1. 16-2 Evolution as Genetic Change

2. Directional selection Form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve.

3. Stabilizing selection Form of natural selection by which the center of the curve remains in its current position; occurs when individuals near the center of a distribution curve have higher fitness than individuals at either end.

4. Disruptive selection Form of natural selection in which a single curve splits into two; occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle.

5. Genetic drift Random change in allele frequencies that occurs in small populations

6. Founder effect Change in allele frequencies such as a result of the migration of a small subgroup of a population.

7. Hardy-Weinberg Principle Principle that allele frequencies in a population will remain constant unless one or more factors cause the frequencies to change.

8. Genetic Equilibrium Situation in which allele frequencies remain constant

9. Key Concept Natural selection on single-gene traits can lead to changes in allele frequencies and thus evolution.

10. Key Concept Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.

11. Key Concept In small populations, 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.

12. Key Concept Five conditions are required to maintain genetic equilibrium from generation to generation: (1) there must be random mating: (2) the population must be very large; and (3) there can be no movement into or out of the population, (4) no mutations, and (5) no natural selection.