1. Chapter 17: Evolution of Populations
Section 17-2: Evolution as Genetic Change in Populations

2. How Natural Selection Works
Evolutionary fitness = success in passing on genes
Evolutionary adaptation = any genetically controlled trait that increases an organism’s ability to pass along its alleles

3. Natural Selection on Single-Gene Traits
Changes allele frequencies
Ex: Body color in lizards

4. Natural Selection on Polygenic Traits
Range of phenotypes
Fitness varies throughout the curve
Natural selection can affect phenotype range, changing shape of bell curve

5. Directional Selection
Occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

6. Stabilizing Selection
Occurs when individuals near the center of the curve have higher fitness than individuals at either end
Keeps center of curve, narrows overall graph

7. Disruptive Selection
Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
Acts against the intermediate phenotype, can create two distinct phenotypes

8. Genetic Drift Occurs in small populations
Allele becomes more or less common by chance
Random

9. Genetic Bottlenecks
The bottleneck effect is a change in allele frequency following a dramatic reduction in population size
Ex: Natural disaster

10. The Founder Effect
Occurs when allele frequencies change as a result of the migration of a small subgroup of a population
Can create new populations very different from original group

11. The Founder Effect

12. Evolution vs Genetic Equilibrium
If allele frequencies in a population remain the same it is in genetic equilibrium
No evolution
The Hardy-Weinberg principle describes the conditions under which evolution will not occur
States that allele frequencies will remain constant unless something causes them the change

13. Hardy-Weinberg Principle
5 conditions can disturb genetic equilibrium and cause evolution:
Nonrandom mating – sexual selection
Small population size
Immigration or emigration
Mutations
Natural selection

14. Sexual Reproduction and Allele Frequency
Sexual reproduction alone does not change relative allele frequency