The farther the flies pressed from their original home in the rain forests, the cooler the weather they encountered. In fact their expansion carried them all the way from the tropical zone to the temperate zone. Lewontin and Birch studied historical records and maps and concluded that the flies had been blocked and slowed in their march across the continent chiefly by this change in climate. Laboratory tests confirmed that the strains of flies in the farthest reaches of their range were more resistant to cold than the flies back home in the rain forest, and there was a neat gradation of cold resistance among the flies in between. These were heritable changes, encoded in the flies’ genes, and all these adaptations had evolved in this species within a single century.
In terms of the adaptive landscape, tryoni was hopping from peak to peak, and as it got farther from the rain forest, each peak was colder and snowier than the one before. Actually the journey was harder on the flies than that, both colder and hotter, since tryoni’s migration from the tropics to the temperate zone exposed it to seasonal swings of temperature that were more and more extreme in both directions.
“Such a process of rapid evolution means rapid genetic change,” write Lewontin and Birch, “and such change, in turn, demands genetic variation on which natural selection can operate. But where did this genetic variation come from?”
It was possible of course that the variation was already present in tryoni back in their primeval rain forest—present in the form of extremely rare genes—and that these genes simply became selected, became more and more common, as the flies hopped from orchard to orchard, farther and farther into the temperate zone. Lewontin and Birch could not rule out that possibility, but they were writing to propose another hypothesis.
Tryoni lives side by side with a second species of flies, Dacus neohumeralis. Tryoni and neohumeralis are sibling species, like Darwin’s finches. The flies share most of the same orchards. Mothers in the two species will even lay their eggs inside the very same apple, which means that tryoni and neohumeralis larvae often grow up side by side, like litter mates.
The only thing that seems to hold these flies apart is sex. Tryoni copulates around sundown, and neohumeralis copulates from mid-morning to mid-afternoon. So the two species are isolated from each other by time though not by space. They look and act so much alike that at least one observer had labeled them as only subspecies. But Lewontin and Birch reason, as the Grants do with Darwin’s finches, that “the clear sexual isolation and the maintenance of their separate identities in nature” warrants calling each of them a separate species.
Tryoni has some bright yellow markings, whereas neohumeralis is plain dull brown. Lewontin and Birch point out that intermediate forms turn up fairly often in collections: flies with a little mosaic of the yellow and the brown. Careful studies proved that these intermediates are, in fact, what they seem to be: hybrids, products of rare crosses between tryoni and neohumeralis. So the separation between the species is not absolute, any more than it is among Darwin’s finches. One fly likes the lights on, one fly likes the lights off, but every once in a while a pair of them gets together anyway.
“This gene exchange has not been sufficient to merge the species,” write Lewontin and Birch, “presumably because of selection against hybrids, but has been sufficient to incorporate foreign species genes into the gene pool of each.”
They are two species as closely related as Darwin’s finches. They often pass genes back and forth, like the finches. They are held apart by natural selection, as the finches were during the first half of the Grants’ study.
With the flies, there seems to be a loose equilibrium in the two gene pools. Alien genes are lost as selection weeds out the hybrids, and more alien genes spill in again as the rare pair meets and mates somewhere along the invisible border that separates their two kinds. Lewontin and Birch suggest that it was this introgression of genes that had led to the rapid adaptation of the flies and allowed them to expand into a radically new physical and adaptive landscape.
To test this hypothesis, Lewontin and Birch performed an experiment in the laboratory. They collected flies of both species and bred them in the lab. Then they set up population cages at three temperatures—20°, 25°, and 31.5° C (68°, 77°, and 89° F)—cool, warm, and hot for these flies.
Lewontin and Birch allowed populations of each species to evolve for two years at each of these temperatures. They witnessed the evolution of a new, combined strain that was more fit than either species is separately. There were marked and rapid genetic changes.
“The introduction of genes from another species can serve as the raw material for an adaptive evolutionary advance even though the original hybridization is disadvantageous,” Lewontin and Birch conclude. “How often this has happened in nature is another question.”
PETER AND ROSEMARY have just seen it happen in nature—on some of the most remote islands in the world. A new vista is opening before them. They are looking at a very broad event, whose action has encompassed the whole of their watch in the Galápagos.
“Under some circumstances,” Peter says, “the populations are maintained as separate entities, because any hybridization that occurs, however rarely, is penalized. The offspring are not as fit. Their chances of surviving to reproduce are not very good. Then comes the rare event.” A terrible drought, or a plague, or a once-in-a-century flood shakes up the island, transforms the adaptive landscape so that the peaks and the valleys are no longer where they were before. The whole adaptive landscape gets shaken like a rug and thrown down again in haphazard new wrinkles and folds. Now the birds that fall in what used to be a valley may find themselves perched on a new, rising peak. Suddenly they are at an advantage. “That causes a very, very slow fusion of the populations,” Peter says. “That’s the direction in which hybridization is pulling.
“But before that goes very far, I think the pendulum will swing back the other way. And at the very least arrest, and at the most reverse, the process.”
“The net result,” Rosemary pronounces: “fusion or fission!”
“At times, we think now, hybrids are at a disadvantage,” Peter says, “and at times, at an advantage. In the last ten years, the hybrids were at an advantage. But in the ten years before that, the hybrid birds were at a disadvantage. So our mental model is one of oscillation, an oscillation of hybrid superiority and inferiority.”
They see a sort of vast, invisible pendulum swinging back and forth in Darwin’s islands, an oscillation with two phases, each phase lasting a decade or more. “Put the two together, and it is very unlikely that the fusion will go to completion before the wheel of fortune is, so to speak, reversed.”
Peter Boag and Peter Grant have projected the consequences of this kind of action for fortis and fuliginosa. The Grants summarize those results in a paper for the Proceedings of the Royal Society of London. “At the observed rate of interbreeding,” the Grants write, with “no hybrid advantage and no selection, it would take more than fifty generations, or more than two hundred years, to eliminate the morphological differences between them.”
Their estimate is conservative. If they factor in the hybrid advantage that the Grants have seen since the flood, then the change would take less time—somewhere between one hundred and two hundred years. If they factor in the increasing rate of interbreeding, it would take less time yet.
In the two decades they have been watching, the Grants have seen the pendulum swing toward drought, toward flood, and back again. They have seen the adaptive landscape heave in slow motion like whitecaps in an invisible sea. When the landscape returns to something like its condition before the great flood, when the land dries out and the cactus and the Tribulus come back into their own, the flow of genes between the species should dry up too. Then, as the Grants write, the hybrids that have flourished in this present interval, in the landscape as it stands now, will be at a disadvantage again, they will be weeded out by natural selection, “and the three species will persist as three separate
species, until the next extraordinary El Niño event occurs. Over the past 500 years El Niño events classified as ‘strong’ have occurred one to three times a century.”
If conditions keep oscillating on the islands more or less as they have in the last half of this millennium, there should be no time for the birds to fuse. The very existence and persistence of the thirteen species argue this. “Surely hybridization must have been selected against,” Peter says. “So, hybridization has not been strong enough to bring these species into a state of panmixia,” he says, pronouncing with some pleasure the exotic lilt of the word.
The Grants need to watch longer and study more to be sure. But this is the view that seems to be opening before them. Whenever the adaptive landscape heaves and flings about, like a sea under heavy winds, the hybrids among Darwin’s finches will be favored. They will intermingle their genes. But when the landscape returns to the pattern it held before the storm, the birds will settle back to their old peaks, and the sharing of genes will slow again.
THE GRANTS HAVE BEGUN to think about how far all this goes beyond the Galápagos. “Hybridization,” as they have written this sabbatical in an article for the journal Science, “provides favorable conditions for major and rapid evolution to occur.” There are a total of 9,672 species of birds in the world today. Back in 1975 a German ornithologist, W. Meise, estimated that about 2 percent of the younger, more recent species hybridize regularly, and about 3 percent more hybridize occasionally. In 1989 a Russian ornithologist, E. N. Panov, compiled a more extensive list, including every species of bird that has ever been seen, even once, to hybridize. As the Grants note, “No other class of organisms of comparable size is known so comprehensively.” And the new numbers look interesting.
The total number of bird species in the world is almost 10,000. Almost 1,000 of them, the Grants write, “are known to have bred in nature with another species and produced hybrid offspring … roughly one out of every ten species.”
Among some orders of birds the incidence is even higher. Hybridization seems to be quite common among grouse and partridges, also among woodpeckers, hummingbirds, and many species of hawks and herons. It is highest of all among ducks and geese. Of the 161 species of ducks and geese in the world, 67 species have been known to hybridize. In other words, as the Grants note, almost one out of every two species of ducks and geese has been seen to interbreed in the wild.
The actual incidence is likely to be much higher. After all, Darwin’s finches have been one of the best-studied groups of birds in the world for most of this century. Yet it is only now, after this extraordinary watch, involving generations of birds and generations of graduate students, that the extent of the mingling of genes among Darwin’s finches has come to light. No one has ever followed a set of species of birds in the wild (or any other kind of animal in the wild) with this kind of near omniscience, with every single individual in every generation identified and tracked, its family tree charted and its fate recorded at last in a waterproof notebook with a cross and an R.I.P.
Not so long ago, hybridization among birds was thought to be very rare. In 1965 Ernst Mayr, one of the finest ornithologists and evolutionists of this century, wrote, “On the basis of my examination of random collections, I estimate that perhaps one out of 60,000 wild birds is a hybrid.” His estimate may be correct for old and well-established species. But now it seems possible that the interbreeding of birds is more common among younger lineages, where, in Darwin’s phrase, we find the manufactory of species still in action. And the process may be important for evolution, the Grants write, “because it produces novel combinations of genes, as well as new alleles [variant forms of the same gene], thereby creating favorable genetic conditions for rapid and major evolutionary change to occur.”
It may seem improbable that the crossing of lines could do so much to shape the tree of life. But the power of intercrossing “is not hypothetical,” to recycle the phrase with which Darwin introduced the power of natural selection. Among plants the intercrossing of species can create new species, and it can do so literally overnight. “As many as forty percent of plant species may have arisen in this way,” the Grants write. That is a huge number of species. Somewhere between a third and a half of all the green things on this earth, and at least half of the world’s flowering plants, arrived by the mixing of genes from separate species.
Traditionally, evolutionists have thought of this kind of intermixing and rapid evolution as the more or less exclusive property of the plant kingdom. Mayr concluded that hybridization was unlikely to play much of an evolutionary role among higher animals. Yet that may not be true. Certainly it is rarer among animals than plants, but among birds and many other groups of animals, it seems, hybridization is widespread. It is common in toads of the large genus Bufo and in many families of insects. It is extensive among fish, which usually spread their sperm and eggs in the water to be fertilized outside their bodies, rather like plants. Mayr himself cites “occasional or extensive hybridization” among lampreys, trout, salmon, whitefish, catfish, pike, goodeid killifish, live-bearers (including John Endler’s guppies), silver-sides, perch, sunfish, and more.
The flowers we enjoy so much in the plant kingdom are really sperm throwers and sperm catchers. “As we delight in the strange and exotic beauty of orchid flowers,” writes a British biologist, “it is salutary to reflect that we are, in essence, looking at their genitalia.” Because they are open to the winds, they catch a lot of alien sperm that animals’ sperm catchers are more likely to dodge. Being animals we find the flowers’ arrangement peculiar. But in the big picture, in the way our lines grow and split on the tree of life, our kingdom may not be so different from theirs. “Animal species may be more like plants than is generally realized,” the Grants write. Animals may mingle their genes almost as freely as the trees and flowers that send out their sperm to drift on every breeze, and open their flowers to catch the sperm from every breeze. Many animals may have their “genetic systems open to invasion, especially early in their existence as quasi-independent lineages.”
For plants the advantage of all this interbreeding is obvious. Mayr has put it succinctly: “Plants cannot move. A seed germinates where it drops and must succeed or die.” So as the plants’ pollen is swept from one plant to another by winds and insects, hybridization is not only inevitable but also desirable, because so many myriads of seeds will fall and sprout in adaptive landscapes that are different from those of their parents. Here natural selection favors great genetic variability, and hybridization is one way to generate it fast. Cross a tree with star-shaped leaves and a tree with spear-shaped leaves, and you can get generations of hybrid leaves with shapes like splayed hands, pyramids, hearts, and arrowheads. That is only the variation that catches the eye—imagine the variation beneath the surface.
Because the Grants are watching so closely, they can see that even on the same desert island, on a lump of rock that looks to the casual eye as changeless as the moon, the adaptive landscape varies with extraordinary energy from decade to decade. So the birds that are bound to this little island, breeding where their line has bred for generations, may often need as great an infusion of fresh variation as plants whose seeds drift hundreds of miles on the wind.
TO THE GRANTS, the whole tree of life now looks different from a year ago. The set of young twigs and shoots they study seems to be growing together in some seasons, apart in others. The same forces that created these lines are moving them toward fusion and then back toward fission.
The Grants are looking at a pattern that was once dismissed as insignificant in the tree of life. The pattern is known as reticulate evolution, from the Latin reticulum, diminutive for net. The finches’ lines are not so much lines or branches at all. They are more like twiggy thickets, full of little networks and delicate webbings. This sort of reticulate evolution doesn’t bind lineages together forever; eventually they part ways or fuse. But it may be a general and hitherto neglected feature of the origin o
f species.
Ever since the Grants and their team published the first news of hybrids among Darwin’s finches, evolutionists have been talking and writing about this new view the finches are helping to open up, the implications of a reticulate tree of life.
“Instead of thinking of the evolutionary chart of the finches as a well-developed family tree with clean branches heading off in distinct directions,” writes the evolutionist David Steadman, who is an authority on the fossil remains of Darwin’s finches, “I find it useful to think of it as a young bush in which branches are so tangled, untrimmed, and interrelated that evolutionary directions remain jumbled and tentative.” He writes, “It is as if, like young adults, they are experimenting with different adult identities, some aspects of which they will keep and some they will discard.”
“In the short term,” writes another evolutionist, Jeremy Searle, “they are not following entirely independent evolutionary pathways.” A novel gene that evolves in one species can spread to others. Life would be so much simpler if lines of animals would only keep to themselves, Searle writes, only half-jokingly. That should not be too much to ask: it is the zoologist’s standard working criterion of a good species. But “things are not so easy for zoologists,” Searle concludes. “It is disappointing that even Darwin’s finches do not seem to quite fit the bill.”
A third evolutionist, Robert Holt, is also greatly struck by this blending of competing lines. “Species that are competitors over ecological time,” Holt writes, “may be mutualists over evolutionary time, each providing a store of genetic variation that can be tapped by the other.
“Maybe we should all be grateful that Mother Nature is a bit slovenly when it comes to reproduction, for this may ultimately permit the unfolding of the bountiful diversity of life on Earth.”
The Beak of the Finch Page 24