mau'umae+environmental+pressures

Cacti may have evolved and continued to evolve in correspondence to the weather, they probably evolved in a dry area, so natural selection would favor those that retained more water, with thicker cell walls, however, in it's current environment it rains nearly everyday. which may cause it to evolve in the opposite direction, because those with thinner cell walls will retain more energy for reproduction and survival with a low chance of losing water because of the wet climate.

The relationship between pollinators and pollinatees. The flowers which produce more of whatever pollinators are looking for in their nectar will pass their genes on to the next generation.