GENETIC DRIFT AND SELECTION

Drawn to Scale Flat Tire High Chair

GENETIC DRIFT AND SELECTION Genetic Drift: Chance events (not Natural Selection) that change the genetic composition of a population  These random events change the allele frequencies over time.  They reduce the amount of genetic variation in the population  Can cause fixation of one allele (One allele occurring over and over) Target: I will be able to define genetic drift, explain how founder and bottleneck effects result in genetic drift, and identify/predict the effects of the 3 selection pressures. Pg. 71

Two types of Genetic Drift: 1. Founder Effect: The loss of genetic variation when a new colony is formed by a very small number of individuals (founders) from a larger population. 2. Bottleneck Effect: The dramatic loss of genetic variation due to environmental effects or human activities.  Ex: Earthquakes, fires, droughts, over hunting, destroying habits

Three types of Selection: 1. Stabilizing Selection- The extreme traits in a population are selected against.  Favors the common, or average traits in a population  Result: Reduces the extremes in a population

2. Directional Selection- One extreme trait is selected for.  Result: The population's trait distribution shifts toward the other extreme

3. Disruptive Selection- The extremes traits are both selected for.  Result: Reduction in the individuals with intermediate traits in the population  This type of selection is not as common as the first two.

Let’s practice: Finches on the Galapagos island of Isabela can have beak sizes that range from small (best for cracking open small seeds) to large (best for cracking open large seeds). Average-sized seeds become less common and larger and smaller seeds become more common. What kind of selection is this an example of? Disruptive Selection How will the population will change: The species will change so that birds have large beaks and small beaks, but not medium beaks.

In a species of plants, the plants that grow too tall get broken in the wind and plants that are too short don't get enough sunlight. What kind of selection is this an example of? Stabilizing Predict how the population will change: The species will change so that the plants will be medium height. Summary:

THREE TYPES OF SELECTION On the top of each flap write the names of the three types of selection. On the underside of the flap describe what is happening during each type of selection. On the notebook paper below, draw a picture of each type of Natural Selection. *Use a minimum of 4 colors* Pg. 70