Chapter 17.2: Evolution as Genetic Change in Populations

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

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

Stabilizing Selection

- Individuals with intermediate (medium, average) phenotypes have higher fitness than others

- This phenotype is favored and selected for

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

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

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

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