Carl Zimmer in his blog, The Loom:
Sometimes a picture can tell you a lot about evolution. This particular picture has a story to tell about how two species–in this case a fly and an orchid–can influence each other’s evolution. But the story it tells may not be the one you think.
Coevolution, as this process is now called, was one of Darwin’s most important insights. Today scientists document coevolution in all sorts of species, from mushroom-farming ants to the microbes in our own gut. But Darwin found inspiration from the insects and flowers he could observe around his own farm in England.
Darwin’s thoughts about coevolution began with a simple question: how do flowers have sex? A typical flower grows both male and female sexual organs, but Darwin doubted that a single flower would fertilize itself very often. Flowers, like other organisms, display a lot of variation, and Darwin thought that the only way flowers could vary was if individuals mates, mixing their characters. (Sex turns out not essential for creating variation, but it does do a good job of creating it.) But in order to have sex, plants can’t walk around to find a mate. Somehow the pollen of one flower has to get to another. Not just to any flower, moreover, but to a member of its own species.