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Lines of Evidence
for Past Change

The Importance of Variation

2. Variation and Selection

What is evolution?

Evolution is a change in frequencies of inherited characters in a population over time.

An Analogy Using the Candy Dish Selection:

Unbeknownst to the teachers, the dish filled with different types of candy was part of an experiment to illustrate selective pressures at work. At the start of the session, there were a variety of candies available in the dish; there was an equal number of each type of candy. At the end of the session, the bowl contained the following amounts of each type:

Reese's cups 0
Hershey's minis 0
Candy corn 5
Good & Plenty's 12 and 14
Candy Canes 15
Candy necklaces, rings, watches 1 of each (started with 2 of each)

a) What were the traits of the candies that you ate?

Chocolate, creamy, size, location in the bowl, nostalgia, wrapped

b) What were the traits that led you to avoid certain candies?

Age, cavity-causing potential, flavor, lack of chocolate, camouflage

Discussion can look at variation of traits and differential survival rates. The traits listed in (a) led to the disappearance of certain types of candy while the traits listed in (b) led to the continued survival of the candies in the bowl. If there were no variation, however, we wouldn't see this difference in selection. The activity can be extended by having all "surviving" candy "reproduce" to exaggerate the effect; then discussion can focus on change in frequencies in populations and changes over time.

Variation leads to different "survival" rates, which leads to changes in frequencies in a population, which is evolution.

More on Variation: Variation within a Population | Activity: Candy Dish Selection | Variation and Genetic Drift

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