The flies’ extreme genetic similarity may suggest that these species are very young, that they evolved very recently. If so, the pressures of sexual selection may sometimes act much faster than the pressures of environmental change—at least, to start the ball rolling, to make a branch start to split and fork.
Evolutionists have argued about sexual divergence for years, as they have about competitive divergence, and this argument runs almost exactly parallel with the former. Does the divergence in sexual taste occur while the species or varieties are apart, separated by physical barriers like cliffs, valleys, and arms of the sea? Or do sexual tastes diverge when the varieties are neighbors—are they driven apart by selection?
Now, in the Galápagos, the finch watchers know that the pressures of sexual selection can sometimes compare with the intensity of natural selection. “And to the extent you’ve got isolation, it’s based on beak shape,” says Dolph Schluter. So both divergences seem to be linked in these finches. Both kinds of Darwinian selection, natural and sexual, have gotten fixed (at least partly) on the birds’ beaks. And both kinds of selection seem to be changing now, judging by the rise of the hybrids.
We think of sexual preferences as constant and unchanging through the generations. But now we really have to ask on what grounds we assume they are constant. We know there is variability—in some cases extreme variability—from one individual to the next, both in sexual tastes and in sexual characters. We know that these features are heritable, passed on from one generation to the next. We know that they are subject to powerful selection pressures. And we know from experiment that selection pressures can produce rapid changes from generation to generation.
So on what basis do we assume these things are more or less stable, like the rocks in the stream, and not merely waves and ripples in the stream? Arrangements between males and females—arrangements and behaviors we think of as primary, as given, fixed, almost as immutable as naturalists before Darwin considered species—these are not permanent at all. Behavior is the product of forces, contending forces that are still contending today, struggling within each generation. The borders between species are continually tested and redefined by the outcorne of each member of each generation’s luck in love—an amazing thought.
Darwin never understood how the intertwining of the two kinds of selection he put on the map, natural and sexual selection, could lead to the creation of these invisible coasts. He wrote relatively little about the subject in the Origin (despite the title). Nor did he treat it at any length in Natural Selection. “The twenty-five pages on species and speciation in his unfinished big book manuscript,” the evolutionist Ernst Mayr wrote recently, “contain so many contradictions that they are almost embarrassing to read.”
Now the picture is becoming clearer, with the help of those who survey both coasts at once, the visible and the invisible, and it is an extraordinary picture. All these forces, a wide field of conflicting forces, play like winds across the adaptive ranges that hold each species apart, or like storm winds at sea. The borders between species are as fluid and adaptable, as sensitive to changes in pressure, as the heaving waves in a high sea. And winds can split the waves, as if splitting the mountains or sending a new mountain or new archipelago up above the rest.
What drives the first widening wedge? It is (to switch metaphors) a little like the splitting of an amoeba: one population goes one way, and one goes the other. You have one vessel, one gene pool, and you end up with two. And the beginning of the split can be a very small thing. A detail can make the difference. Even a detail that has no adaptive significance can make all the difference in the world. In other words, the origin of species can lie in the kinds of small, subjective decisions and revisions that in our species come under the heading of romance.
We are exquisitely sensitive to one another’s features, and they can be more fateful than we dream. “Cleopatra’s nose, had it been shorter, the whole face of the world would have been changed,” says Blaise Pascal in his Pensées. A little less from bridge to tip (or a little more) and Julius Caesar and Marc Antony might not have fallen in love with her. If Cleopatra’s nose had been shaped a little less like the Grecian ideal, and a little more like Cleopatra’s Needle, there would have been no Alexandrine war, no sea-fight at Actium. The whole arc of the Roman Empire would have been reshaped by Cleopatra’s beak.
When Darwin met Captain FitzRoy for his job interview, the captain took an instant dislike to Darwin’s nose. The captain was an amateur phrenologist and physiognomist, and prided himself on his ability to judge the character of his men by their skull bumps. FitzRoy felt sure that he was looking at the nose of a lazy man. He almost sent Darwin home. We might have lost the Origin and The Descent of Man. The whole face of human thought was almost changed by Darwin’s beak.
ON THE ISLAND of Genovesa, sheltered by Darwin Bay, there is a lagoon that is one of the loveliest and even sexiest spots in the Galápagos. It is a pale blue pool of clear water and white coral sand, with no leeches and no sharks. Masked boobies gather in the bushes that fringe the lagoon, and perch on the lava bluffs above it, among the prickly-pear trees. Great frigatebirds with deflated red balloons (gular sacs) dangling from their chests fly overhead. In courting season these males stand around together among the salt bushes. Each time a female flies by, they shake their wings, puff out their shockingly red balloons, throw back their heads, and howl.
The Grant family camped by this lagoon for years. Nicola and Thalia may be the only human beings ever to have been more or less raised in this strange paradise, beneath the phantasmagoria of booby cries and finch songs, and the big wing-rustles of circling frigatebirds. Here they read The Swiss Family Robinson—and hated it. They thought it was ridiculous that everything the family needed simply washed ashore. They liked Robinson Crusoe better, because that was more like life in the clamorous solitudes of Daphne and Genovesa. The girls built driftwood rafts and did their homework in the middle of the lagoon (home schooling on Genovesa). Laurene Ratcliffe remembers Nicky one year playing the violin to the lava gulls. And it was here at the lagoon that Thalia learned to draw.
The Grants first began camping here in 1978, the year after the great drought. They pitched a tent on the cliff above the lagoon and began to measure variation in the beaks of the large cactus finches. As part of their standard measurements they looked at the two groups of singers on the island, the A singers and the Β singers. The As were singing variations on a theme that sounded like chuh-chuh-chuh.… The Bs were singing variations on a shorter theme that sounded like chrrrrr.
That first year by the lagoon the Grants were surprised to find that the As and Bs differed slightly in their beaks. The As’ beaks were on average narrower, shallower, and longer than the Bs’. The difference in length was only about a millimeter, but of course a millimeter can make a world of difference. In fact that year in the dry season they saw the As, with their longer beaks, drilling holes in cactus fruits and pulling out the seeds. They never saw any of the Bs doing that; instead the Bs were concentrating on fallen cactus pads, ripping and tearing them open with their beaks, eating the pulp, and picking out the grubs. The Grants never saw any of the As doing that. So when it came to feeding, the single millimeters difference was forcing the two groups of singers somewhat apart.
No one had been posted to watch the finches on this island the year before, during the drought. But the drought had struck here, as it had on Daphne Major, and although the Grants could not be sure, the stress of the drought might have caused this disruption in the large cactus finches, much as it had caused a split in fortis, the medium ground finches, on Daphne. In that split, fortis with the very biggest and the very smallest beaks were favored. The biggest were favored because they could handle the big seeds that are the province of magnirostris; a few of the very smallest female fortis were favored because they were able to poach the smallest seeds, the niche of fuliginosa.
So the drought might have caused a split here too, alt
hough no one had been here to watch it happen. The Grants found another surprise. Among the mated males there was not a single pair of neighbors singing the same song. Yet among the bachelors, the unmated males, the territories were a mix, sometimes A next to A, sometimes A next to B. This suggested to the Grants that the females were choosing to settle down only with males whose neighbors sang the other song type.
When the first rains come to the Galápagos, large females are ready to mate a few days earlier than small females. These large females, who get first choice of the males, tend to choose males whose neighbors—whose nearest rivals—are not singing their song. Now there were more Bs than As. So the As stood out and were most attractive to the big females. In this way, the year before, during the drought, more big females might have mated with the A singers, and their offspring might have had larger beaks than the Bs’. Then in the intense drought the differences in beak length could have been widened further by disruptive selection. Those with the longest beaks could drill the fruits best, and those with the shortest could rip the pads best. Selection would favor the longest and the shortest beaks and widen the difference between them.
The Grants will never know for sure if that is what really happened. They still kick themselves. If only they had been at the lagoon the year before! An evolutionist has written, “It is a fundamental difficulty of an historical science like the study of evolution that one can never establish the cause of a past event.” In this case the past event was less than a year before.
Whatever had happened during the drought year, the Grants could see that the population had split slightly apart as if by the thin edge of the wedge. The separation in breeding also showed up in a more direct way, in the nest. When Darwin’s finches first hatch, their beaks are always either pink or yellow. It is much the same with barnyard chickens. The beaks of a brood of hatchlings may be all pink, all yellow, or some of one and some of the other. The color lasts for their first two months of life.
As the Grants went around banding nestlings that year, they noticed that the nests of the song A males had more than twice as many yellow-beaked chicks as the nests of the song Bs. If the two groups had been mating and mixing at random, that would not have happened; they would have fathered roughly equal frequencies of yellows and pinks. So it seemed the gene pools of the As and the Bs were partly divided too.
Thus the Grants were looking at two groups that showed signs of developing the kinds of fixed differences in beaks, songs, and survival skills that mark the separate species of Darwin’s finches. The difference in the beak measurements of the two groups, the As and the Bs, was about 6 percent. The difference in separate species of Darwin’s ground finches averages about 15 percent.
“Subdivision is incipient speciation,” the Grants write, and then add, with their usual caution, “or at least it provides the potential for speciation.” At the time they thought that might be what they were about to see.
Unfortunately, the dry season of 1978 was very dry on Genovesa. Of the 120 nestlings the Grants banded by the lagoon, only six were still alive a year later. So the Grants could see that they would need to keep watch a long time on Genovesa, as on Daphne, to understand what might be going on.
It would not have taken much for the split between the As and the Bs to widen. The daughters of these two lines would have to keep on choosing males that sang the same song as their fathers. Then the division would have kept on growing.
But the next year the females around the lagoon chose males that sang the opposite song as their fathers as often as they chose males who sang the same song as their fathers. They chose males with the long form of the beak as often as males with the short form. (Maybe a difference of only a millimeter is not enough to make or break a couple.) Because they interbred this way, the beaks of their sons and daughters were in between the beak lengths of the two lines, and the tantalizing differences in beak length disappeared and fell apart. Their offspring no longer ate in two different ways, and they were as likely to have yellow or pink beaks as anybody else.
By 1979 the checkerboarding of the territories of the As and Bs was gone, although it may have appeared again briefly once or twice, just for a week at a time, in later years. (The Grants feel sure it did; but to a disinterested observer their evidence seems softer than usual here. The boundaries of birds’ territories are harder to measure than the shapes of their beaks, and of course the Grants were looking very hard to see if that checkerboard would come back.)
In any case, by 1981 the link between the shape of the beak and the songs of the As and the Bs was completely gone. A millimeter still made the difference between the fruit borers and the pad rippers, but the birds no longer divided into As and Bs.
In 1985 another drought came to Genovesa, and this time the Grants were there to watch. Not a single drop of rain fell that year. But this time, the drought did not partition the birds into two groups, as the Grants believe it may have done in 1977. Of course the island was a different place in this drought than it was in the one before, because of the crazy Niño of 1982–83. Most of the cactus trees had been overrun and toppled by vines. There were hardly any fruits and seeds to be had anywhere in anyone’s territory. So the pressures of the drought favored short, deep beaks for ripping up dried cactus pads, but there was no separate niche for skinny, long beaks. The drought of 1985 pushed the whole population in the direction of the only niche that was open to them. The survivors had significantly deeper beaks.
In a sense the future evolution of this population of cactus finches is cramped by its neighbors. These cactus finches are not alone by the lagoon. There are sharp beaks to pick up the small seeds, and there are big beaks to pick up the really big seeds. So the cactus finches are hemmed in. There is no big new empty niche waiting for a group of them, like adventuresome colonists, to split off and explore; at least, there is no niche the Grants can see. These finches have been on Genovesa for a long, long time. There is no new world out there by the lagoon that is just sitting there, ripe for the taking.
Thus the Grants suspect that the finches here are perpetually being forced slightly apart and drifting back together again. A drought favors groups of one beak length or another. It splits the population and forces it onto two slightly separate adaptive peaks. But because the two peaks are so close together, and there is no room for them to widen farther apart, random mating brings the birds back together again.
These two forces of fission and fusion fight forever among the birds. The force of fission works toward the creation of a whole new line, a lineage that could shoot off into a new species. The force of fusion brings them back together. What the Grants saw was the population cinching at the waist like an amoeba in the act of dividing, and the division was just a millimeter in the birds’ beaks.
Chapter 12
Cosmic Partings
I was of three minds
Like a tree
In which there are three blackbirds.
—WALLACE STEVENS,
“Thirteen Ways of Looking at a Blackbird”
In the Origin, Darwin gives a sketch of a few growing twigs on the tree of life. It is the only illustration in the book, and it is highly schematic and abstract. A dozen lines of life, labeled A through L, rise upward through a series of horizontal lines numbered I through XIV.
These horizontal lines represent “long intervals of time,” Darwin says, and as he talks about the diagram he keeps stepping farther and farther away from it, contemplating wider and wider expanses of time.
“The intervals between the horizontal lines in the diagram, may represent each a thousand generations,” Darwin writes, “but it would have been better if each had represented ten thousand generations.…”
“In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or hundred million generations, and likewise a section of the successive strata of the earth’s crust.…”
What Darwin does n
ot do, either in this “Diagram of the Divergence of Taxa” or anywhere else, is to step closer and contemplate the moment of divergence itself, the point where one line splits in two, where two roads diverge, the cosmic parting.
He could not look at the point of origin in any particular case, because he did not have one to offer; he did not try to contemplate the mechanics of divergence even in the abstract, because he did not know enough.
Today, as more investigators take up the study of evolution in action, they watch for these cosmic junctures, the point where lines diverge, the parting of ways. What has already emerged from these studies is how fast divergence can happen, how much we can actually see in real time. We are not talking about the lapse of ages.
Some years ago, for instance, the evolutionists Theodosius Dobzhansky and Olga Pavlovsky published a now-famous article in the Proceedings of the National Academy of Sciences entitled “Spontaneous Origin of an Incipient Species in the Drosophila paulistorum Complex.”
“It has been questioned, by His Holiness Pius XII among others, whether biology has really succeeded in making a species from another species,” the evolutionists began. The process of speciation “is generally too gradual and slow to be observed directly. An exceptional situation … is reported in the present article.”
The Drosophila paulistorum complex is what is known as a superspecies. It is a cluster of populations that are partly but not completely isolated from one another by their build or their behavior—like Darwin’s finches, but more promiscuous, interbreeding more freely. The paulistorum complex consists of six populations of fruit flies that are, in the words of Dobzhansky and Pavlovsky, “too distinct to be considered races of the same species but not distinct enough to be regarded [as] full species.” Their ranges lie across much of Central and South America, from Guatemala to the Andes.
The Beak of the Finch: A Story of Evolution in Our Time Page 21
