Introduction to Biology
Natural Selection Lab Activity
Natural Selection, the Darwinian mechanism for evolution, proposes that the environment “selects” those
phenotypes that provide enhanced survival, allowing individuals endowed with more advantageous traits to
leave behind more offspring, thus changing the gene frequency in a population. The ability to survive well
and produce more offspring is measured as “fitness”.
This mechanism could work to increase the numbers of “average” individuals for a trait if that phenotype
provided the highest degree of fitness (stabilizing selection); or it could move the average phenotype
toward one extreme (directional selection), or it could split the population into two or more groups of
advantageous phenotypes (disruptive selection).
In this exercise you will “measure” the fitness of several phenotypes and predict any change in the
frequency of phenotypes in a population over time.
After completing this exercise, you should be able to:
1. Understand the concept of Natural Selection;
2. Understand the concept of Fitness;
3. Predict how selection pressure may change the frequency of genes governing advantageous or
PROCEDURE: PART A: Different beaks, same environments. (use the checkboxes to track
1. Close your eyes and pick a beak out of the box.
2. Sketch and describe your beak on the data sheet, Part A.
3. While your partner times you for 20 seconds, pick up as many food items from the table as you
can using your beak.
o You may only LIFT the food off the table, one item at a time. NO SCOOPING!
4. Count the number of beans you have collected and fill in the table on the board and copy onto
your data sheet.
5. Repeat 20 second rounds 4 more times for a total of 5 rounds.
DISCUSSION: PART A: Different Tools, Same Environment
1. What does the grab box represent in terms of evolution, genetics, and inheritance?
2. What does each beak type represent (remember, this is a single species population)?
3. What do the beans represent?
4. What did the number of beans collected represent? What is the average number of beans collected by all
phenotypes (use class data from table on board)?
5. Your fitness equals the average number you collected divided by the class average. Record your
fitness on the board.
6. What would happen to the numbers of individuals with your “phenotype” in the population after several
generations as a result of the number of beans you collected?
7. Of the phenotypes tested, was there more than one type that performed its task well? What types were
they, and what attributes did they share?
8. If the most fit phenotype had already evolved to account for the largest number of individuals in a
population (say X number), and the other phenotypes represented fewer individuals in proportion to their
fitness, what would a bar graph look like that represented the number of individuals of a given phenotype
(dependent variable) measured against the variations within the trait (independent variable)?
If the most fit type had a fitness of 1.2, and another type had a fitness of .8, then the less fit type should
represent .8/1.2 or .67 (67%) of the number X assigned to the most fit individual. Let’s make X = 100.
PROCEDURE: PART B: Different Tools, Different Environments
1. Select a beak from the box based on your perceived notion of its ability to enhance your fitness.
Oh, by the way, you won’t know your environment until after you have a tool! Sketch your beak on
the data sheet, Part B.
2. Now you will be assigned an environment. Describe the changes in your new environment
compared to the first environment.
3. Develop an hypothesis which predicts whether your type will survive or die out in the new
4. Collect food for 5 rounds of 20 seconds each in the new environment. Record your results on the
board and in the data table, Part B.
1. What was the average number of items collected by all individuals?
2. Compute your fitness. Record it on the board.
3. Did your fitness confirm or refute your hypothesis? Explain.
4. What “phenotype” worked best in its environment? Would it have worked best in all the environments?
5. If the population at the beginning of this part had been composed of proportions of phenotypes the same
as that described in #8, Part A, would you expect the numbers of each phenotype to 1) remain the same,
2) change toward one extreme or the other, or 3) divide into more than one phenotype of higher fitness in
your new environment?
ASSIGNMENT: Natural Selection Lab Activity
Analyze the several ways for an environment to select phenotypes that aid in survival and increase fitness
(stabilizing, directional, and disruptive selection – see text for help). Use the information you developed in
the two parts of this lab to support your analysis.
Answer the following two-part question at some point in your analysis:
1. Do phenotypes necessarily arise (evolve) as a result of the environment, giving the impression that a
species somehow perceives what it needs to be like in order to survive better in that environment?
2. Or, are some randomly occurring phenotypes better at performing a specific task than others
depending upon in which environment they find themselves? Is there a “guarantee” that a better
phenotype will appear?