Ebert’s findings did not come as a surprise to some biologists. They had built mathematical models of the relationship between hosts and parasites, and they had discovered theoretical reasons why familarity could breed contempt. Natural selection favors genes that can get themselves replicated more often than others. Obviously, a gene that makes a parasite instantly fatal to its host won’t go very far in this world. Yet, a parasite that is too well mannered won’t have any more success. Because it takes almost nothing from its host, it won’t have enough energy to reproduce itself and will come to the same evolutionary dead end. The harshness with which a parasite treats its host—what biologists call virulence—contains a trade-off. On one hand, the parasite wants to make use of as much of its host as possible, but on the other hand, it wants its host to stay alive. The balancing point between these conflicts is the optimal virulence for a parasite. And quite often, that optimal virulence is quite vicious.
The way virulence works is nicely illustrated by mites that live on the ears of moths. Moths have to be on constant guard against bats, which seek them out with echolocating shrieks. When moths hear the bats sending out their ultrasonic signals, they immediately start dodging and weaving through the air to avoid an attack. If the mites colonize the full extent of a moth’s ear—on both its outside and its inside—they will have enough room to produce a lot of offspring. But as they root around, damaging the delicate hairs that the moth uses to hear, they leave the moth deaf in that ear. With one ear out of commission, the moth will have a harder time escaping bats. If both ears shut down, the moth is doomed.
Nature has settled on two solutions to this dilemma. Some species of mites take up residence in the entire ear, both on the outside and on the inside. But they live in only one of the moth’s ears, leaving their host with enough hearing to keep it from being devoured. Other species of mites live on the outside of both ears. But because they forgo all the inner-ear real estate, they reproduce less than the deafening mites and are transmitted more slowly from moth to moth.
To test theories of virulence, biologists can make predictions about how real-world parasites behave. In the forests of Central America, several species of parasitic nematodes live inside wasps. These wasps are exceptional creatures: the female lays her eggs inside the flower of a fig tree and dies. The flower transforms into a plump fruit, and the eggs of the wasps hatch, the wasp larvae feeding on the fig. They mature into adult males and females and mate inside the fruit. The females then leave the fig to find a new one to lay their eggs in. As they leave they gather pollen on their bodies, and when they find a new fig flower, they fertilize it, triggering the production of a new seed.
It’s a pleasant symbiosis for both plant and animaclass="underline" the fig depends on the wasp to let it mate, and the wasp depends on the fig for a place to raise its young. But into this happy scene intrudes the nematode. Some figs are riddled with these parasites, and when an egg-bearing female wasp prepares to leave, a nematode crawls onto her to hitch a ride. By the time the wasp has arrived at a new fig, the nematode has penetrated her body and is devouring her guts. The wasp enters the fig and lays her eggs, but the parasite has laid its own eggs inside her body as well. By the time the wasp has finished laying her eggs, the parasite kills her, and out of her body emerge a half dozen or so new nematodes.
The wasps and nematodes have been living together as host and parasite for over 40 million years—a long, venerable association. From species to species the wasps have different egg-laying habits: some will lay eggs only in a fig untouched by other wasps so that their young will have the fig to themselves. Other species don’t mind laying eggs alongside those of other wasps. Virulence theory makes a prediction about the nematodes that live in fig wasps. Nematodes that infect a wasp that lays its eggs alone must handle their host delicately. If they ravage the wasp too quickly, she may be able to lay only a few eggs, or none at all. The nematode’s own offspring would then have fewer potential hosts in their fig, and they’d have worse chances of surviving.
The same doesn’t hold for parasites of more neighborly wasps. When a nematode’s offspring hatch in a fig, they’re likely to find other wasps there that they can parasitize. What a nematode does to its own host doesn’t pose any risk to its offspring, so you’d expect these parasites to be far nastier. The biologist Edward Herre studied fig wasps and their parasites in Panama for over a decade, and when he looked over his records for eleven species, he found that they did indeed fall into the predicted pattern—a powerful vindication for the theory of virulence.
To study the laws of virulence, parasitologists can work with just about any parasites, whether they are mites, nematodes, fungi, viruses, or even rogue DNA. The host can be a human, a bat, a wasp, an oak tree. The same equations still apply. When scientists look at parasites from this evolutionary point of view, suddenly the walls that traditionally divide them tumble away. Yes, they all occupy different branches of the tree of life; yes, they are all descended from radically different free-living ancestors. But those gulfs make their similarities all the more remarkable. Darwin himself noticed that different lineages can independently evolve toward the same form. A bluefin tuna and a bottlenose dolphin are separated by over 400 million years of divergent evolution. Yet, the dolphin, whose ancestors looked like coyotes only 50 million years ago, has evolved a teardrop-shaped body, a rigid trunk, and a narrow-necked tail shaped like a crescent moon—all of which are possessed by the tuna. Biologists call this coming-together convergence, and parasites are the most spectacularly convergent organisms of all. Free-living nematodes have moved from the soil into the roots of trees, where they have evolved the ability to switch on and off individual genes and turn individual plant cells into comfortable shelters. Another lineage of nematodes produced Trichinella—a parasite that does the same thing to the cells in muscles of mammals. The lancet fluke has evolved chemicals that can force an ant to climb to the top of a blade of grass and clamp itself there. The same feat is accomplished by fungi. To find the last common ancestor of lancet flukes and fungi, you’d have to explore the oceans for some single-celled creature that lived a billion years ago or more. Yet, after all that time, they both managed to come across the same tactic to control their hosts.
The laws of virulence are also built on convergence, and they promise to change the way we fight diseases. A virus such as HIV needs to go from host to host in order to propagate, just as a nematode does. If it becomes easier for a strain of HIV to travel, it can reproduce more quickly in a given host (and cause him or her more harm). That’s how the AIDS epidemic has played out: in populations where people have many sexual partners, the virus destroys its host’s immune system faster. Cholera is caused by a bacterium called Vibrio cholerae, which travels through water and escapes its host by causing diarrhea. In places where water is purified and Vibrio’s odds of infecting a new host are low, the disease is milder. In places without sanitation, the bacteria can afford to be more vicious.
The history of parasites, stretching over billions of years, is just beginning to emerge, but already it has made clear that degeneration isn’t its guiding force. Parasites may indeed have lost some traits over the course of their evolution, but then again, in our own history we have lost tails, fur, hard-shelled eggs. Lankester was appalled at how Sacculina lost its segments and appendages as it matured. He could just as easily have been disgusted by the way he himself had developed the vestiges of gills in his mother’s womb and then lost them as he grew lungs. As parasites colonized Earth’s third great habitat, they did lose some of their old anatomy, but they evolved all sorts of new adaptations that scientists are still trying to understand.