Sexual Go‑Betweens

The concept of coevolution dawned on Darwin in the 1830s as he pondered the mystery of how plants have sex. A typical flower grows both male and female sexual organs. The male anthers hold pollen grains, which enter the female pistils and fertilize their seeds. In order for plants to mate with other plants, they cannot pull up their roots and look for a partner. Somehow the pollen from one plant must get to the eggs of another. And not just any other plant: it has to reach another member, of the same species.

For some plants, setting their pollen on the breeze is all that’s necessary. But Darwin discovered that some plants use insects to carry the pollen for them. He watched bees come to scarlet kidney bean plants to drink the sweet nectar they produced. As a bee climbed up the petals of the bean’s flowers, it invariably rubbed its back on the plant’s anthers, picking up its pollen. And when the bee visited another plant, it unloaded the pollen on its female pistils. Darwin realized that flowers use bees for sex, giving them nectar for their services.

Another scientist might have been content with making such a remarkable discovery. But Darwin was never satisfied to understand nature as it was; he wanted to uncover clues of its history. In the case of flowers and their pollinating insects, he realized that evolution must have taken a complex path. Here Darwin was not dealing with a species adapting to physical conditions like the force of gravity or the viscosity of water. Here he was dealing with two species that were adapting to each other. While the force of gravity doesn’t change, a species can change with every new generation.

In Origin of Species, Darwin sketched out one example of how coevolution might shape two species. Common clover is normally pollinated by bumblebees. But imagine that one day bumblebees suddenly went extinct. If the common red clover didn’t get a new pollinating partner, it would be unable to reproduce and disappear as well.

Honeybees might fill the void. Normally, honeybees only pollinate a different form of clover, called incarnate clover. But some honeybees might start taking advantage of all the nectar in the common red clover that was now going to waste. At first the honeybees would have a difficult time, because their tongues aren’t as long as those of bumblebees, so they wouldn’t be able to get as much nectar. Any honeybee born with an unusually long tongue would be richly rewarded with lots of common red clover nectar, so natural selection might gradually lengthen the tongue of the honeybee.

Meanwhile, the common red clover might adapt to its new pollinator, the honeybee. A common red clover flower with a shape that made the efforts of a honeybee a little easier would be rewarded with the spread of its pollen. Gradually the red clover flower and the honeybee would evolve to suit each other.

“Thus I can understand,” Darwin wrote, “how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by continued preservation of individuals presenting mutual and slightly favorable deviations of structure.”

Not long after Darwin finished Origin of Species, he discovered just how drastically flowers and insects could affect one another. He began studying orchids, crouching in the fields around Down House to observe native species or studying exotic species at his greenhouse that he had sent from the tropics. In Darwin’s time, most people considered orchids creations designed purely to please the human eye. But Darwin recognized that their shapes were not beauty for beauty’s sake, but elaborate devices for luring insects into their sex lives.

As a mechanic might dismantle a car, Darwin figured out how the parts of an orchid worked together. One of the species that fascinated him was the South American orchid Catasetum saccatum. It keeps its pollen loaded on a disk attached to a flexible shoot, which is bent back so that the disk is lodged within the flower itself. There the disk remains, cocked like a crossbow. When an insect arrives at the orchid to drink its nectar, it has to land on a cup‑shaped petal that extends horizontally out from the plant. To get to where the nectar is hidden, it has to walk across the petal, grazing its back against antennae hanging down overhead. The antennae are attached to the flexible shoot, and they act like a trigger, dislodging the shoot and letting it fling its pollen disk down against the back of the bee.

Moving the antennae, Darwin recognized, was the only way to set the pollen free. The specimens of C. saccatum that he studied had been delivered to him by train, and yet the jostling of the ride hadn’t made the pollen explode. He poked at the orchids in various places with a quill, and nothing happened. “After trials made on fifteen flowers of three species,” he later wrote, “I find that no moderate degree of violence on any part of the flower, except on the antennae, produces any effect.” The orchid, Darwin realized, had coevolved with its insect visitors.

Darwin described these orchids, and many others, in a long‑titled book: The Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects, and on the Good Effects of Intercrossing. Like Origin of Species, it was an argument for evolution, but it was subtler than its predecessor. Darwin guided the reader from orchid to orchid, showing how each flower’s design represented an elaborate form of sex. Just as he had previously demonstrated how barnacles were highly evolved crustaceans, he now showed that orchids were highly evolved flowers. Evolution had stretched and twisted and transmogrified the parts of ordinary flowers to create the crossbows and other devices that orchids use to spread their pollen.

Darwin was so confident that coevolution created the shapes of orchids that he made a bold prediction in his book. Explorers had already discovered the Madagascar orchid Angraecum sesquipedale, with its 16‑inch‑long nectary. As outrageous as it might sound, Darwin predicted the existence of a suitably long‑tongued insect on Madagascar. The orchid’s pollen, he predicted, “would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop.”

Darwin held out hope for that huge, wonderful moth even as decades passed without anyone discovering it. It was not until 1903 that entomologists reported the existence of just such an insect, which they named Xanthopan morgani praedicta–praedicta in honor of Darwin’s prediction. Biologists now know of many other species of moths and flies with long tongues, which they use to drink the nectar of equally long‑nectaried flowers. They can be found not just on Madagascar but in Brazil and South Africa as well. It’s a lucky scientist whose most bizarre predictions get confirmed even once.