1. Chapter 17.2: Evolution as Genetic Change in Populations

2. Natural Selection on Single-Gene Traits
Natural selection selects for traits
Genes create traits
There can be a wide variation in traits and genes
Some traits are a better fit for the environment than others, so the genes that create them are also a better fit

3. Directional Selection
Polygenic traits, like height or size, have many variations
One extreme phenotype in the range of variation has higher fitness than others
This phenotype is favored and selected for

5. Stabilizing Selection
Individuals with intermediate (medium, average) phenotypes have higher fitness than others
This phenotype is favored and selected for

7. Disruptive Selection
Individuals with both types of extreme phenotypes have higher fitness than others
These phenotypes are selected for, and the intermediate phenotype is selected against

9. Genetic Drift Is a random change in allele frequency
An allele can become more or less common in a population by chance
Occurs more frequently in small populations
Can be caused by human actions, disease, or natural disasters

11. Bottleneck Effect
Is a change in allele frequency following a catastrophic reduction in the size of a population
Can sharply reduce a population’s genetic diversity

13. Founder Effect
Is when a small group from one population leaves and founds a new population elsewhere
The founding group that left may have different allele frequencies than the original population