The process of biological evolution can be accurately defined as “descent with modification.” This definition includes microevolution (changes in allele frequency of a population over time) and macroevolution (the descent of different species from a shared common ancestor over many generations). Evolution relies on four processes that function as the basic mechanisms of evolutionary change:

1. Mutation. Mutations are the ultimate source of variation in a population, resulting in changes in the genetic makeup of an individual.
2. Migration. The allele frequency of a population can change if members of an existing population leave, or new members join.
3. Genetic Drift. Genetic drift happens when allele frequencies change due to purely random factors. For example, if a person accidentally stepped on a population of beetles and randomly killed all the brown beetles in the population, the allele frequency of the population would certainly change, but the cause of the change is completely random. This is an example of genetic drift. It is most significant in small populations.
4. Natural Selection. Charles Darwin based his theory of natural selection as the driving force for evolution from the following observations:
   1. Reproduction. Species reproduce in excess of the numbers that can survive.
   2. Variation. All sexually reproducing species vary in characteristics.
   3. Heredity. Traits can be passed from one generation to the next.
   4. Fitness. Those individuals with hereditary characteristics that have survival value, i.e., improved fitness, are more likely to survive and reproduce compared with less fit individuals. Be careful here, because the word “fitness” does not refer to physical fitness or healthiness! This word is being used in a very specific way to mean “successful reproduction.” Fit individuals make more babies. This is not necessarily true of our common use of the word!

   Put Darwin’s observations together and you get natural selection, a process favoring survival and reproduction of the most fit individuals in a population. Natural selection can be viewed as differential reproduction. Individuals with favorable characteristics have a competitive advantage and are more likely to pass their genes on to the next generation.

If these four processes are coupled with reproductive isolation, then speciation (the formation of a new species) can occur. Reproductive isolation occurs by some mechanism that can isolate diverging populations so as to prevent interbreeding. Given sufficient time, a population that is isolated from the original population can diverge physically and/or behaviorally to the point where it is a distinct species. There is a variety of isolating mechanisms that can prevent gene flow from occurring. One example is the presence of geographical barriers such as mountain ranges or islands that prevent gene flow between separated populations.

Part 1: Natural Selection Exercise—Generation 1

This exercise illustrates the effect of natural selection on populations of predators and prey. Students, in groups of four, will represent predators, each with a different adaptation for capturing their prey. The prey will consist of different species represented by different colored beans.

Procedure

1. Each team of 4 students will count out exactly 100 dried beans of each color.
2. Thoroughly mix the beans and spread them evenly over your “habitat.” Your habitat depends on the weather.
   1. If the weather is poor, it is dark outside, or your instructor would rather, your habitat will be a tray of sediment in the classroom.
   2. If the weather is lovely, or your instructor is adventurous, you will do this lab outside. Each team will mark off a 1m × 1m “habitat” in the grass using yarn, a meter stick, and wood stakes.
   3. All “prey” are confined to the habitat, wherever it is!
3. Each student (predator) will have a different feeding apparatus: A fork, spoon, knife or forceps.
4. When everyone is ready, predators will spend 60 seconds capturing prey with their devices and depositing them into a cup while obeying the following rules:
   1. Predators must only use their capture device to capture prey.
   2. Predators may not scoop prey up with their cup.
   3. If predators “eat” too much of the environment, they will become constipated and DIE.
5. Each predator determines the number of prey captured and records results in Data Sheet: Generation 1.
6. Calculate and fill in the remaining statistics on the data sheet (see example below).

Data Sheet: Generation 1

Example of Data Collection and Analysis for Generation 1

# of This Bean That Survived = population size – total kills

% of This Bean That Survived = (# survived/population size) x 100

% Total Population = (# survived/total survived) x 100

Part 2: Natural Selection Exercise—Generation 2

The predator with the lowest capture percentage will go “extinct” and will not participate in the next exercise. The predator with the highest capture percentage will reproduce itself and the “offspring” will participate in the next exercise. The surviving prey will also survive and reproduce.

Procedure

1. The person with the lowest capture percentage (as calculated in the previous exercise) will “die” and turn in their feeding device.
2. The person with the highest capture percentage will reproduce by having the “dead” person use their same feeding device in the next round.
   1. If the Fork won the first round and the Spoon lost, then in the second round, there will be two Forks and zero Spoons. There will also be one Knife and one pair of Forceps.
3. Now the surviving prey will reproduce and double.
   1. If there are 40 surviving black beans, you will add another 40 black beans to the habitat, so there are a total of 80 black beans in the habitat for round 2.
4. Repeat the procedure you carried out in Part 1. Collect data for Generation 2.

Data Sheet: Generation 2

Note: For population size in generation 2, multiply the number that survived in generation 1 by two.

Part 3: Natural Selection Exercise—Generation 3

The winning predator will reproduce again and the surviving prey will also reproduce (just like they did in the previous exercise). Collect and record new data.

Data Sheet: Generation 3

Part 4: Pie Chart Analysis of Predator and Prey Populations

Now that you have collected data from three generations of predator and prey populations, you will use the data to create a set of pie charts to help you interpret your results. The first pair of pie charts represent the data from the original predator and prey populations. Use these examples to create your own charts using your group’s data.

End of First Generation

End of Second Generation

End of Third Generation