With a few strokes of the keyboard, Rosemary can also locate all of the females that this old finch has mated with: the life and loves of 3425. Rosemary puts her index finger to her cheek, studying the screen. “He bred in 1983 with the same female as in ’82 … seven times. But in the middle of the season he had another female, 4629.” Like so many other finches in the flood year he turned fickle and swapped mates. “She died in 1987. And in ’84 he bred with yet another female, 5538. She’s also dead now.”
Peter pokes his head in Rosemary’s door. In place of the black binoculars he wears in the Galápagos, a pair of black reading glasses hangs by a cord around his neck. He is deeply tan, and dapper in a tropical khaki shirt. If anything, he looks even more like Charles Darwin here at Princeton than he does in the islands. Some of his colleagues wonder if he nurtures the resemblance. As if to deflect this suspicion, Peter has taped to his office door not one but two identical yellowing newspaper photos of a bearded Scottish bagpipe player. Peter looks even more like the bagpipe player than he looks like Darwin.
“How’s it going?” Peter stoops and glances at the monitor, which is still displaying the love life of 3425, with the running heads above the columns, SPECIES, EGGS, NESTLINGS, FLEDGLINGS, SECTOR.… He straightens briskly, without studying the screen, and pronounces with a positive pleasure: “I make it ten fledglings produced over his lifetime before this year. And this year he produced two. And none of those were recruits.”
“Oh, you have that in your head, do you?” Rosemary says.
“I’m practically certain of it,” says Peter. “Swapping mates didn’t do him any good. Of all the fledglings he’s produced—two of the ten in 1984, and the other eight in 1983—not one has reproduced. So this bird is a loser.”
Rosemary casts up onto the screen the lifetime batting average of another old male of the same generation, 2666.
“He’s been a good producer of fledglings, 2666,” Peter says, this time without even looking at the screen. “In the region of thirty. And some of them are still around and breeding. At least two. Maybe better than that.”
“He wasn’t the most successful breeder, though,” says Rosemary. “That was 720—was it he, Peter? With ten recruits. Quite an achievement.”
“Ah, yes, 720. God bless his soul; rest in peace! The most successful of all time.”
The sun has gone behind a cloud now. A high of 87° F is predicted for today, but in Rosemary’s office the air is cool. Tote umbrellas and raincoats hang from a peg rack by the door. Otherwise there is very little personal in the room: only the row of computers, the shelves of diskettes, a great wall of books, a few framed portraits of cactus finches, and Nicola Grant’s phone number taped to the telephone.
Now that the girls are grown, Rosemary’s role in the research has expanded. The first monograph of the finch watch, published in 1986, is by Peter R. Grant. The second monograph, published in 1989 and focusing on Genovesa (“Rosemary’s Island”), is by B. Rosemary Grant and Peter R. Grant. “They work as a couple—they work as a unit,” says one old friend. “The world will take much of what they do as Peter. Yet they really do something that transcends either one of them. That’s developed over a long period of time, probably to a greater point than even they themselves are aware.”
Peter vanishes, then pokes his head back in Rosemary’s door: “I’m setting up lunch, darling.” But Rosemary keeps going. She calls up the life story of 5608, which was the other rogue finch on the north rim that morning: the Princeton bird, banded orange over black. Another loser; but then, the losers are so much more common than the winners.
“So,” she says at last. “For any bird, if we want to know the dimensions of its beak, or who were its parents, or when was it born, or where was it born, or who were its siblings in the nest”—she recites this list almost in a tone of incantation—“we have them all right here, four or five generations, going right back.” That is, four or five generations counting by life-spans, more than twenty generations counting by begats: a Book of the Chronicles, or a Book of the Kings, all devoted to Darwin’s finches.
“Then we have the vegetation,” Rosemary says, with a little laugh, waving her hand at another set of little boxes. “It’s all there. Here are the cactus data, for example.” There is the Star Trek sound of a diskette ejecting from the Macintosh, and then the thunk of “Compiled Cactus Data” going in. By pressing a few more keys she can produce an instant graph of cactus yields on Daphne Major for the last fifteen years.
“And then the seeds, the yearly seeds,” she says. “We have seed files across all the years up to this.” Rosemary loads and displays one of these disks, an enormous file called SEEDYEARN.
“Then of course there’s rainfall, temperature.…” More boxes of diskettes. “And we have the notebooks, of course, and the daybooks.” (The daybooks are fair copies of the notebooks, made in the evenings on the islands.) She riffles one of the latest daybooks: pages and pages of neatly inked numbers.
“And these are the songs.” This morning the songs of all these generations of Darwin’s finches are neatly stacked on a table beneath Rosemary’s bay window. Students have transcribed the songs from tape recordings using a machine called a sound spectrograph. The machine takes each song on the tapes and represents it in rows of thin, finely spaced vertical lines. Each picture is a snapshot-sized piece of white paper, bearing a single finch’s song.
“We’re getting them bound, just today,” says Rosemary, flipping through one stack of songs. She laughs again. “And so on, and so on, and so on! It all takes a surprising amount of time, but it’s fun. As of this moment, every finch on the island has been banded but one. That one is right up at the top of Sector Four. He’s been there for years. A terrible place, really horrible. Everyone has tried to catch him. He’s right up here,” she says, pointing to the spot on a map of the island. “It’s very steep, going down into the crater. Impossible to put a net there. Also he flies. He’s a very difficult bird.”
THE GRANTS HAVE CARRIED the islands home in their pockets. Now, sitting in Princeton, they can look for action that was invisible from the rim of Daphne Major: secrets they were too close to see, when they were measuring the beak of one finch while three others perched up and down their wrists to watch.
Although their sabbatical has just begun, and they are still taking turns at the computers, checking and rechecking their data, they feel an excitement they could not explain very clearly to anyone else. Some of the latest figures from the field are puzzling. Peter and Rosemary have found a set of statistics that does not fit, the suspicion of a surprise. They first noticed this anomaly while they were out in the islands. In fact they have been half noticing it for a few years. Now the more they study it, the richer and stranger it seems.
Back in 1983, for instance, during the mating rush of the super Niño, a male cactus finch on Daphne Major, a scandens, courted a female fortis. This was a pair of truly star-crossed lovers. They were not just from opposite sides of the tracks, like the Prince and the Showgirl, or from two warring families, like Romeo and Juliet: they belonged to two different species. Yet during the chaos of the great flood they mated, and they produced four chicks in one brood.
“For most families of four,” says Peter, “you get zero, or one, and the rest are never seen again. Maybe occasionally two. In this case all four fledged: 5626, 5627, 5628, and 5629. The first three were sisters, and the last a brother. The brother bred in one year and then disappeared. But the sisters have done extremely well.”
“And their offspring have bred,” says Rosemary.
“And their offspring have bred,” Peter agrees. “I think 5629 turned up in an owl pellet. But they all bred.”
“Here they are,” says Rosemary, opening a file called HYBRIDNEST. “Born in 1983. Their father was a scandens, 4053. Their mother was a fortis, 1536. Three sisters and a brother.”
Rosemary opens the brother’s file first. “Okay,” she says. “The first time he bred, he wa
s the father of four eggs, which produced three fledglings.” So the brother was off to a strong start before he vanished into the owl.
Next Rosemary opens the sisters’ file, and the numbers rush down the computer screen like an invading army. “Now look at this,” she says. “We have some naughts in a few of these years, but if you add up the totals … forty-three grandchildren from the three sisters! As of this year. That does not make them the all-time champions on Daphne Major, but it puts them high. The four were born only in 1983, and they have forty-six grandchildren altogether. That’s very high.
“If I follow those through, there will be great-grandchildren. There’ll be a lot of great-grandchildren, because I know a lot of those forty-six went on to breed. Some of the children bred even in 1983! So we’re going to add up to a lot of birds.
“Goodness! This year, 5626 had six eggs. She produced six eggs and six fledglings. And 5627 produced six eggs and five fledglings. And 5628 produced five eggs and four fledglings.”
According to conventional wisdom, this should not be happening. When David Lack was in the islands half a century ago he tried very hard to spot a finch of one species pairing off with a finch of another, and he did not detect a single case. None of the ornithologists who visited the islands before Lack had ever reported such a thing either. Lack shipped a multitude of caged finches to San Francisco, where Robert Orr, of the California Academy of Sciences, tried to breed them. The birds mated in San Francisco, but only like with like, only within their own species. “Clearly hybridization between species is rare, if not absent,” Lack concludes in Darwin’s Finches.
It was the Grants’ Finch Unit that spotted the first hybrids among the finches. Peter Boag and Laurene Ratcliffe made the discovery in 1976, the year before the great drought. By then the finch watchers knew enough birds that little oddities and rarities could begin to stand out. In that one year Boag and Ratcliffe noticed five male fortis, medium beaks, mating with five female fuliginosa, small beaks. Altogether these five couples produced a dozen eggs. None of the fledglings made it, of course—not a single one of the birds in 1976 survived the drought of 1977. But when the survivors of the drought of 1977 paired off, one of the new couples on the island was, once again, a male fortis and a female fuliginosa.
In nature, the isolation between sibling species is seldom absolute. Among closely related plants, hybridization is more the rule than the exception, as Darwin notes: even annuals and perennials, or deciduous and evergreen trees, “can often be crossed with ease.” Among animals crossing is less common, but it happens. Mallard ducks and pintail ducks interbreed freely. In zoos, lions and tigers can be made to hybridize and produce tiglons. Zebras and horses can hybridize and produce striped sterile zebroids.
If two species intercross very rarely, the mixing will not dramatically alter their gene pools. Often their offspring will be less fit than purebred offspring. Horses and donkeys interbreed all the time, and some of their offspring are famously sturdy, hardier than their parents. But the hybrids are unfit in the technical sense, the sense used by evolutionists, who define and measure fitness in terms of numbers of offspring bestowed on future generations. Crossing a female horse and a male donkey will never change the gene pools of horses and donkeys by even a single gene, because all mules are sterile. Crossing a male horse and a female donkey will not change their lines much either, because hinnies are small, and less useful around the farm; so farmers seldom let them breed, though not all hinnies are sterile.
That was the aspect of hybridization that interested Darwin. His chapter on “Hybridism” in the Origin is mostly about hybrid sterility. He assumes most species would be sterile if they interbred, “for species within the same country could hardly have kept distinct had they been capable of crossing freely.”
The Grants have always assumed, with Darwin, that the offspring of their Galápagos hybrids must be relatively unfit. They thought the hybrids would be rejected, culled, weeded out by the hand of selection. So would the tendency to interbreed. The rare and peculiar tastes that lead to a star-crossed marriage would remain peculiar and rare. For if the products of these mixed marriages had any advantage in the struggle for existence, then the tendency to interbreed would be favored, and hybridization would become more common. And if it became common, and stayed common long enough, all of these species would fuse. The family tree of thirteen finches would taper to a single twig. Their celebrated beaks, their ingeniously varied tool kit, would become one beak.
Since there is not just one species of Darwin’s finches in the Galápagos, Rosemary and Peter were positive that the hybrids and their experimental beaks must be at a disadvantage. And they thought they knew why. Everything they had learned about the struggle for existence of Darwin’s finches in Darwin’s islands argued that the hybrids should do badly. They had seen again and again that the slightest variations in the beaks of these birds can change their fates. Half a millimeter can decide who lives and who dies. Since these slight variations are passed down from one generation to the next, the brood of a small beak and a medium beak would be likely to have intermediate beaks, equipment that would sometimes differ from their parents’ not by one or two tenths of a millimeter but by whole millimeters, maybe by many millimeters. If two hybrids mated, their nest would be a mother’s nightmare, a little novelty shop of strange, assorted beaks. Daphne Major is not a forgiving place. A line of misfits should not last.
During the first half of their study, the years before the great flood, hybrids were so rare that it was impossible for the Grants and their team to draw any definite conclusions. But the mixed bloods did seem to be doing less well than the purebred birds.
That is why the Grants are so puzzled now. There have been too many cases to put down to chance. On Daphne, one of the Grants’ favorite dynasties began some years ago when yet another fuliginosa female paired off with a fortis. “She was the smallest bird on the island,” Peter says. “She’s the very smallest fuliginosa we’ve ever had on the island. And she has outproduced all others of her kind, breeding with a fortis. Never paired up with another fuliginosa.”
This tiny fuliginosa was Number 006. She paired up with fortis Number 459. They nested along the inside wall of the crater, near the top, just where it begins to slope down, in the territory the Grants call “Downstairs.” “And they just, you know, bred very well,” Peter says. “Some of the birds we have on the island right now are the grand- or even the great-grandchildren of that pair.”
The misfits are not dying out; they seem to be thriving. In place of a few interesting individuals, Peter and Rosemary are seeing outstanding families. During their ten-year watch on the island of Genovesa (“Rosemary’s Island”) some of these hybrid families were like Genovesa royalty. They turned out fledgling after fledgling, recruit after recruit, when families all around them were dying out and disappearing. “It went on for three generations!” says Rosemary.
IN ENO HALL there are more distractions than on Daphne Major. Even on sabbatical the work is stop and go. Peter and Rosemary often work through lunch, sitting at a desk in the corner of Peter’s office, jumping up now and then to scrawl on the blackboard, then back to their posts with the door ajar between them. It makes an island of a sort, but there is still the telephone.
“From New Zealand,” Peter calls through the door. “They didn’t receive the fax.”
“From North Carolina Biological. I told them you placed the order.”
By fits and starts they analyze their numbers, compiling and debugging the data sets, collecting all their records on hybrids since the beginning of the great flood. Peter has to fly to Hungary to give a paper at the International Congress of the European Evolution Society in September, and he and Rosemary scramble to finish the last of the calculations before he goes.
They compare the hybrids’ survival rates to the rates of the normal birds in their cohorts. They compare the hybrids’ success in the nest. They compare the successes and failures of several
generations, one against the next.
“Hey, we’ve got it!” Peter cries at last. The grand totals are as weird as they thought.
Before 1983, the year of the flood, pairs of medium beaks and small beaks on Daphne Major produced thirty-two fledglings. None of these hybrid fledglings bred at all until the flood. In fact, only two of them lived long enough to see the flood. These misfits were unfit.
But since 1983, which can truly be called a watershed year, the hybrids have done better. Those that hatched after that year were more likely to survive than the antediluvian hybrids, and they were more likely to breed. They were also slightly more successful than the offspring of purebred fortis or fuliginosa pairs. And these odd couples went right on producing for the rest of the 1980s.
In all demographic studies, one of the crucial statistics is the rate of replacement: the accounting of births versus deaths. There are always three possibilities. A cohort can replace itself over time, births keeping up with deaths; the cohort can grow, births exceeding deaths; or the cohort can dwindle, deaths exceeding births.
When the Grants look at the very latest of this year’s breeding statistics for Daphne, they can see that the purebred fortis and fuliginosa born in 1987 have not managed to replace themselves. They have not produced enough fledglings to keep up with the rate at which they themselves are disappearing. As the Grants put it, the starting number of fledglings in 1991 is smaller than the starting number of fledglings in 1987.
But the crosses between these two species are doing much better. The hybrids born in 1987 have more than replaced themselves: they have increased by a factor of 1.3.
Crosses between fortis and scandens, the medium beak and the cactus finch, are doing better yet. For these hybrids the contrast with the years before 1983, the antediluvian years, is striking. Before the flood, there was only one such pair on Daphne. This pair produced only one fledgling, and their fledgling survived less than one year. But by the end of the breeding season in 1987, the Finch Unit had discovered and monitored a total of five different pairs of fortis-scandens hybrids, and the five pairs had produced twenty-three fledglings. Many of those fledglings lasted longer than purebred offspring of either species; they even outlasted the fortis-fuliginosa hybrids.
The Beak of the Finch: A Story of Evolution in Our Time Page 15
