In other words, the offspring of hybrids will be novel and bizarre, and they will require habitats as novel and bizarre as they are themselves. Anderson showed how this might happen in a study of two species of spiderwort that grow wild in the Ozarks. One grows in deep dark woods at the foot of cliffs, he says, while the other grows in full sun at the top of the cliffs. In many ways these habitats are polar opposites. The species at the base of cliffs requires:
rich loam
deep shade
leaf-mold cover
while the species at the top of the cliff requires:
rocky soil
full sun
no leaf-mold cover
The two spiderworts, he says, “are well-differentiated species; neither one of them is by any means the closest relative of the other.” He could cross them readily in experimental gardens, where he learned to recognize the distinctive appearance of the hybrids. But he seldom found one of the hybrids growing in the wild, because the Ozarks rarely provided an intermediate habitat (not even the very middle of the cliff would do).
Anderson did find a few of these hybrids growing wild in the woods. Imagine what would happen if these hybrids crossed, he writes. Their offspring, by shuffling their diverse sets of genes, would now require six new habitats, besides their parents’ two:
rich loam
rocky soil
full sun
deep shade
no leaf mold
leaf mold
rich loam
rocky soil
full sun
full sun
leaf mold
leaf mold
rich loam
rocky soil
deep shade
deep shade
no leaf mold
no leaf mold
The actual number of differences between any two species is much greater than in this schematic example. What is more, Anderson points out, the number of different habitats the hybrids require “will rise exponentially with the number of basic differences between the species. With ten such differences, around a thousand different kinds of habitat would be needed to permit the various recombinations to find a niche somewhere.… With only twenty such basic differences (and this seems like a conservative figure) over a million different recombined habitats would be needed.”
You don’t find situations that chaotic under natural conditions, but you do find them in the havoc that human beings bring in their train. Our arrival, Anderson says, “can provide strange new niches of hybrid recombinations.” Thus, our disturbances hybridize both the environment and the species. We are hybridizing the planet.
Botanists have actually watched this happening. Farmers on the Mississippi Delta treat their lands slightly differently, and botanists have discovered that different wildflower hybrids flourish in the different conditions on each farm. The land is the same, the climate is the same, but the hybrids, as Anderson writes, sometimes go “right up to the fenceline at the border of the farm and stop there.” The same sort of pattern has been documented in the fields and pastures of the St. Lawrence Valley and elsewhere. One botanist has looked at two species of sage that grew in the chaparral of the San Gabriel Mountains. In the chaparral itself he found no hybrids. But he did find them flourishing next to it, in an abandoned olive orchard. “In this greatly disturbed area, new niches were created for the hybrid progeny, which are apparently always being produced in the chaparral but at very low frequency,” Anderson writes. “In this strange new set of various habitats some of the mongrels were at a greater selective advantage.” The sage in the deserted olive orchard was composed of hybrids, and almost none of the original species grew there.
This kind of thing must have happened whenever new islands were colonized by windblown or seabird-carried seeds, as in the Galápagos. It must have happened whenever new continents were colonized by new plants and animals. “At such times and places,” Anderson argues, “introgressive hybridization must have played an important role in evolution.”
In a paper he wrote with G. Ledyard Stebbins, “Hybridization as an Evolutionary Stimulus,” Anderson argues that what we are seeing now in the world is something the world has seen in one form or another many times before. We are ecological dominants, the two evolutionists write, but we are not the first to bestride the world in conquest. “When the first land vertebrates invaded terrestrial vegetation they must have been quite as catastrophic to the flora which had been evolved in the absence of such creatures. When the large herbivorous reptiles first appeared, and also when the first large land mammals arrived in each new portion of the world there must have been violent readjustments and the creation of new ecological niches.”
This would have happened in the Hawaiian Islands, the Galápagos, Lake Baikal, wherever “species belonging to different faunas and floras were brought together” and “physical and biological barrier systems were broken down.” It would have happened most recently in the upheavals of the Pleistocene when ice sheets swept across the continents of the Northern Hemisphere and there were icecaps even on the summits of the island of Hawaii.
So in chaotic times, after each deluge, hybridization may help the standard Darwinian mechanisms to run faster. By intensifying the pressure of selection everywhere on the world and disturbing habitats all over the world we may be creating conditions in which evolution is running at its maximum rate. Anderson and Stebbins argue that “the recent rapid evolution of weeds and semi-weeds is an indication of what must have happened again and again in geological history whenever any species or group of species became so ecologically dominant as greatly to upset the habitats of their own times.”
Under our influence, as Anderson and Stebbins write, “evolution has been greatly accelerated. There has been a rapid evolution of plants and animals under domestication and an almost equally rapid evolution of weed species and strains in greatly disturbed habitats.”
Normally we think of the intrusions of weeds and introduced species as distractions from the great work of nature, the creative process of Darwinism. But this kind of upheaval has happened again and again, though never through the advent of a conscious dominant like ourselves, a conqueror come to watch. So what we are seeing is a specimen of evolution as it has happened again and again throughout the ages. “Far from being without bearing on general theories of evolution,” write Anderson and Stebbins, these events “are of tremendous significance, showing how much more rapidly evolution can proceed under the impact of a new ecological dominant (in this case, Man).”
“The enhanced evolution which we see in our own gardens, dooryards, dumps and roadsides may well be typical of what happened during the rise of previous ecological dominants,” they write. We are doing what the dinosaurs did before us, only faster. We bring strangers together to make strange bedfellows, and we remake the beds they lie in, all at once.
DARWIN AND HIS shipmates collected fifteen finches on the island of Floreana. Five of them were an unusually large race, magnirostris magnirostris: the biggest of the biggest of Darwin’s finches. They must have been extremely common on the island at the time; in old owl lairs on Floreana, the bones of this giant among finches are more than twelve times more abundant than any other finch.
The evolutionist David Steadman has made a study of the great Galápagos finch. He notes that when Adolphe-Simon Neboux and Charles-Rene-Augustin Leclancher of the French frigate Venus collected birds on Floreana in 1838, only three years after Darwin, they did not find a single magnirostris magnirostris. Neither did Thomas Edmonston of HMS Herald in 1846, or one Dr. Kinberg of the Swedish frigate Eugenie in 1852. In fact magnirostris magnirostris has never been seen since. It seems to have gone extinct almost immediately after Darwin’s visit to the islands. Darwin was the first naturalist to collect a specimen and the last to see one alive.
What happened to the giant finch of Floreana? No one on the island was observing it the way the Finch Unit is now watching Daphne. But it is certain that the birds w
ere not wiped out by a volcanic eruption, because Floreana has been dormant for as long as human beings have been in the islands. The finches almost certainly died because of another kind of eruption, the upheaval that human beings brought to their island. The culprit in their extinction is almost certainly the prison colony, which had been established only a few years before Darwin’s arrival. There were two or three hundred Ecuadoran prisoners on the island when the Beagle stopped there, as Darwin recorded in his journal. They had cleared farms in the highlands where they tended sweet potatoes and plantains. They hunted wild pigs and goats and feasted on the giant tortoises, and at least one family ate finch soup.
In some ways the arrival of the farmers was a bonanza for the finches. They made pests of themselves on the farms, as they are doing now on Santa Cruz. The “Gross-beaks” that Darwin saw digging up seeds in the fields were almost certainly magnirostris magnirostris.
Unfortunately for the finches, however, the prisoners imported more than seeds to the island. They also brought cattle, goats, pigs, cats, and rats. And though the prisoners were soon gone (the colony lasted only a few years after Darwin’s visit), their retinue of animals remained and multiplied. When Captain A. H. Markham of HMS Triumph visited the islands in 1880, he found it to be “in undisturbed possession of the so-called wild cattle.” He also saw “donkeys, dogs, pigs, and other animals that had been left to run wild on the abandonment of the island by the former inhabitants.”
Magnirostris magnirostris were big birds, and probably didn’t fly much, even by the standards of ground finches, says Peter Grant. They must have been easy for rats and cats to jump on. To make matters worse for the finches, the local cactus went extinct. Magnirostris magnirostris was probably dependent on this cactus, which was an equally unusual local race called megasperma megasperma, meaning big, big seed. The seeds of this cactus are the largest and hardest of any cactus in the Galápagos, about half an inch in diameter. A relic population of the giant cactus survives on a small outlying island, Champion, off the coast of Floreana. The Grants have tested its seeds with their McGill nutcracker. It rates a 20 on their scale for size and hardness, while the next-toughest cactus seed in the Galápagos rates only just under 11. Megasperma megasperma produces these giant seeds in prolific numbers. Giant mags probably could crack those seeds, says Peter Grant, and for a relatively small investment of time, get a big return. But any ordinary finch would find them impossible to crack.
Almost certainly the doomed magnirostris magnirostris specialized in megasperma megasperma. In fact their beaks and the cactus seeds may have evolved in a kind of arms race, the finches’ predations driving the cactus to make bigger and bigger seeds, and the bigger seeds driving the finches to bigger and bigger beaks.
When the prisoners left Floreana their abandoned cattle and donkeys learned to knock over the cactus and chew its flesh for water. In a very short time there was almost no megasperma megasperma left on the island. Without it, conditions for magnirostris magnirostris may have become unlivable. Mockingbirds went extinct on Floreana too, at about the same time. Galápagos mockingbirds nest in cactus.
TODAY THE SCIENTISTS at the research station are trying hard to rescue many of the best-beloved animals in the Galápagos, including iguanas, tortoises, and dark-rumped petrels. Living things on islands are more vulnerable than their relatives on continents, because they are small in numbers, and because they evolved apart from the dizzying welter of competition on the continents. Around the world almost one hundred species of birds are known to have gone extinct since the seventeenth century, together with more than eighty subspecies; more than nine out of ten of these lost species and subspecies lived on islands.
“If goats got onto Daphne, for instance, they would eat out the cacti, which would certainly drive scandens extinct,” says Rosemary. “And if cactus and Tribulus were both wiped out, that would finish all the finches with big beaks and medium-sized beaks.”
Cactus finch.
Drawing by Thalia Grant
That is why the Grants take such extraordinary precautions each time they travel from Santa Cruz to the Landing on Daphne, or to Darwin Bay on Genovesa. They do not want to bring even one pregnant fire ant with them.
“We’d be horribly upset—” says Peter.
“—Oh, my God, yes—” says Rosemary.
“—because that is not the island’s natural state,” says Peter. “And if we were responsible, we’d be devastated.” Every time they land on the island, they wash all the fresh food in salt water, and the mist nets’ poles too. Sometimes they tow the poles in the water all the way to the island, to make sure stowaway ants are drowned. “We always wash the tents and the bird bags, and we just sort of sanitize everything,” says Rosemary. “Usually I take a can of spray just in case, but we’ve never had to use it.”
“Fire ants would kill off all the scorpions, and probably the spiders,” Peter says. “And since these are small islands, the scorpions and spiders would probably go extinct.”
A lot of people would be happy to lose the scorpions and spiders. But then, as Peter retorts, “A lot of people live in cities.”
“If they killed the scorpions, that would affect the Tropidurus, the lava lizards,” Rosemary says.
“But it is the general principle, more than concern for any one species,” says Peter. “If any new species invades, the old balance is gone.”
“The changes would reach through the food chains,” says Rosemary.
“Yes, changes of unpredictable sorts. It would be very difficult to be sure of the consequences for organisms higher up in the food webs. The habitats we are watching are so sensitive and fragile,” Peter says. “It’s easy to imagine major changes taking place on both islands.”
“So few places have not been touched by humans,” says Rosemary. “You can imagine if a big hotel, a Holiday Inn were built on Daphne or Genovesa—with the introduced plants, and insects, and rats, and cats.…”
“Parrots, budgerigars.…”
“You’d get Daphne and Genovesa just totally destroyed.”
ON THE HAWAIIAN Islands, which were once the most isolated archipelago in the world, human beings arrived near the beginning of the first millennium. Many other aggressive species landed with us, in accelerating waves, including pigs, goats, rats, mongooses, mosquitoes, army worms, bookworms, cockroaches, centipedes, and scorpions, not to mention the English sparrows and more than 4,500 species of alien plants. Beneath these tidal waves the adaptive radiations of native birds in the archipelago have been crushed. Species of finches, hawks, owls, flightless ibis, and flightless rails have gone extinct; the finches have been hit particularly hard. “Even casual observers have noted a kaleidoscope of shifting dominance by different species of alien birds over the past thirty years,” says one ecologist, Peter Vitousek; “the one constant has been the near absence of natives.”
In the far north and west of the Hawaiian archipelago there is an atoll called Laysan. The adaptive landscapes of this remote island were turned upside down in the nineteenth century by the arrival of European rabbits and rats. In 1967, in a sort of Noah’s Ark project, the U.S. Fish and Wildlife Service took away more than one hundred specimens of an endangered species of Hawaiian finches and carried the birds by boat about 500 kilometers (more than 300 miles) to a small cluster of Hawaiian Islands known as Pearl and Hermes Reef.
After the Laysan finch was introduced to this reef, Sheila Conant, an ornithologist at the University of Hawaii, inspired by the Grants’ work in the Galápagos, began watching the birds in their new home islands. The Laysan finches are as tame as Darwin’s, and Conant and a few field assistants have been able to band most of them. She could not watch the birds almost continuously the way the Grants and their teams of watchers have managed to do. But she has made a few expeditions to the islands, spaced several years apart.
Conant found that within twenty years of their introduction to the islands of Pearl and Hermes Reef, the finches had begun t
o go off in different directions. On Laysan their ancestors had had shallow, wide, medium-length beaks. Now, in their new home, the finches on Southeast Island have longer beaks, while the finches on North Island have shorter, deeper, narrower beaks.
A Galápagos finch hunts flies on an iguana.
Drawing by Thalia Grant
These are grain-eating finches. The seeds they take range from very small and soft to big and hard, the toughest of all being, once again, Tribulus, which probably got to Hawaii the same way it got to the Galápagos, hitching rides on sailors and whalers. Tribulus is not common on Laysan, but it is one of the main plants on Pearl and Hermes Reef. Correspondingly, the finches in their new home spend much more time working on Tribulus. And the Tribulus mericarps on Southeast Island are larger than on North Island, which may explain the divergence in the beaks of these finches.
The speed with which these finches have adapted is amazing, and it shows how fast the primoprogenitors of Darwin’s finches may have adapted when they first came to the Galápagos. “Are we tinkering with evolution?” asks Sheila Conant. The answer is of course, yes. As more and more species are endangered, more and more well-meaning attempts to save them involve introducing them to new homes or reintroducing them to their old homes. What happens next in these cases will seldom be monitored as closely as the Grants are watching Daphne, and Conant is watching Laysan. But it is clear from these studies that the introduced species, if it survives, can evolve rapidly and unpredictably in its new homes. To Conant this is intriguing and perplexing. Conservationists may be frustrated to find that by moving a species, instead of helping it to survive, they have helped it evolve into a new form.
The Beak of the Finch: A Story of Evolution in Our Time Page 29
