection Evolution favors the most fit species. Causes an increase in the number of individuals with favored trait to one end of a curve.

Stabilizing Selection When individuals with the desirable traits are towards the center of the curve. Major Point: The extremes don’t survive.

When the individuals at the extremes have the highest fitness. Disruptive Selection When the individuals at the extremes have the highest fitness.

Genetic Drift In a smaller population, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. This can be caused by the bottle neck effect.

Four things that lead to speciation: Reproductive isolation Behavioral isolation Geographic isolation Temporal isolation

Reproductive Isolation When members of two populations cannot interbreed and produce fertile offspring.

Ex. Eastern Meadowlark/Western Meadowlark Behavioral Isolation When two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. Ex. Eastern Meadowlark/Western Meadowlark Their songs are different and so don’t reproduce.

Two populations are separated by geographic barriers. Geographic Isolation Two populations are separated by geographic barriers. Ex. Rivers, mountains, etc.

When two or more species reproduce at different times. Temporal Isolation When two or more species reproduce at different times.