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The Natural Selection of Forks and Beans

Mike Basham
El Dorado High School

561 Canal St.
Placerville, CA 95667

This lab was developed from an activity conducted in Dr. Martin Cody's Ecology class at UCLA in 1975.

Type of Activity:

  • Hands-on (active) simulation of predator- prey and population dynamics.

  • Students must form hypotheses and use data to support conclusions regarding the interrelationships between several populations within a community.

Target Audience: Biology

Background Information:

This activity is scheduled during either of the units on Evolution or Ecology. This activity could easily be conducted as an introduction to the unit on Evolution. In either case, the effect that the environment has on phenotypic characteristics (favorable, unfavorable or neutral) is explored. Students should probably understand what a phenotypic character or adaptation is and have some understanding of how the genetic code controls phenotypic expression. Another concept that the student should be familiar with is the concept of biological carrying capacity .

In this lab, a simulated but recently changed community of several populations of predator and prey species will be examined. As a new equilibrium is established, population levels of both predator and prey will be explored. The predators (students armed with either plastic knives, forks and spoons will try to capture four different prey species; pinto beans, red beans, white beans, and split peas. The habitat for each of these populations can be either a lawn (green or brown) or a field. A different habitat space, and hopefully type, should be used for each different class.

Preparation Time

The preparation time for this lab is minimal and involves nothing more than counting which can be conducted by students or student aides.


Within every ecological community there exists a variety of several species. In stable communities each of these species tends to maintain a relatively stable population size, particularly when viewed over the long term. This condition is known as equilibrium. However, during times when environmental conditions fluctuate, a new equilibrium must be established. Such is the case in this lab where some sort on environmental perturbation has recently occurred. This lab will explore (simulate) how the forces of natural selection operate to favor certain phenotypes while limiting the success of others as a new equilibrium is established. In this experiment four different prey species (pinto beans, red beans, white beans and split peas) and three different predator species (students equipped with a cup and a plastic knife, fork or spoon) will begin at equal levels and as the forces of natural selection act on each, a new equilibrium will be established. The overall goal of this simulation will be to observe the process on natural selection as new population levels are established.

Initially, it will be necessary to insure that equal numbers of each prey species are evenly distributed within the "foraging habitat". A patch of lawn approximately 2-3 times the size of a classroom serves nicely as a "habitat". However a variety of foraging substrates should be utilized, perhaps by different classes, so that a variety of results can be obtained. Dry, brown lawns and gravel areas serve as good contrasts to healthy green lawns. Different lighting conditions at different times of day can also lead to different results. It will be necessary to use a different patch of habitat for each class. To insure that equal numbers of prey species are used, simply have your students or students lab aides count out 500 of each prey type. Alternately, 100 beans of each prey type may be counted out and massed using a balance. Conversion factors of #g/100 beans and #g/500 beans of each type will be obtained. All of the beans can then be mixed in a single bag and the teacher can then scatter them by hand, as evenly as possible over the "habitat".

To determine an equal distribution of predator species simply divide the total number of students in the class by 3. If there are extra students, one or two of them can serve as "captains" in charge of determining the population levels for subsequent generations or the teacher can also become involved as a predator. Following each predation session, it will be necessary to determine and adjust the population sizes for both predators and prey for the next generation. This can be accomplished by performing the calculations below and adding the proper number of beans of each type to the habitat for the prey species and having some students change their roles to different predator species.

To run this lab for 3-4 generations approximately 2 class periods are required. Leaving beans on a lawn until the next day usually does not present significant problems unless heavy watering or rain occurs.

To determine the population size for both predators and prey for the next generation following each predation session, and as reproduction occurs, the following formulas are used:

To illustrate the concept of "carrying capacity", the total number of prey individuals will always remain constant from generation to generation. For 4 prey species the total number will always be 2000.

% of prey species for next generation =

(the number of individuals of that species who successfully avoided predation)
divided by
(the total number of prey individuals of all species who avoided predation)

# of prey species for next generation =

% of prey species for next generation x 2000

The total number of all predator species will also remain the same from generation to generation; if there are 30 students in the class then there will always be a total of 30 predators.

% of predator species for the next generation=

(number of individuals that were captured by that predator species)
divided by
(total number of prey individuals captured by all predators)


In this activity, students actively participate as one of three predator species as they attempt to capture as many individuals as possible of four prey species. By maintaining the total number of both prey and predator individuals and by allowing reproduction to occur for successful species, the concept of biological carrying capacity can be simulated. The overall goals of this activity are as follows:

  • Students will observe how population size can vary from generation to generation in response to changing environmental conditions.

  • Students will determine which phenotypes (for both predator and prey) are most successful in a given type of environment.

  • Students will determine how populations of various predator species affect populations of various prey species and visa versa.

  • Students should prepare graphs comparing population size and generation for both predator and prey populations.

  • plastic knives, forks, and spoons (15 each per class)
  • paper cups (one per student)
  • suitable "patch" of habitat (any outdoor area)
  • 1-2, 1 kg bags each of pinto, red, and white beans and split peas
Each foraging session lasts 3-5 min.

Student Information | Generation 1 | Generation 2 | Generation 3 | Generation 4 |

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