Technology Lesson: Virtual Experiment in Evolution
Premise: This lesson is designed to be used as part of an introductory biology course. It is formatted to be used in a 1.5 hour block period and should take place during a unit on evolution. Prior to this lesson, students will already have been introduced to Darwin’s theory of evolution, along with the ideas of adaptation and natural selection. This lesson will give students the opportunity to review what they know about evolution, natural selection and adaptation. They will use computers to carry out a simulation of evolution at work in guppy populations. Students will also be introduced to the idea of sexual selection. The lesson will cover material required under the NC Standard Course of Study biology sections 3.05 and 4.03 (sections covering evolution and adaptation). It will involve internet resources that can be found at http://www.pbs.org/wgbh/evolution/
Goals:
1. Students will review and understand the theory of evolution by natural
2. Students will apply the concepts of evolution, adaptation and natural selection to an internet simulation of guppy populations.
3. Students will become familiar with the concept of sexual selection.
Objectives: Following this lesson the student will be able to…
1. use an internet simulation to explore guppy evolution.
2. name at least three predators of guppies.
3. list at least two benefits/disadvantages of different coloration for a male guppy.
4. analyze the results of a guppy simulation.
5. explain the difference between natural selection and sexual selection.
6. apply knowledge of male guppy coloration to other situations.
7. use the terms evolution, natural selection, sexual selection and adaptation to discuss the results of a simulation.
Anticipatory Set: Show an internet video clip of a scientist explaining how a species of praying mantis has adapted to look like a leaf. This clip can be found at: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_03.html
Following this short (1 min.) video have students offer ideas of why the praying mantis species would have this adaptation, and how this is an example of natural selection at work. As students brainstorm ideas, jot them down on a whiteboard/blackboard. (time: 10 minutes; supplies: data projector, laptop computer, pens or chalk)
Statement of Goals: Tell students that during this lesson they will get the opportunity to apply their knowledge of evolution, adaptation and natural selection to the analysis of virtual populations of guppies (small fish that live in streams). More specific objectives will be listed for students on the worksheet accompanying the simulations. Do not mention sexual selection at this point, as students will be discovering this phenomenon as the simulations progress and you will discuss it as a class after they have completed the simulations. (time: 5 minutes)
Computer Activity/Guided Practice: Pass out the worksheet to be used with the guppy simulations. Take the class to the computer lab. Depending on the number of computers available, students can work individually or in pairs. Students should follow the directions on the handout, which will direct them to go to the website: http://www.pbs.org/wgbh/evolution/sex/guppy/index.html
At this website, students will read about research that evolutionary biologist John Endler did on populations of guppies. Endler noticed that the coloration of male guppies living in streams in Trinidad seemed to differ depending on what part of a stream they lived in and what predators they were exposed to. He decided to set up experiments in controlled streams to see if he could get the coloration of the males in guppy populations to change over successive generations by exposing them to different combinations and numbers of predators. After reading background information about Endler and guppies, students will answer a few questions on their worksheets. Then, they will carry out three different simulations on experimental populations of guppies (directions for the simulations are found on the worksheet). Following the simulations, students will answer a number of questions about the guppy experiments and their implications. (time: 45 minutes; supplies: worksheets, computers)
Checking for Understanding: As students work, circle the room making sure that they are not having trouble setting up the simulations. If students are confused with the website/simulations, the teacher might use a data projector to pull up the website on a big screen and model how to get to the various sections of the site. (optional supplies: data projector/computer)
Full Class Discussion/Instructional Input: Once students have completed the simulations and answered the questions on the worksheet, bring the class back together for a discussion of the simulations. Go through the questions on the worksheet and ask students to share their responses. (time: 25 minutes; supplies: data projector/computer, white board on which to write down students ideas about the simulations)
Over the course of the discussion, be sure to clear up any misconceptions and make sure that students understand the central concepts:
1. During simulation #1, when a guppy population with males displaying an even mix of bright and drab coloration types were exposed to 30 rivulas (a predator fish), the average coloration of males over the course of many generations became brighter. Why? This may seem counterintuitive to students at first. However, it can be explained by sexual selection. The female guppies prefer to mate with brightly colored males. In simulation #1 there were not enough predators (or the rivulas were not likely enough to eat the guppies) to offset the reproductive advantage of being a brightly colored male guppy.
2. In simulation #2, a guppy population that again started out with males displaying an even mix of bright and drab coloration types were exposed to 30 rivulas and 30 acara (predator fish). After a number of generations, the favored coloration types are “bright,” followed by “drab.” However, the number of “brightest” and “drabbest” males has gone down. In other words, more average coloration is favored rather than the extremes, and being bright is still generally to a male’s advantage. This indicates that the number of predators was still too low (or the types of predators not hungry enough for guppies) to give very drab males a genetic advantage over their brighter counterparts. However, in this simulation it is better to be a slightly drab male than it is to be very bright.
3. In simulation #3, a guppy population that again started out with males displaying an even mix of bright and drab coloration types was exposed to 30 rivulas, 30 acara and 30 cichlids (predator fish). This time something new occurs. After a number of generations, the majority of males are very drab in coloration. This indicates that in the presence of this larger number and greater variety of predators, it is no longer to a male guppy’s advantage to be bright at all; he is too likely to be eaten that way. In this situation, the natural selection working against bright coloring is more powerful than the sexual selection working in favor of bright coloring.
4. Explain the concept of sexual selection. Show a video clip (4 minutes) that demonstrates sexual selection at work in peacocks: http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html
5. Be sure that students understand the difference between sexual selection and natural selection. Natural selection occurs when members of a population that have a more favorable trait variety are more likely to survive, reproduce and pass on the favorable trait to offspring. Sexual selection is a special type of natural selection that occurs when competition for mates drives the increase in frequency of a trait among one sex (usually male) that causes it to appear more desirable to the other sex (usually female).
6. In the guppy simulations both sexual selection and more traditional natural selection are occurring and driving evolution.
7. Can the students think of reasons why female guppies would always be drab in coloration?
8. Can students think of other species where sexual selection plays a role?
9. Be sure that students understand how coloration worked as an adaptation for the male guppies in the simulations. Sometimes, drab coloration was a beneficial adaptation that acted to camouflage the guppies from predators. Other times, bright coloration was a beneficial adaptation that acted to make the male guppies more attractive to females.
Closure: Before students leave, pass out slips of paper. Ask each student to think about an adaptation that might develop in an animal species through natural selection (this can include sexual selection). For example, the manes of male lions help to attract mates. Have students write their examples on the slips of paper and hand them in as they leave class. These ideas can be compiled on a transparency and used as a review during the anticipatory set for the next class. (time: 5 minutes; supplies: small slips of paper)