The cacti which have spikes keep the predators away. The cacti which have leafs, attract predators to eat it.
The cacti which have spikes are the fit ones which survive and reproduce. The cacti with leaves get eaten.
In the future, most of the population would be cacti with spikes and fewer cacti with leaves because the spikes on them let the predators keep away. The leaves are dying and the spikes are surviving and reproducing.