Co-evolution occurs when closely interacting species influence each other’s development. In predator-prey or parasite-host relationships, two species can lock horns in an evolutionary arms race. Take the common garter snake, for example. Its favourite prey is a tasty but toxic newt which progressively evolves more potent toxins to deter its predator. As snakes with lower immunity to these toxins are killed or injured, the garter snake population evolves greater resistance to the poison. Increased predation means the newt is once again under pressure to become more poisonous, and the cycle repeats. Other species can co-evolve to co-operate, as occurs commonly with plants and their pollinators. Over 40 million years, the yucca moth and the yucca plant have become entirely dependent on one another.