Candy Bowl Predation

In this activity, students will:

1. experience how selective predation is a form of natural selection.
2. create and interpret predation data.

• fitness: the ability of an organism to survive and produce viable offspring in a particular environment; or the reproductive success of an organism in a particular environment
• genotype: the genetic makeup of an organism; all of the genes and alleles of an organism
• natural selection: the process by which organisms that are better adapted to their environment survive and reproduce at a higher rate than those that are less adapted, resulting in the survivors’ characteristics being more prevalent in subsequent generations. Therefore, natural selection produces changes in the genetic composition of a population from one generation to the next so that alleles that are beneficial for survival become more common, while alleles that are not beneficial decrease. Originally proposed by Charles Darwin, natural selection forms the basis of the process of evolution.
• predation: the act of an organism capturing and feeding on others. 
• selective predation: a behavior in which a predator attacks only individuals with certain characteristics in a population of potential prey.
• phenotype: all observable physical and functional characteristics of an organism; it is the product of the genotype and environmental influences

1. Make the candy dish accessible in advance so students can pick candies over a period of time, or you can pass the dish around the room a couple times. You can avoid commenting about it at all, or you can make very innocent remarks about providing a treat for the students. But, do not draw too much attention to the candy.
    
2. After more than half of the candy has been removed, gather the class together. Start the discussion by pointing out that there is often great variation among individuals of animal species (and candy). For example, students can look around the room and list the characteristics that vary among humans. Then, ask the students why variation might be significant. (One reason variation is important is that variation allows for differential survival of individuals, but your students may not reach that conclusion just yet.)
    
3. Reference the candy bowl and the remaining candies, informing students that—perhaps unbeknownst to them!—these candies represented prey items. Because of our predation, some of the prey survived and some died.
    
4. Count which candies remain and list them on the board. Ask the students if they remember which candies were originally available. Make a list on the board of the original set of candy.
    
5. Inform students that you secretly counted the number of each candy before class.
    
6. Make a bar graph that compares the candy before and after students made their selections.
    
7. Now ask them to list the traits of the candy they chose from the candy dish. (Examples include: chocolate flavor, large size, favorite brand, etc). These are the traits that led to the removal of certain candies.
    
8. Make a list now of the traits of the candies that were not chosen (examples: bad flavor, small size). These are the traits that allowed the candies to survive being passed around the room.
    
9. Ask students what their predation did to the population. (It decreased the candy population size, but it also changed the composition of the candy population.)
    
10. So, the fact that there were different candies with different traits resulted in some candies being eaten and others surviving. This is what selective predation (a form of natural selection) does with individuals in a population and between populations.

Evolution is a two step process. First, there must be variation among individuals in a population. Secondly, natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes. The genotype of the favorable phenotype, will increase in frequency over the following generations Some individuals are more fit in a particular environment and are thus able to pass on their genes to subsequent generations more successfully than other individuals. This process drives evolution within a population. Since it is crucial to both survive and reproduce in order to pass on genes to the next generation, natural selection results from both the differential survival and the differential reproductive abilities of individuals.

There are many forces that affect natural selection. For example, an organism’s environment can change, making some individuals more fit to survive or reproduce than others. Or, a predator can exert selective pressure on a population. Because predators do not capture their prey randomly, but often exhibit preferences, predation can change the composition of a prey’s population. Predator preference is thus an agent of natural selection.

• Adapted from Tang, C. Candy Dish Selection. This activity was first suggested to Dr. Tang by Jack Laws, associate at the California Academy of Sciences.
   
• Purves, W. K., Sadava, D., Orians, G. H., & Heller, H. C. (2001). Life: The Science of Biology (6th ed.). Sunderland, Massachusetts: Sinauer Associates, Inc.