Divide the students into different species for the first round of the game: about 10% of them are wolves (wearing ears); the rest of them split between rabbits (wearing white tails of cloth tucked into the back of their pants) and grass (holding construction paper grass).
Divide the gym or other open area into three areas. The wolf territory is in the middle, the rabbits on one side, the grass on the other side.
When the teacher signals the start of a day (or more realistically, a month), the rabbits try and run across the wolves territory to get to the grass. The wolves try and catch them by snatching their tails as they run across. If a rabbit does make it across the wolf territory, it needs to find a grass of its own.
Initially at least, once the rabbits have made it across, or not, the teacher signals the end of the day, and the class discusses what has happened and resets the characters as follows:
If a rabbit is caught by a wolf, the wolf gets to eat and reproduce: the rabbit becomes a wolf for the next round.
If the rabbit makes it across the wolves territory, and finds its own grass to eat, it reproduces: the grass it pairs with becomes a rabbit for the next round.
If a wolf does not catch a rabbit, or a rabbit does not find a free grass, they die, and recycle into the earth: they become grass for the next round.
If the students get the hang of becoming a new species each round, try running the game continuously: they switch out their ears/tail/grass as they become a new species each time. Rabbits can walk back across the wolf territory unchallenged before attempting to run across without being caught to reach the grass.
Run the game for several rounds, maybe stopping earlier if one of the living things dies out (or not - to see how the other populations oscillate back and forth).
Discuss what happened: the species that are able to eat reproduce more successfully - they are selected for. If there are a lot of one species, the food that they eat becomes depleted, then there is competition for the less food available, and not as many of that species are able to eat and survive, so their population decreases again. This game shows natural selection in action. Sometimes a species may die out, and the populations of the other species it interacts with in the food chain take over.
Ask students for ideas of an adaptation that each species has to help it survive, then discuss more adaptations:
The wolves have teeth and claws to catch their prey, large ears and good noses to locate prey etc
The rabbits have long legs to escape from predators, camouflaged colours to hide from predators etc
The grass is green to catch the energy from the sun, and has dormant seeds that can grow in rich soil (from recycled living things) when the conditions are right.