Both of these plants have thick, water-filled branches and sharp spines. You might guess that they are closely related — but they are not. In fact, one “cactus” is more closely related to a common weed, and the other is more closely related to a carnation. Their similarities are analogies — independently evolved adaptations that aid survival in the desert: succulent, waxy stems help store water, and spines provide shade and protect the plant from herbivores.