The Natural Selection of Forks and Beans
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.
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.
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
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.(the number of individuals of that species who successfully avoided predation)
% of prey species for next generation =
(the total number of prey individuals of all species who avoided predation)
% of prey species for next generation x 2000
# of prey species for next generation =
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)
(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.
Each foraging session lasts 3-5 min.
- 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
Student Information |
Generation 1 |
Generation 2 |
Generation 3 |
Generation 4 |