Weaving the Web of Life

One of the many reasons we need to save endangered species from extinction is because we can find among them lessons about how evolution works. In the rapidly shrinking forests of Madagascar, one such species clings to existence: an extraordinary orchid named Angraecum sesquipedale. One of the petals on its pale white flower is shaped into a 16‑inch‑deep shaft, and nestled in the bottom of the shaft are a few drops of sweet nectar.

What could this deep nectary possibly be for? What evolutionary force created it? Wait long enough and the answer will arrive on the wing. A species of moth visits the orchid, slowing to a hovering stop over the flower. Its tongue, coiled up like a watch spring, begins to fill with blood, and as it does, the pressure forces it to straighten out. It grows to be 16 inches long, far longer than the moth’s entire body. The moth snakes its tongue down the tube until it reaches the sweet nectar. As it soaks up the nectar, it buries its face in the flower, and as it does, its forehead rubs against pollen grains. When the moth is finished with its drink, it curls up its tongue again and flies away with the pollen smeared over its head, to find another orchid to drink from. The pollen on its forehead brushes off on the new orchid, where it can fertilize that flower’s eggs.

It may be hard to believe that a pair of species could be so tightly linked together, and yet nature is filled with such intimate partnerships, whether they are the beneficial ones between flowers and their pollinators or hostile ones, such as predators and the prey they hunt. Life consists for the most part of a web of interacting species, adapted to one another like a lock and key.

Partners such as the orchid and its moth did not spring into existence already linked together in the relationships they have today. They gradually evolved into closer and closer intimacy, and they continue to evolve today. Every generation of plants, for instance, adapts its defenses against insects, but the insects are also evolving ways to overcome their defenses at the same time.

Scientists are discovering that the way in which the evolution of one species drives the evolution of another–known as coevolution–is one of the most powerful forces shaping life. Coevolution can create anatomy as remarkable as a moth’s 16‑inch tongue. It is also responsible for creating much of life’s diversity, as the spiraling coevolution of partners spawns millions of new species. Coevolution is also a fact that we ignore at our own peril. The crops we eat, the paper on which these words are printed–every plant we depend on is coevolving with intimate partners, both life‑sustaining and life‑destroying. If we alter their coevolutionary dance, we may have to pay a steep price.