Lab 2: Simulating Evolution by Natural Selection SYNOPSIS

In this lab you will simulate the process of natural selection by taking the role of predators hunting for prey. Predators (you) will have varied feeding morphologies, and prey (beans) will have varied colors and shapes. As predators hunt for prey and as both predators and prey “reproduce,” you will observe and interpret how the distribution of traits in each population changes.


At the conclusion of this lab you should have a clearer conception of how the process of natural selection works, and you should better understand how this process can drive evolutionary change in populations. You should also be able to graph the change in frequency of a trait over time.


Evolution is the central idea that unifies all of biology. As the geneticist Theodosius Dobzhansky once remarked, “Nothing in biology makes sense except in the light of evolution.” As you have learned in lecture, natural selection is one of the major processes driving evolutionary change. Before Darwin, many biologists had considered the idea that populations of organisms evolve, or change in their characteristics through time, but these early scientists lacked a feasible mechanism or causal explanation. Charles Darwin and Alfred Russel Wallace then put forth a theory, evolution by natural selection, which was both logical and testable. Once combined with Gregor Mendel’s insights about genetics and inheritance, this enabled us to understand how and why organisms look and behave they ways they do, how organisms adapt to their environments, and how traits in populations change through time.

In order for a trait to evolve by natural selection, three conditions must hold:

1) Theremustbevariationinthetraitamongindividualsinapopulation.

2) The trait must be heritable (able to be inherited). This means it must be

encoded in the genes, and can be passed from parents to offspring.

3) There must be differential reproductive success among individuals, based upon the variation noted in condition (1), such that those with one form of the trait survive to produce more or more fit offspring than those with another form of the trait.


These three conditions are necessary and sufficient to result in evolution by natural selection; if these conditions hold, then natural selection will take place. Individuals that leave the most descendants also pass the most genes to future generations, so the genes of the more successful individuals will be better represented in the population in the future.

Thus, while natural selection acts on an individual’s phenotype (its morphological, behavioral, or physiological traits), this causes change through the generations by affecting the genotype (the alleles, or gene copies, that underlie phenotypic traits). As particular genotypes become more prevalent in a population through time, their corresponding phenotypes become more prevalent as well. We most often notice the effects of evolution by natural selection by observing changes in phenotypes. However, most phenotypic traits are determined by some combination of genetic, environmental, and developmental influences, so strictly speaking, evolution by natural selection is best documented by measuring change in gene frequencies through time. The extent to which a phenotype reflects a genotype determines how effectively natural selection can change allele frequencies in a population.

A characteristic that promotes reproductive success (directly or through longer survival) is termed an adaptation. Evidence for natural selection is all around us in the adaptations of animals, plants, and microbes to their environments. Some of the evidence most familiar to us comes from artificial selection, natural selection guided by human influence. By selectively breeding organisms, we have selected for specific traits in dogs, cats, cattle, pigeons, and the countless crop plants we depend on for our diet. By directing the dynamics of selection, we have invented agriculture and have created hundreds of varieties of organisms that are useful and appealing for our society.

While the end results of selection are readily observable all around us, actually witnessing natural selection as it takes place in real populations is tricky. Evolutionary changes generally accumulate over many generations, which can be a long period of time (on a human time scale). Moreover, natural selection usually acts in concert with other evolutionary forces, such that it can be difficult to isolate and identify.

Running a simulation lets us observe how natural selection works in a real population. All we need is a system that shows the three conditions for evolution by natural selection outlined above. With a little imagination we can get almost anything to replicate these conditions….

Imagine that beans are organisms that live out their lives in two habitats: (1) in the mountains, on alpine slopes that are often covered by snow, or (2) within the adjacent sun-dappled forest. The only concern our hypothetical bean creatures have is the disquieting presence of ravenous predators (that would be you). Beans vary in size, shape, and color, and any given bean’s phenotype could potentially influence the likelihood that it gets snatched away by a hungry predator. The predators in our simulation vary as well, having different implements with which to capture the beans. Different feeding morphs — different predator phenotypes — might potentially vary in their ability to capture beans. Both predator and prey “reproduce”. Beans and


predators born into their populations “inherit” the traits of their parents. With this simple set-up, we should be able to replicate evolution by natural selection.


You and your classmates are all individuals in a population of Beaneaters (Biostudentis beanivorus), a species of predator that survives by hunting for a prey species known as Beans (Beanus polymorphus). In this predator-prey simulation you will attempt to capture and “eat” as many Beans as you can within the time allotted.

Getting Set Up

Your Beaneater population will be split into five subpopulations (one per lab table), with each table containing an area of habitat that is home to your tasty Bean prey. Three tables are of one habitat (HABITAT A = sun-dappled forest), while two tables are of the other habitat (HABITAT B = snowy alpine slope). Four Beaneaters will compete for food at each table, and any additional students will serve as “Roamers” who will help the T.A. patrol the groups watching for infractions of the rules.

The beans come in four different phenotypes that vary in color, size, and shape. Examine these bean morphs in conjunction with the habitat of your table. Formulate a hypothesis as to which bean type will be easiest to prey upon. Now propose a hypothesis as to which predator will be most successful. Write your hypotheses in the spaces allotted for Questions 1 and 2 in this lab handout, and explain why you made the choices you did.

Your T.A. will give each table a supply of beans that has equal numbers (100) of all four phenotypes (for a total of 400 beans). Mix and spread these evenly across your table’s patch of habitat.

Now it’s time to arm yourselves as predators! Beaneater phenotypes are determined by their feeding appendages. In our simulation, we will have four different feeding types: forks, knives, spoons, and forceps. Your T.A. will distribute the feeding appendages so that there are equal numbers of the four phenotypes in the initial predator population (this will mean one of each type per table).

Now, pick up a cup. This will serve as your “stomach.” You will forage by picking up beans ONE AT A TIME using ONLY your feeding appendage and placing each bean in your “stomach”. You must follow the rules below for feeding.

Using the data sheets provided, record the initial number and frequency of each prey and predator phenotype at your table.


Running the Simulation

The rules are as follows:

Stomachsmustbeheldinonehandandfeedingappendagesintheotherhand at all times.

Stomachs must remain upright. You may not tilt them or use them to scoop up beans.

Beans cannot be swept off the table into stomachs. Stomachs must be kept above the level of the table.

Beans must be deposited in stomachs ONE AT A TIME. If you accidentally place multiple beans into your cup at once, you must stop and remove all beans that you captured on that particular scoop.

Onceabeanislegallyinastomach,itcannotberemovedbyacompeting Beaneater.


Don’t be shy about ruthlessly competing with your fellow predators . . . like spooning a bean away from a clumsy fork. Remember — you are in a race to survive and reproduce, and only the fast, agile, and clever will succeed!

At your T.A.’s signal, start feeding!
When your T.A. calls TIME, stop feeding. (Each feeding bout last 60 secs.)

Count the number of each type of bean in your stomach. Enter these data and those of your table’s fellow Beaneaters into the data sheet provided for “Prey Consumed”.

Now figure out how many beans of each phenotype remain “alive” (uncaptured in the habitat) at your table, and enter these numbers into the data sheet.

It is now time for the beans to reproduce. For each bean of a given phenotype that remains uneaten on the table, add one additional bean of that phenotype. Thus, the number of beans of each type will double. For example, if 7 lima beans remain after your first round of predation, the number of lima beans would double to 14 to begin the second generation. Record the number of each phenotype of bean starting this second generation and enter it in your data table.

Among Beaneaters, the individuals who caught more prey than average in the first generation get to survive and reproduce, passing their genes to the second generation. In contrast, the individuals who caught less than average die of starvation, and do not reproduce. Each surviving predator “reproduces” one offspring by bestowing a replicate of his or her feeding appendage on an empty-handed classmate. For example, if Knife and Forceps do better than Fork and Spoon at your table, then in the next generation the Fork student and the Spoon student are “reincarnated” as a Knife and Forceps. (Alternatively, “Roamer” students can be given a chance to compete as Beaneaters, and losing Beaneaters can take a turn as Roamers.) Your T.A. will distribute the new


feeding appendages accordingly. Calculate and record the new number of each feeding type in this next generation of predators.

Run the second round of feeding as you ran the first round, and repeat all the steps of bean reproduction, Beaneater reproduction, and data entry as described above.

Finally, run a third round of hunting using the new distributions of bean and Beaneater phenotypes as calculated for the third generation, and enter all your data. At the end of the simulation be sure to count out beans and replace into bags as they were at the beginning of lab.

Interpreting the Data

Once all three generations of the simulation are over, your T.A. will reconvene the class so that tables can share their results. Your T.A. will lead you through the compilation of data from each of the two habitats. Follow along and record the compiled data, and determine frequencies of each of the different phenotypes.

Across the three generations you should notice changes in the frequencies of some bean phenotypes and some predator phenotypes. To clearly visualize these changes, you will graph some of these data in line plots on the axes provided:

Plottheincreaseinfrequencyofthemost-successfulpredatorphenotypeinthe first habitat type over the three generations.

Plottheincreaseinfrequencyofthemost-successfulpredatorphenotypeinthe second habitat type over the three generations.

Plotthechangesinfrequencyofthefourphenotypesofbeansinthefirsthabitat type over the three generations.

Plotthechangesinfrequencyofthefourphenotypesofbeansinthesecond habitat type over the three generations.

Foryourhabitat,usehistograms(barcharts)tographthechangesindistribution of the four phenotypes of beans over the three generations.

Finally, your T.A. will lead a discussion of the questions below. As you discuss these issues, think about your experiences in this lab simulation, and consider how the dynamics of the game may be similar to (or different from) the forces of natural selection acting on organisms in the wild. Write in answers to these questions during or after this discussion. You will hand in your typed answers to these questions (15 pts) and your graphed data (5 pts) at the beginning of your next lab. Your TA will provide you with additional instructions regarding your lab write-up.


Answer the first two questions before you begin the simulations:

(1pt)Formulateahypothesisastowhichbeantypewillbeeasiesttopreyupon, and explain why you think so.

(1pt)Proposeahypothesisastowhichpredatormorphologywillbemost successful, and explain your choice.

Answer questions 3-8 after graphing the data compiled from the simulations:

(2pts)WasyourhypothesisinQuestion1supportedorfalsifiedbythedata? Which bean type was least successful (preyed upon most easily) in your habitat, and why? What happened to the frequency of this phenotype in the population?

(2pts)WasyourhypothesisinQuestion2supportedorfalsifiedbythedata? Which predator type was most successful in your habitat, and why? What happened to the frequency of this phenotype in the population?

(2 pts) Which prey type was most successful in your habitat, and why? Was this true in both habitats? Explain these results.

(2pts)ArethereanycharacteristicsofsuccessfulBeaneatersthatyouwouldcall an adaptation? Can you name any adaptations of successful Beans?

(2pts)Doesnaturalselectiontendtoincreaseordecreasevariationwithina population?

(3pts)Howdideachofthethreeconditionsfornaturalselectiongiveninthe introduction to the lab play out in the simulation?