Meanwhile, Kraaijeveld continued to raise his wasps on the other flies, D. subobscura. When the next generation of D. melanogaster had matured, he took some of the wasps and transferred them to their chamber. The wasps would then attack the new generation of D. melanogaster, and once again, Kraaijeveld would raise the survivors to produce a new generation.
By raising the wasps and the flies in this way, Kraaijeveld was blindfolding one of the boxers in his host-parasite match. With each generation, the D. melanogaster flies were able to adapt more and more to the wasps. But the wasps, which Kraaijeveld raised on another species of fly, didn’t have any chance to match the evolution of their D. melanogaster host. The mismatch let D. melanogaster steadily improve their fight against their parasites. In only five generations, the proportion of flies that could kill the wasp larvae rose from one in twenty to twelve in twenty.
Hosts and parasites may evolve together in a continual escalation (what biologists call an arms race), but in many cases their evolution can look more like a merry-go-round. Parasites evolve over time to do a better and better job of recognizing their hosts, finding weakness in their defenses, and thriving inside them. But a host species is never genetically uniform—it instead comes in strains, each with its own set of genes. Parasites have variations of their own, and some of them may help parasites against particular strains of hosts. Over time strains of parasites emerge, each adapted against strains of hosts.
Biologists have built mathematical models of these intimate relationships. If one strain of host is more common than the rest (call it Host A), any parasites that are adapted to it will have a rosy future. After all, they can hop between a wealth of hosts, replicating along the way. The problem is that, as parasites, they will kill or disable a lot of their hosts. From generation to generation, Host A will fade as its parasites undermine their success.
The attention that parasites pay to the most common host gives rarer host strains an advantage. Since the most common parasites aren’t adapted to attack them, they get the opportunity to multiply. As Host A declines, another host, say Host B, rises. But then parasites that can adapt to Host B get rewarded by natural selection and multiply as well. They eventually drive down Host B’s numbers, letting Host C ascend, then D, and E, and so on, maybe even back to Host A again. Every now and then a mutation creates a rare new strain of host. It simply becomes Host F and falls into the rotation.
This endless rise and fall would probably have appalled the biologists of Lankester’s day. They saw the history of life as a march of progress, always threatened by degeneration. In this new kind of evolution there is no progress forward or backward. Parasites force their host to go through a huge amount of change without going anywhere in particular. One variant rises, then it falls, and another variant rises to take its place, only to fall in turn. This sort of evolution isn’t the stuff of epic poetry but of surreal children’s stories. Biologists came to call it the Red Queen hypothesis, referring to the character in Lewis Carroll’s Through the Looking Glass who took Alice on a long run that actually went nowhere. “Now, here, you see, it takes all the running you can do, to keep in the same place,” the Red Queen declared.
Yet, there’s a paradox to the Red Queen hypothesis. While it’s all about running to stay in place, it may have allowed evolution to take one crucial step forward: it may have brought about the invention of sex.
In the early 1980s, Curtis Lively found himself in New Zealand wondering about sex. He had just finished earning a Ph.D. in evolutionary biology by studying the barnacles of the Gulf of California. One of the questions he had to answer on his qualifying exams was, Why did evolutionary theory have such a hard time accounting for sex? He had no idea.
It’s not a question that most people are accustomed to asking. “If you go into a class of sophomores and ask, ‘Why are there males?,’ they look at you as if you’re crazy,” Lively says. “They’ll say you need males to reproduce and that each generation produces more males. Well, that may be true for mammals, but for many species that’s not true. It’s just staggering to them to think that anything could do that, could reproduce without males and sex. Sex and reproduction are just fused in most people’s brains.”
Bacteria simply divide themselves in two when the time seems right, as can many single-celled eukaryotes. Many plants and animals have the ability to reproduce themselves on their own quite comfortably. Even among the species that do reproduce sexually, many can switch over to cloning. If you walk through a stand of hundreds of quaking aspen trees on a Colorado mountainside, you may be walking through a forest of clones, produced not by seeds but by the roots of a single tree that come back up out of the ground to form new saplings. Hermaphrodites, such as sea slugs and earthworms, are equipped with male and female sex organs and can fertilize themselves or mate with another. Some species of lizards are all mothers: in a process called parthenogenesis, they somehow trigger their unfertilized eggs to start developing. Compared with these other ways to reproduce, sex is slow and costly. A hundred parthenogenetic female lizards can produce far more offspring than fifty males and fifty females. In only fifty generations, a single cloning lizard could swamp the descendants of a million sexual ones.
When Lively was learning about the mystery of sex, there were only a handful of good hypotheses to explain why it existed at all. Two of the favorites were nicknamed the Lottery and the Tangled Bank. According to the Lottery hypothesis, sex helped life survive in unstable environments. A line of clones might do well enough in a forest, but what if that forest changed over a few centuries to a prairie? Sex brought the variations that could allow organisms to survive change.
According to the Tangled Bank hypothesis, on the other hand, sex gets offspring ready for a complicated world. In any environment—a tidal flat, a forest canopy, a deep-sea hydrothermal vent—the space is divided into different niches where different skills are needed for survival. A clone specialized for one niche can give birth only to offspring that can also handle the same niche. But sex shuffles the genetic deck and deals the offspring different hands. “It’s basically spreading out progeny so that they’re using different resources,” says Lively. The progeny wouldn’t have to fight with each other over food as much, and thus a mother would be more likely to become a grandmother. While the Tangled Bank hypothesis might work in theory, it wasn’t very likely. The different kinds of bodies built by the different sets of genes had to be quite distinct from each other in order for it to work. Nevertheless, it was the dominant idea at the time.
Lively found himself in New Zealand in 1985 because his wife, Lynda Delph, wanted to study evolutionary biology at the University of Canterbury. Lively got a job there as a postdoctoral researcher, and he wondered if New Zealand might offer him a way to test the different explanations for sex. In evolutionary biology, ideas tend to bubble up fast and easily, and often turn out to be miserably untestable. To test explanations for sex, Lively would have to find the right species to study. It would have to be a mix of sexuals and asexuals. Among some animal species, for instance, there are populations of males and females that live alongside clones. Other species are hermaphrodites, and they can choose to have sex with themselves or with another animal. Only in these sorts of animals could the generation-by-generation effects of evolution be seen, because a biologist could compare how the sexuals and asexuals fared. “If you’re dealing with something that’s all sexual,” says Lively, “it’s hard to know what selection would be for or against an asexual. But if you have a system where you have both, now you have the basis for comparison.” He couldn’t test an idea about the persistence of sex in humans, for instance, because we all do it. There is no lost tribe out there who can have children with natural cloning. In our own evolutionary lineage, the race between the sexuals and the asexuals ended hundreds of millions of years ago.