1. Begin the lesson by explaining that all living things have the following characteristics in common: they're made of cells, they use energy, they grow and develop, and they respond to their surroundings. Then ask students to name one other characteristic that all organisms have in common. (All organisms reproduce, for example.) Following this brainstorm, focus the rest of the discussion on reproduction, reminding students that one of the most important things an organism can do is reproduce. Ask:

• What is the result of reproduction?
• Why do organisms bother to reproduce? Why don't they just live forever?
• What would eventually happen to a species if every member suddenly lost its ability to reproduce?

2. Show the Single-Celled Organisms video and discuss the following:

• What type of reproduction -- asexual or sexual -- do most single-celled organisms use?
• What must a single-celled organism do before it can reproduce?
• When a single-celled organism reproduces, what is the result?
• In what ways, if any, does a single-celled organism differ from its parent?

3. Show the Floral Arrangements video and discuss the following:

• What type of reproduction -- asexual or sexual -- do most plants use?
• What nonliving force do plants rely on most often for pollination?
• What are some of the ways in which plants encourage or trick animals into carrying their pollen to other plants?
• What proportion of each parent plant's genetic material does each offspring plant have?

4. Ask students to explore the The Mating Game Web activity in pairs. Recommend that students play two rounds of the game and then choose four of the species highlighted in the activity's Dating and Mating Gallery. Ask students to take notes on the information provided about each species, focusing on the reproductive similarities and differences among them. Have pairs of students present their findings to the class while you record the various reproductive strategies on the board.

5. Ask the class to rank the reproductive strategies on the board in order of relative difficulty, or "expense," to the animal. For example, the tube sponge's strategy of casting out clouds of sperm or egg cells into the open water is relatively less expensive than the bowerbird's efforts to attract a mate by building an elaborate bower. Ask students:

• What are some of the things that animals can't do when they're focusing so much time and energy on finding or attracting a mate?
• What proportion of each parent's genetic material would the offspring of any of these species have?

6. Show the Asexual Reproducers video and discuss the following:

• What type of reproduction -- asexual or sexual -- do the whiptail lizards in the video use?
• How many parents do whiptail lizards have?
• How do young whiptail lizards differ from their parents, if at all?
• How much of their parent's genetic material do whiptail lizards have?

7. Ask students to consider why some species might have evolved reproductive strategies that require a lot of energy and that allow individual organisms to pass only half of their genes on to their offspring. Ask students what benefits they think sexual selection might have.

8. Show the The Red Queen video and discuss the following:

• What are the differences between the two species of minnows featured in the video?
• Which species -- the asexual or the sexual reproducers -- tends to be more heavily parasitized by the worm that causes black-spot disease?
• How are the sexual reproducers able to evolve defenses against parasites more quickly and more effectively than their asexual counterparts?