Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (9 page)

Being mismatched, however, is different from being stuck. Instead of asking how we can overcome our Stone Age genes, let us ask which traits have changed quickly, which slowly, and how we can tell the difference.

3

Crickets, Sparrows, and Darwins—or, Evolution before Our Eyes

W
hen we think about rapid change in a species, humans are often the first to come to mind, perhaps because we are so used to the idea that the modern world is very different from the one in which we evolved. But our anthropocentrism betrays us here, as it does in so many other places. Scientists are discovering more and more examples of evolution occurring in the span of just a handful of generations in animals large and small. What’s more, some of those examples illustrate the practical implications of evolution, and why fishermen and farmers, not just scientists, should take heed of its findings.

My own firsthand experience with rapid evolution reminded me of taking our cat William with us in a U-Haul when my husband and I moved from New Mexico to Ohio. William was generally a stalwart and pragmatic animal, but like most cats, he greatly disliked riding in a vehicle, and he spent much of each day’s trip howling in his crate, waiting for the horror to end. We would smuggle him into the motel room each evening, letting him out to use the litter box and eat. He would immediately dart under the bed, collect his nerves for an hour or so, and then emerge to go about his business.

This routine worked for the first two nights, but on the third we happened to stay in a room with a bed that had a frame extending all the way down to the floor. William exited the crate with his usual alacrity and headed for the bed. He stopped; there was no opening for him to creep into. He circled the bed. Still no hiding place. He circled it again. He leapt on top of the mattress, as if ascertaining that it was, indeed, a bed, and then tried again. No dice. Repeat. It was as if his brain kept cycling through the same set of incompatible conclusions:
It’s a bed, so it must have an under-the-bed. But there is no under-the-bed, so it must not be a bed. But it seems to be a bed, so it must have . . . etc.
He couldn’t resolve the cognitive dissonance except by tiring himself out by jumping on and off the bed and eventually taking refuge in sleep. We made it to our destination the next day, to a house with a proper bed, for which we were all grateful.

My own story of one sense contradicting another involves less jumping, but just as much befuddlement, and it also was my introduction to just how quickly evolution can happen in the wild. For many years I have been studying a species of cricket that lives in Australia and much of the Pacific, including islands such as Tahiti, Samoa, and the Marquesas. At least 150 years ago, the crickets were also introduced to the Hawaiian Islands, where they can be found in grassy areas like the lawns around buildings. They behave much as any other field cricket does: males call to attract females, and females select mates on the basis of which songs they prefer. Unfortunately for the crickets, in Hawaii—though not their other haunts—they must contend with parasitic flies that can also hear the chirping. A female fly is incredibly good at locating a male cricket using his song, and when she does, she dive-bombs him, leaving in her wake not missiles but something far worse: her tiny voracious larvae. The maggots burrow into the cricket and develop inside his still-living body, gradually consuming his flesh. After about a week, the maggots emerge from the withered, dying shell of their cricket host, to pupate in the soil and finally buzz away as adult flies.

A field cricket with
Ormia ochracea
, the parasitic fly that locates male crickets by their songs. In Hawaii, these flies seem to have spurred extremely rapid evolution in crickets, rendering them silent.
(Courtesy of Norman Lee)

This gruesome life history is remarkable for many reasons, but as a specialist in the evolution of the signals used in animal sex—peacock tails, cricket chirps, or elk bellows—I became interested in the dilemma it poses for the male cricket. Obviously, the more you sing, the more likely you are to attract a mate—an evolutionary jackpot for any animal. Singing is more or less a male cricket’s raison d’être. But in this case, the more you sing, the more you expose yourself to the deadly parasites, and dead crickets cannot sing at all (a truism I have always thought would make a good title for a mystery novel). Selection thus operates in precise opposition, acting both for and against the same trait, and I have spent much of the last two decades trying to figure out how the crickets resolved their evolutionary conundrum.

My colleagues and I discovered many interesting parts to the solution, but the real stunner happened during what I thought would be a somewhat rushed and routine visit to Kauai. We had been doing our research on the Big Island, Oahu, and Kauai, and for several years I had been catching and dissecting crickets in each place to see how the males infested with parasites might differ from the ones that had escaped. It was always a small thrill to open up the body cavity under the microscope and see, in addition to all the normal cricket organs and tissue, a plump white maggot glistening under the cold fiber-optic light we use for dissection. The tiniest maggots have little black racing stripes along the sides, giving them a cheerful perkiness that belies their more sinister character.

None of my colleagues saw the charm of any of the fly larvae, large or small, but we all could see that the percentage of males harboring the maggots was always highest on Kauai. Nearly a third of all the males we caught were infested, and we wondered for years whether the relentless hunting by so many flies would eventually wipe out the cricket population entirely. We don’t know how long the flies have been on the islands, but we know the crickets did not evolve there, so the relationship is relatively recent, suggesting that it might not be stable.

Indeed, starting in the late 1990s, it became increasingly difficult to find any crickets at all on Kauai. A field that once held dozens grew more and more silent, and by 2001 we heard only a single cricket calling in our usual field site. That doesn’t mean others weren’t around, of course, but the relative silence was symptomatic of a major decline in the population.

Thus, when my husband and I returned to Kauai in 2003, I was not optimistic. It was certainly possible that the flies had finally proven the death knell for the crickets on the island. And sure enough, when we drove up to the field where we usually found our samples, the night was silent. But having come so far, I figured we might as well get out of the car and take a look. We put on our headlamps and started walking up the road.

And that’s when I felt like William the cat. Because in front of me, on each side, hopping on the ground and perching on the grass, were crickets. Lots of crickets. More crickets than I had seen on Kauai for years—maybe ever. I caught one. It was one of our species all right, and it was even a male, easily distinguishable from the females by the absence of a long, straw-like ovipositor used to lay eggs. But I didn’t hear a thing.

Remember that singing is, well, what crickets do. Except for a few oddball species that definitely did not include my study subjects, the definition of a male cricket is an animal that gets out there every night, lifts his wings, and rubs them together to make noise. Individuals may refrain from calling sometimes for one reason or another—it’s too cold, they are injured, or they mated so recently that they cannot produce sperm for the next female—but chirping is part of the cricket identity. So my mind started going through puzzled loops, much as I imagine William’s had done years before:
These can’t be crickets, because they aren’t calling. But look, there are lots of crickets. Maybe they are a different kind of cricket that just suddenly appeared here on Kauai? Nope, they are my kind of crickets. But these can’t be crickets, because they aren’t calling. Maybe I’ve gone deaf? No, that’s not it. But these can’t be . . . etc.

I would like to be able to say that I resolved the disharmony of the situation more quickly than William had done in giving up his futile perusal of the motel bed and falling asleep, but the truth is that although I shook myself out of my own cognitive dissonance and actually tried to use science to solve the puzzle, it took a few months before we had the answer.

The crickets, it turned out, weren’t silent because they could have called but chose not to; they were silent because they lacked the apparatus to produce any sound at all. In the space of fewer than five years, or about twenty generations (taking a conservative estimate that the mutation appeared a few years before we noticed it, and with the crickets producing three to four generations per year), a new form of the cricket that we dubbed “flatwing” had become so common that we now estimate only about 10 percent of the crickets can still sing. (We just happened not to hear any on that first night, but later visits turned up a valiant handful of callers.) Additional research in my lab showed that the flatwings have a mutation in just one gene, but that single gene changes their wings so that they lack the equivalent of a fiddle and bow for making music.

Ordinarily, of course, not being able to call would be a tremendous liability for a male cricket, and if the mutation arose under normal circumstances, it’s virtually certain it would be an evolutionary dead end for its bearer, who would be unable to mate and hence unable to pass on any genes. But the flies change the rules of the game, and the flatwings are protected from detection by the lethal parasites by their silence, a veritable cloak of inaudibility that provides an enormous advantage.

This leaves one major question: How can a female cricket find her silent mate? The answer is turning out to be complicated, with the females apparently willing to mate with the silent males as long as the males are near one of the few remaining callers. But for our purposes, the point is that the crickets are an example of one of the fastest cases of evolution in the wild, taking not hundreds or thousands of generations, but a mere handful. In human terms, twenty generations is only a few centuries.

Although my crickets evolved more quickly than many species, they are by no means alone in changing during a relatively short period. Contrary to the commonly held notion that evolution is a ponderous process, requiring geological spans to produce any detectable change, scientists are now discovering that, as Andrew Hendry and Michael Kinnison note, “The fundamental conclusion that must be drawn is that evolution as hitherto considered ‘rapid’ may often be the norm and not the exception.”
¹

The birds who came in from the cold

Perhaps because people are always interested in how fast events can happen, and also because people seem to have differing views on how long Earth and its inhabitants have been around, the rate of evolution has piqued our interest virtually since Darwin. One of the first to suggest a speed record for genetic change in a population, the most basic definition of evolution, was an extravagantly mustached scientist named Hermon Bumpus. Like the other residents of Providence, Rhode Island, at the end of the nineteenth century, Bumpus, an assistant professor of comparative zoology at Brown University, experienced some record-setting bad winter weather. But unlike most of them, he decided to profit from the misfortune of others in the name of science.

Somehow—his 1899 paper is mute about the source—the day after a particularly severe storm, Bumpus was brought 136 dead or stunned house sparrows.
²
I have searched in vain for the person or persons who thoughtfully provided Bumpus with his subjects, and their reason for doing so; was he well known for a general interest in dying birds? Did his friends and neighbors know that he had wanted all his life to record the miseries of house sparrows, those English invaders of North America? Was the purveyor of the bird bodies also interested in natural selection, only to be summarily dismissed from Bumpus’s subsequent work and denied a coauthorship? You don’t just cart around 136 sparrows in your pockets in the hope that they will come in handy sometime.

Alas, the backstory to this experiment seems destined to be shrouded in mystery, but regardless, once Bumpus saw that about half the birds recovered after they were warmed, while the others died, he knew he had a gold mine on his hands. Natural selection requires some individuals to survive and reproduce while others fail, and here was an opportunity to see whether the sparrows that gasped to life in his laboratory had any characteristics that the others did not. So Bumpus methodically measured the size of bills, wings, legs, and other body parts in the survivors and those not so fortunate, and then compared the two groups. They differed substantially; birds that were either unusually large or unusually small fared worse than those clustered around the average size of the group, and Bumpus postulated that stabilizing selection, a kind of natural selection that winnows out the extremes and favors those in the middle, had been at work.

His results have been scrutinized many times since he published them, and scientists still argue about the best way to analyze his measurements, but the fact remains that Bumpus’s sparrows are among the earliest examples of evolution occurring before our eyes, at least within the local population of house sparrows. Bumpus had not only confirmed that evolution happens in the wild; he had shown that it could, literally, occur overnight.