This is a good game for outdoors.
Give the students their ears.
When their ears are on their head they are a wolf.
To be a rabbit, students turn one of the ears down, floppy bunny ear-style.
To be grass, students put their ears around their necks out of the way, squat down and put their fingers up like blades of grass.
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 (with floppy bunny ear, or ears around necks) and grass (ears around necks and fingers up).
Divide the area into three parts. 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 tagging them 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 their own grass, they die, and recycle into the earth: they become grass for the next round.
At the end of the round, all rabbits walk back across the wolf territory unchallenged to their starting side.
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.
Add in an adaptation:
One rabbit gets a random mutation in its DNA that makes it run a little faster than the other rabbits. Signify this by adding a clip or other marker to the headband of one rabbit. In the game this rabbit gets to cross the wolves territory without being tagged. If it finds its own grass it can reproduce to make another rabbit, also with the "fast" mutation.
As the days past by, more and more of the rabbits that survive have the fast mutation. It is a beneficial adaptation that has spread through the population.
Discuss other adaptations in wolves and rabbits:
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, also large ears to listen out for danger etc
Both animals are camouflaged to blend in with their surroundings. For the rabbit, a prey animal, it can hide in the grass better and not be spotted by a wolf. Wolves are camouflaged so that they can sneak closer their prey without being seen by them.