One 45-50 minute class period. (Could be shorted, depending on level of students)
Sexual vs. asexual. Sex was "invented" near the beginning of life and both sea urchins and humans use this strategy. What are the adaptive advantages of sexual reproduction?
This simulation is designed to demonstrate that sex is simply a way of mixing up the gene pool and why this is an advantage.
Simply put, the disadvantage of sex is the need for males. They consume resources but cannot reproduce and they are never around when you need them. (Remember in asexual reproduction every individual can reproduce). See the Calculator for sexual vs. asexual.
The advantage of sexual reproduction is that mixing the gene pool produces diversity, thereby thwarting parasites and predators who can not adapt instantly to the altered host or prey.
The case of the cichlid in the book, The Beauty of the Beastly "Plenty of Fish in the Sea", shows how one species can become 300 in as short as 200,000 years. This rapidly evolving diversity limits a parasite or predator's success.
To understand how it is thought that sexual reproduction developed see A Walk Through Time, p89-94. More information on the above books may be found here.
What follows is a proposal for a "game" to help illustrate some of the advantages and disadvantages of sexual reproduction. This game is highly adaptable and can be played as class activity led by the teacher, as a class split up into competing teams, or as a single individual working on their own. The background information should be given in a lecture or a discussion ahead of time so students can be looking for examples in the game (brainstorm to asses prior knowledge?). The teacher should stop play when a point is illustrated and discuss the implications/significance.
We had a great deal of trouble writing this lesson. The idea of "mixing the gene pool" is fairly straightforward, the trick is to devise a way of showing this simply. Many variations were tried before this final version. The trouble was in the biology that was not explicitly shown and not intended to be part of the lesson.
TIP: Make sure all of the biology for the lesson is correct. It will not help your students if when you get your point across, unknowingly, you teach them three others that are wrong.
There are three "pieces" to the game:
- The Game Cards – there is one for sexual and one for asexual. These can be enlarged or projected easily. Print the cards onto a transparency material to use as an overhead.
- The "Genes" – the various phenotypes are represented by a "cast of characters". A character sheet is provided so you can make the many game pieces you will need.
- Scenarios – print these out and use as is or adapt to help make the point you are trying to get across.
There are a number of ways to run this game. It is very adaptable to different learning styles and level of students. You will want to model a short run up front before letting them go on their own. Probably best to do this as a whole class activity.
This is not an easy game.
We will need feedback from you to improve. We will continue to post responses as they come in from teachers who have tried the game and will make adjustments based on suggestions. Check in once in a while to see what's happening!
You may or may not want to go over the Background material in class. Sometimes it is better for them to discover the "rules" themselves. This will depend on the thinking level of the class.
- Assemble all of the pieces described earlier.
- Starting Genes. Lots of possibilities here, depending on the points that you want to make. You may even want different groups in the class to start with different strategies. You can have the students pick randomly from the gene sets, or the students can try and pick the "ultimate" gene strategy. One way is to pick the same combination for the asexual and the sexual reproduction cards so as to have a more controlled comparison. Another strategy is to compare a diverse gene population with one that has very few different individuals.
- Playing. Start reading the scenarios one step at a time. You may want to stop play at points to get feedback to be sure the students see what is happening. You can add your own scenarios!
- End Game. lots of ways to define the end. a) the first to fill their card b) the last one with live pieces c) the one with the greatest diversity (most gene types) d) the one with the least diversity (one gene type) In each case, was the winner sexual or asexual in reproduction?
Tables and graphs can and should be made of the results. Brainstorming sessions and active debriefing sessions help to insure they get the points you are trying to get across.
example table: (see scenarios for example data)
|event||# of asexual||# of sexual||hypothesis, why?|
NOTE: hypothesis need not always be filled in.
Here are some sample questions to get things started.
- Why are there an equal number of males and females, if only "one" male is needed and they are such a waste of resources anyway?
[in humans they are not a waste if they help in taking care of the offspring]
[finding mates is much harder if one sex is low. also you loose the advantage of recombination if there is only "one" male around]
[in some species if there are not enough of one sex some of the remaining can change their sex to help equal things out]
- If bacteria are asexual, how come they are so successful?
[bacteria are not totally asexual and can exchange genetic material sometimes even across species boundaries]
- Wouldn't the ideal organism be one that could use either method depending on circumstances?
[maybe, as some have adopted this strategy, the problem is, is that it is not always easy to anticipate the best path]
- Explain why setting out lots of sterile male fruit flies would help rid an area of fruit flies.
[mating with a sterile male would render that female unproductive]
- What about the people who have multiple "bad" genes. Should we have allowed these to die at birth?
[Hitler, or who is the judge]
[sickle cell anemia gives immunity to malaria]
[on the other hand what about the huge expense of care in an overloaded healthcare system?]
- Should we allow only the "good" gene people to reproduce?
[remember, it is the diversity that gives sexual reproduction the edge. If you narrow the gene pool you set yourself up for a plague, predator, etc. that no one has immunity to, no matter how good the rest of the genes are]
[again, who is the judge of the "good" genes?]
A number of ways to evaluate. At the simplest level, have students fill in already prepared or suggested tables and answer a few of the easier questions in the Implications section. In this case the students are given the Background information ahead of time. More advanced students should be able to synthesize the "goals" themselves and answer the more difficult questions in the Implications section. The format of a lab report may be used, or a handout "exam" type format may be used.