Variation, Heritability, competition, and different survival.
Offspring different from parents would belong in sexual reproduction. Sexual reproduction makes the offspring different from the parent(s), while asexual reproduction makes somewhat of a clone of the parent.
The floor of the rain forest will dry up and die, causing the animals that live down on the floor to die from no food or water, that would then cause the animals that live in the canopy to die from lack of food. and a lack of water would eventually kill the taller higher canopy.
