17. Below is a series of pictures representing changes in a population of cacti.

Scene 1	Scene 2
Scene 3	Scene 4

Why would a deer be more likely to eat the left front cactus than the other two shown?

In figure 3, two of the cacti have flowers, but the cactus that has been eaten by the deer is too damaged to make flowers. Figure 4 shows the situation several months later. What has happened? Do you think that evolution by natural selection is occurring in this cactus population? Explain why or why not.

What adaptation seems to increase the fitness of the cacti? How do you think this adaptation increases the ability of cacti to both survive and reproduce?

18. "Survival of the fittest" is a common expression. What do you think most people mean by this expression? How would you explain this expression to help someone understand how natural selection actually functions? (Remember that an adaptation is any characteristic that increases fitness, which is defined as the ability to survive and reproduce.)

19. What characteristics of humans do you think helped them survive and reproduce during the hundreds of thousands of years when they were hunter-gatherers?