Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (10 page)

How many darwins does it take to screw in a lightbulb?

After Bumpus, several other scientists tried their hand at calculating rates of evolution, and at coming up with a unit of measurement that could be applied to it. It’s fine to say that the sparrows’ size changed after a storm, or that the silent crickets came to predominate after just five years, but how do you compare, say, a 10 percent increase in leg length after thirty generations with a 40 percent decrease in swimming speed after a hundred generations? Which is faster?

The scientist who first tried to formalize measuring the rate of evolution was the great British geneticist J. B. S. Haldane. Born into an aristocratic family, Haldane became an enthusiastic scientist at an early age, performing physiology experiments on himself (he once drank hydrochloric acid to see how it would affect his muscles), as well as a public intellectual, writing a dystopian science fiction novel published in 1924, and an avid Marxist. Haldane’s paper outlining the technique for assessing how fast evolution occurs is regrettably much less colorful than his fiction, but it does suggest using the percentage of change in a characteristic—for example, leg length—and combining that measure with the characteristic’s standard deviation, a statistical measurement of how much a trait varies within a population.
³
He suggested a unit of measurement called, appropriately enough, a “darwin,” which means that if we had lightbulbs being installed at two different times, we could, at least in theory, ask the question in the title of this section.

Haldane’s method, however, is best applied to changes that take a long time, like the interval between the appearance of different kinds of teeth in fossil horses from different time periods (the example he used in his 1949 paper). Long after Haldane’s death from cancer in 1964 (while ill he wrote a poem called “Cancer’s a Funny Thing”), paleontologist Phil Gingerich proposed a different unit, which he dubbed the haldane.
⁴
Both measures are currently used by scientists, although neither has made it into the popular vocabulary.

If that is how evolution is measured, what do we mean when we say that evolution is fast? These days, evolution is considered rapid if a population shows a genetic change over tens of generations or fewer, or sometimes as much as a hundred generations. Rapid evolution is sometimes called “contemporary evolution,” to emphasize that it happens within a modern time span, or “evolution in ecological timescales,” to emphasize that evolution can be important to occurrences within the lifetime of animals or plants, while they are undergoing ecological events like being eaten by a predator or attacked by a parasitic fly.

Beaks of eagles, and finches

In his 1936 poem “The Beaks of Eagles,” Robinson Jeffers rather sanctimoniously chided that man needs “to know that his needs and nature are no more changed in fact / in ten thousand years than the beaks of eagles.”
⁵
It is probably just as well that Jeffers found eagles a more suitable subject for poetry than finches, given the latter’s place in the rapid-evolution hall of fame. Of course, we don’t know for certain that eagles have been all that unchanging, which casts doubt on the whole “needs and nature” of man idea as well. But the finches, at least the ones that live in the Galápagos Islands off the coast of South America, have certainly been changing, in beaks as well as other body parts, for at least the last several decades.

The finches of the Galápagos, also known as Darwin’s finches because they contributed substantially to his theory of how several different species can evolve from a common ancestor, have been studied by biologists Peter and Rosemary Grant since the 1970s. When I was in graduate school at the University of Michigan, where Peter held a professorship until he moved to Princeton, the “Finch Group” was a well-oiled machine, with students trooping down to the islands to measure and monitor the birds almost year-round. I was friends with several of the students whom Peter advised, so I got to exchange letters with them and listen in on the discussions of the field trips after the students returned, improbably tanned, in the Ann Arbor winter.

The first thing these interactions did was disabuse me of any notion that field work is glamorous (a valuable revelation, given my later predilection for crawling around on my hands and knees in the dark catching crickets). I got lots of letters about the monotonous diet, with moldy and weevil-ridden oatmeal receiving a particular mention, and the routine of getting up each morning on a tiny nubbin of rock, finding the birds, noting their stage in the nesting cycle, and keeping track of which birds had how many chicks and when. I also heard about measuring. A lot of measuring. The Grant crew measured the height and number of shrubs, the size of the seeds the shrubs produced, the width and length of the bills, legs, and wings on the birds that ate the seeds, and pretty much anything else they could lay their hands, or at least their calipers, on.

While the students were making their treks to the Galápagos in the 1980s, the area experienced a phenomenon called El Niño, an oceanographic event that drastically alters the usual rainfall patterns. Ordinarily the Galápagos Islands are dry, even desolate, with brief rains that fall during only part of the year. But between November 1982 and July 1983, an El Niño brought record-breaking rain, and the plants responded with flourishing growth followed by a bonanza of seeds. The Grants and their students were gobsmacked by the effect of El Niño, showing an enthusiasm that I must admit I did not entirely share; the lab meeting after they brought back the first photographs showing evidence of the rains involved scrutinizing each bush and rock on each slide, with detailed commentary along the lines of, “Did you see that big branch on the top of that shrub on the left? It’s incredibly green, isn’t it? I remember when that whole branch was brown, don’t you? But it’s green now!” After listening for half an hour or so, I got up and crept out of the room, unnoticed.

The rains and their subsequent effect on the foliage did, however, help to demonstrate evolution in action. The lists of measurements made their way back to the lab in Ann Arbor along with the students, and when the data on the birds were matched with the information on long-term weather patterns, an astonishing result emerged. Before El Niño, when the climate was relatively dry and fewer shrubs produced seeds, finches that were larger, with bigger bills, had survived and reproduced better than their daintier companions, probably because a large bill enables a bird to crack large, hard seeds more easily and the smaller, easier-to-eat seeds had all been consumed. Afterward, smaller individuals had their chance, until the more usual dry seasons came around once again. Over the three decades that the Grants and their coworkers studied the finches, the average body and beak size of one of the finch species, the medium ground finch, first became smaller, then rapidly became larger, and then decreased again, but more slowly, as the conditions favoring large or small bills and bodies changed and selected for different traits.
⁶

What this means is that populations, and species, can and do change rapidly, over and over again, as the forces of nature change around them. They don’t always change in the same direction, because evolution is more of a drunkard’s walk than a purposeful path, as I discussed in Chapter 1. This means that the future success of any single individual depends on the circumstances into which it is born; a female ground finch that hatched in 1983, after the El Niño had passed, had to live for six years, and produce ten chicks, if she was to be guaranteed to replace herself and her mate with two of them. Most of her effort would have been in vain, with her offspring dying when they could not find enough food. But if she had been hatched just before the rains, in 1978, she could have been assured of leaving those two replacement young in just two and a half years, with only five babies. Either way, it makes you wonder whether Jeffers would have found consolation in the finches.

One fish, two fish, old fish, new fish

To continue in a literary vein, albeit a somewhat more lowbrow one, the poet Ogden Nash observed in “The Guppy” that while we have special names for many animal youngsters—cygnet, calf, cub, kitten—“guppies just have little guppies.”
⁷
Indeed they do, and quickly; maybe we don’t bother with names for baby guppies because they become adults in a matter of a few months. A female guppy can be sexually mature at two months of age and have her first babies just a month later. This unstinting rate of reproduction makes guppies ideally suited for studying the rate of evolution, and David Reznick, a biologist in my former department at UC Riverside, has been doing exactly that for the last few decades.

People usually think of guppies as colorful aquarium fish, but they also have a life in the real world, inhabiting streams and rivers in tropical places like Trinidad, where Reznick has done his fieldwork. As with the finches, guppies can experience different kinds of conditions depending on the luck of the draw, though for the fish it is the presence of predators, not the amount of rainfall, that makes the difference between who lives and who dies. A lucky guppy is born above a waterfall or a set of rapids, which keep out the predatory fish called pike cichlids found in calmer downstream waters. As you might expect, the guppy mortality rate—that is, the proportion of individuals that die—is much higher in the sites with the rapacious cichlids than in those without them.

Reznick has shown that if you bring the fish into the lab and let them breed there, the guppies from the sites with many predators become sexually mature when they are younger and smaller than do the guppies from the predator-free sites. In addition, the litters of baby guppies produced by mothers from the high-risk streams are larger, but each individual baby is smaller than those produced by their counterparts.
⁸
The disparity makes sense because if you are at risk of being eaten, being able to have babies sooner, and spreading your energy reserves over a lot of them, makes it more likely that you will manage to pass on some of your genes before you meet your fate. Reznick and other scientists also demonstrated that these traits are controlled by the guppies’ genes, not by the environment in which they grow up.

How quickly, though, could these differences in how the two kinds of guppies lived their lives have evolved? Because there are numerous tributaries of the streams in Trinidad, with guppies living in some but not all of them, Reznick realized that he could, as he put it in a 2008 paper, “treat streams like giant test tubes by introducing guppies or predators” to places they had not originally occurred, and then watch as natural selection acted on the guppies.
⁹
This kind of real-world manipulation of nature is called “experimental evolution,” and it is growing increasingly popular among scientists working with organisms that reproduce quickly enough for humans to be able to see the outcome within our lifetimes.

Along with his students and colleagues, Reznick removed groups of guppies from their predator-ridden lives below the waterfall and released them into previously guppy-free streams above the falls. Although small predatory killifish occurred in these new sites, these do not pose anything close to the danger of the cichlids. Then the scientists waited for nature to do its work, and they brought the descendants of the transplanted fish back to the lab to examine their reproduction. After just eleven years, the guppies released in the new streams had evolved to mature later, and have fewer, bigger offspring in each litter, just like the guppies that naturally occurred in the cichlid-free streams.
¹⁰
Other studies of guppies in Trinidad have shown evolutionary change in as few as two and a half years, or a little over four generations, with more time required for genetic shifts in traits such as the ability to form schools and less time for changes in the colorful spots and stripes on a male’s body.
¹¹

More members of the biological rapid-response team

Although guppies have been particularly well studied, more and more animals are turning out to be capable of extremely quick evolution. The mounting examples are significant not simply because they are fascinating in their own right, but because, taken together, they put to rest the notion that evolution requires millennia. It turns out that even relatively complex traits can evolve quickly—a discovery that has led to increased study, and new understanding, of the way genes themselves interact. This research can in turn be applied to humans, enabling us to understand which of our own traits are likely to evolve quickly.

Most populations of blackcap warblers, European songbirds with males bearing the rakish head ornament of their name, sensibly spend the winter in southern Europe or northern Africa. But starting in the 1960s, a handful, and then a steady stream, of blackcaps could be found migrating to England, where they survive in people’s gardens. Peter Berthold and his colleagues took some of the Britain-wintering blackcaps into aviaries and bred them; their offspring stuck to the new migration route, and further work showed that the birds’ migration routes are inherited.
¹²
It would seem that the migratory behavior itself evolved in just thirty years.

Other examples of rapid evolution include quick changes in cold tolerance by sticklebacks,
¹³
small fish that occur in both marine and freshwater bodies of water, and a shift to longer straw-like mouthparts in the endearingly named soapberry bug when it encountered an invasive weed with fruits that held deeply buried nourishing sap.
¹⁴
In about a dozen generations, anoles, those small lizards sometimes erroneously labeled chameleons in pet shops, changed their body shape and the length of their hind legs when researchers put them in new places within their native Bahamas, with different competitors for food and places to hide.
¹⁵
The spires of snail shells evolved new curves in less than twenty years.
¹⁶

A recently discovered, and rather clever, case of rapid evolution concerns some of everyone’s favorite animals: toad-eating snakes. Okay, that is not exactly correct. More accurately, they are snakes that sensibly
avoid
eating toads, since toads, like other amphibians, are quite toxic. Poison dart frogs get their name because indigenous South Americans used the secretions from their skin to make their arrows or spear tips more deadly. Even if less lethal, other types of frogs and toads should generally be handled with caution, and you are well advised to wash your hands thoroughly after touching them.