The Song of The Dodo, page 24
Still, Darwin didn’t claim that disuse and its heritable effects were the sole factor accounting for Wollaston’s data. Even more important, he thought, was natural selection.
In a typical mainland situation, natural selection will favor robustness, and one standard of robustness is the proficiency of flight. As suming that flight is useful toward finding food, mates, and protection from predators, individual beetles that are good flyers will leave more offspring than poor flyers, and the flying ability of a species will gradually improve. But on an island like Madeira, natural selection might instead punish the big-winged, strong-flying individuals. How so? Because those strong flyers face a higher probability of being swept out to sea by winds. If they are swept out to sea, their genes disappear from the island’s gene pool. Consequently the genes for strong flight will be winnowed away from the insular population. Only the weaker flyers, and the occasional mutants that are totally flightless, will be left on the island to breed. Embracing this hypothesis, along with the more dubious one about acquired characteristics, Darwin concluded that “the wingless condition of so many Madeira beetles is mainly due to the action of natural selection,”—on which point he was right—“but combined probably with disuse”—on which point he was wrong.
Darwin set the notion of disuse to one side and focused more confidently on his main subject, natural selection. He described how it might lead to flightlessness among an insular population of insects: “For during thousands of successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight will oftenest have been blown to sea and thus have been destroyed.” With or without corroborative evidence, coming from Darwin it sounds good enough to be true. Certainly it seems more plausible than the explanation by disuse. But the plausibility of a hypothesis and the high authority of the man offering it don’t guarantee its correctness.
The Origin of Species is a book of encyclopedic richness and inexhaustible tendentiousness, a great potpourri of argument and fact in which a reader can find almost anything a reader might want: Lamarckism, animal husbandry, geology, ethology, experimental botany, the kitchen sink, island biogeography. But Darwin didn’t answer every question, and every answer he gave wasn’t infullible. Evolutionary biologists of the twentieth century have rejected his general idea about the heritable effects of disuse. And one modern researcher, Philip Darlington, has challenged his view of flightlessness among Madeiran beetles.
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DARLINGTON WAS a museum curator and a taxonomist who specialized in the Carabidae, a family of Coleoptera commonly known as the ground beetles. His paper on flightlessness was published in 1943, despite what must have been difficult and distracting circumstances—Darlington served as an army entomologist during World War II, moving from Australia to New Guinea to the Philippines to Japan. He may have written this gentle monograph as an escape from wartime reality. I picture him in a tent, surrounded by jungle, scribbling over a sheaf of smeary pages. “Lieutenant Darlington, what the ding-dong you doin’?” says his noncom assistant. “We got mosquitoes and lice to kill, sir. We got three hundred GIs to dip.” Maybe his beetle manuscript smelled of DDT and half-rotted boots. He mailed it to the journal Ecological Monographs, in which it eventually appeared under the title “Carabidae of Mountains and Islands: Data on the Evolution of Isolated Faunas, and on Atrophy of Wings.” Based on his survey of ground beetles from all over the world, Darlington begged to disagree with Mr. Darwin.
The Carabidae family encompasses about twenty thousand known species of mainly unspecialized, mainly predaceous beetles, abundant on all continents (except Antarctica) and on most islands. Many species are dark, shiny, and flat. Many live on the ground, hiding among stones and plant debris. When disturbed, even a normal-winged carabid is more likely to run than to fly; flight does have its purposes for the typical carabid, but those purposes don’t seem to include routine locomotion and escape. Although the entire carabid lineage has evolved from normal-winged ancestors, species with nonfunctional wings are now common. One-fifth of the carabid species in eastern North America, one-seventh of the species in South America, and almost half of the Australian species are shrunken-winged. The Carabidae lend themselves well to a study of flightlessness because it’s such a recurrent part of their family tradition.
Darlington was careful to focus his analysis on observable wing anatomy, not on presumed flightlessness, since flightlessness is a negative fact (like innocence, ignorance, virginity, or strict vegetarianism) that can be hard to prove. Still, in the cases where wings were drastically reduced, flightlessness could safely be deduced. Darlington used symbols (a plus sign for normal wings, a minus sign for shrunken) to designate wing morphology, but for our purposes it’s as easy to talk about full-winged and shrunken-winged species. His full-winged species presumably flew—regularly, sometimes, or at least on occasions of desperate necessity. His shrunken-winged species were undoubtedly flightless. On the island of Madeira, Darlington figured, nearly two-thirds of all the carabid beetles were shrunken-winged.
On mainlands, he had noticed, there was a similar pattern in the distinction between carabid species of mountaintops and carabids of lowlands. Wing reduction was more common on the mountaintops than in the lowlands. Since mountaintops often hold islandlike patches of habitat, isolated ecologically and swept by winds, it might seem logical to apply Darwin’s explanation—that mountaintop beetle populations tend to be flightless because the good flyers have been blown away. But that wasn’t correct, according to Darlington. “Darwin’s explanation was too simple. The problem is a complex one which cannot be solved by generalities but only by careful analysis of exact data.” So he had gathered some exact coleopterous data from the White Mountains of New Hampshire, the Appalachians of North Carolina, the Sierra Nevada de Santa Marta of Colombia, the Blue Mountains of Jamaica, and from Hawaii, Saint Helena, the Seychelles, and Bermuda, among other places. Shrunken-winged, full-winged, shrunken-winged, full-winged, percentages, percentages, percentages. To make sense of the data, Darlington argued, you’ve got to know a little about carabid ecology.
Some species live almost exclusively on the ground. He called them geophiles. Some species live along stream banks, slow rivers, and ponds. He called them hydrophiles. And there is an arboreal group, living in trees. These three categories helped him refine his perception of the pattern of flightlessness.
On mountains of the mainlands, most carabid species are geophiles, living low to the ground, and most of those mountain-dwelling geophiles are shrunken-winged. The two traits correlate with each other—geophilic, wings shrunken—but they don’t correlate with exposure to wind. It turns out that the most mountainous areas are not necessarily the most wind-raked. Many of Darlington’s mountain-dwelling carabids lived in dense, stable forests on moist mountain slopes, with good protection from wind. But if the winnowing wind wasn’t serving as the mechanism of natural selection, causing flightlessness to predominate among the survivors, then what was?
At least part of the answer seemed to be that geophilic species, within stable forests, didn’t need wings. They lived in dense local concentrations, which eased the problem of finding mates, and since their food also was localized they had no reason to go wandering widely. Furthermore, they had few enemies. So why should they fly? To them flight had become a superfluous enterprise. And because the struggle for existence, as adjudicated by natural selection, mandates strict economy of resources, the energy resources that had previously been allocated to wing development were gradually reallocated toward other ends. Shrunken-winged carabids may have been able to afford stronger legs for running and burrowing, say, or more powerful jaws for seizing prey. Given the lives they led, wings were useless extravagances.
This is different from Darwin’s notion of disuse. Disuse alone can’t cause an organ to disappear from the genetic heritage—but natural selection, judging harshly the misallocation of resources, can.
On islands as on mainland mountaintops, the pattern was more complicated than Darwin had guessed. Darlington teased out its puzzling components. First, he found that carabid species on islands belonged mainly to the geophile group, the dirt-lovers. Stream-loving species and tree-loving species, by contrast, were mostly absent. Second, he found that mountainous islands (such as the volcanic Madeira) tended to support shrunken-winged species, whereas lower islands (such as coral atolls and cays) tended to support full-winged species. Third, he found that warm tropical islands supported more full-winged species than colder islands. Colder islands supported more shrunken-winged species.
Darlington offered his own explanations for the carabid pattern, involving such factors as wing-muscle physiology, population density, and the instability of habitat. Sparing you those details, I’ll just note that Darlington nodded toward the fundamental yin and yang of island biogeography: evolution and extinction playing complementary roles in determining biogeographical patterns. And he touched on an important empirical truth I’ve mentioned earlier: the correlation between limited habitat area and limited species diversity. Darlington’s beetle work was another intellectual advertisement for something called the species-area relationship, which I’ll discuss later in the context of the yang.
Have you forgotten the ratites? Good. Now I invite you to forget also the geophilic carabids. In all honesty, the notion of a dirt-loving ground beetle seems blindingly redundant even to me. But remember the name Philip Darlington, and the fact that in 1943, while quibbling with Charles Darwin’s interpretation of flightlessness among insular coleopterans, Darlington wrote: “Limitation of area often limits both number and kind of species of animals in isolated faunas.” In other words, small places support fewer species than big places. He knew he was talking about more than the beetles of Madeira.
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LOSS OF defensive adaptations, another item from the Insular Menu, shares some logical overlap with loss of dispersal ability, insofar as dispersal ability is equivalent to mobility and mobility is one means of defense. A flightless beetle, besides being hampered from dispersing across stretches of ocean, is hampered also in escaping danger. A flightless bird, likewise. But flightlessness isn’t the only form of secondarily acquired defenselessness; others are loss of protective coloration, loss of warning mechanisms, prolonged infancy, and loss of wariness. The last of those is what makes some island species seem remarkably trusting in the presence of humans.
Loss of protective coloration shows up gaudily in the lizard Uta clarionensis, carrying bright blue markings that make it perilously conspicuous against the dark lava of its native habitat, on Clarion Island off the west coast of Mexico. Loss of a warning mechanism has occurred in Crotalus catalinensis, the rattleless rattlesnake of Santa Catalina. Prolonged infancy is seen in New Zealand’s kiwis, whose eggs require about seventy-five days of incubation before hatching. Loss of wariness is sometimes manifest as ingenuous nesting behavior: In the Galápagos, the blue-footed booby puts its eggs onto a bare patch of ground, unprotected, unconcealed, not even cushioned by a cradle of vegetation. Another form of ingenuous nesting involves building a nest in plain view on a tree limb, where it can easily be raided by a climbing predator. The Mariana crow practices that sort of reckless behavior on the island of Guam. A more cautious bird might at least conceal the nest, or place it beyond reach at the end of a thin branch, or suspend it in an elaborate woven pouch, as the tropical oropendolas do. But oropendolas are mainland species, surrounded by predators and obliged to be more cautious. Boobies can be boobies. As for the generalized reduction of wariness—it turns up, misleadingly called “tameness,” among a wide range of insular animals throughout the world.
The Falkland Islands once supported an endemic species of fox, but the Falkland fox was “incurably unsuspicious of man,” according to one authority, and in lieu of being cured it was extinguished. Aldabra at the start of this century harbored not just giant tortoises but also an endemic species of ibis that was “tame, stupid, and extremely inquisitive,” and it may or may not still exist. The Galápagos hawk also gives the impression of being tame.
The Galápagos boobies seem tame. The Galápagos tortoises seem tame. The herons and warblers of the Galápagos seem tame. The famous Galápagos finches seem tame. In fact, virtually all of the Galápagos fauna seems tame. Anyone who visits the archipelago is bound to be struck by this phenomenon—the tranquil indifference with which the native animals tolerate human intrusion—and Charles Darwin was no exception. He ended the Galápagos section of his Journal by noting the “extreme tameness” of the birds that he had seen.
Darwin had found this trait among all the terrestrial avifauna—the finches, the flycatchers, the one species of dove, the mockingbirds, and others. “There is not one which will not approach sufficiently near to be killed with a switch, and sometimes, as I have myself tried, with a cap or hat.” It was drastically unlike the tropical forests of mainland South America, in which a bird collector couldn’t expect to bring down many specimens by flailing his hat. The Galápagos offered shamefully easy pickings: “A gun is here almost superfluous,” Darwin wrote, “for with the muzzle of one I pushed a hawk off the branch of a tree.” On another occasion, a mockingbird landed on the lip of a water pitcher he was holding and proceeded to help itself to a drink. “I often tried, and very nearly succeeded, in catching these birds by their legs,” he reported. As much as their incautious behavior surprised him, Darwin believed that the birds might once have been less cautious still. He cited an earlier traveler, identified only as Cowley, who had described the Galápagos back in 1684. “Turtle-doves were so tame,” Cowley wrote, “that they would often alight upon our hats and arms, so as that we could take them alive: they not fearing man, until such time as some of our company did fire at them, whereby they were rendered more shy.” And those that weren’t rendered more shy were rendered dead.
“Tameness” is the wrong word for this phenomenon, implying that the animals have been conditioned by experience to feel friendly toward humans. The reality is different: Evolution, not individual experience, has leached them of wariness. Those “tame” Galápagos animals are simply unwary, not merely toward humans but toward any potential enemy. The Galápagos hawk that was so vulnerable to Darwin might have been equally vulnerable to a mongoose. The Galápagos mockingbird that trusted Darwin might have been equally trusting toward a tree snake. Why? Because that species of hawk and that species of mockingbird evolved in a fool’s paradise. Throughout a million or so years of evolutionary history, their ancestors inhabited an archipelago that was blessedly empty of mongooses, tree snakes, and balding young English naturalists.
We can do better than “tameness.” While we’re at it, we can do better than “loss of defensive adaptations.” My own preferred term for this whole set of traits (including the loss of protective coloration, the loss of warning mechanisms, the prolonged infancy, the ingenuous nesting behavior, and the loss of inherent wariness) is ecological naïveté. These animals aren’t imbecilic. Evolution has merely prepared them for life in a little world that is simpler and more innocent than the big world.
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DURING HIS Galápagos stopover, Darwin’s attention was captured at one point by the marine iguana, Amblyrhynchus cristatus, a singular beast even by island standards. This is the only lizard on Earth that dives in the ocean to feed. It grows large for an iguana—four feet long, more than twenty pounds in weight—with a muscular tail that serves well for swimming. Darwin found it abundant throughout the archipelago, always near the water along rocky coastlines, never so much as ten yards inland. “It is a hideous-looking creature, of a dirty black colour, stupid and sluggish in its movements,” he wrote ungenerously. But he admitted that it is graceful when it swims.
He cut open the stomachs of several marine iguanas and found nothing but “minced sea-weed” in thin wisps of bright green and dull red. “I do not recollect having observed this sea-weed in any quantity on the tidal rocks; and I have reason to believe it grows at the bottom of the sea, at some little distance from the coast. If such is the case, the object of these animals occasionally going out to sea is explained.” Ever the astute naturalist, he was right. Marine iguanas live on a diet of algae, which they browse from exposed rocks when the tide is low or dive to when the tide is high. Forty feet, fifty feet down, they can be found feeding on underwater meadows. Ordinarily a dive might last just a few minutes, but sometimes it can stretch beyond a half hour. Bigger individuals are capable of swimming through modest surf, and occasionally a marine iguana has been seen a half-mile offshore. Nevertheless, Darwin noticed, “there is in this respect one strange anomaly; namely, that when frightened it will not enter the water.” He didn’t get that on hearsay. He cornered an animal, grabbed it, and threw it into a large tide pool left by the ebbing sea.
Evolutionary biology, as I’ve said, is not generally an experimental science. But there are historic exceptions.
This interaction between Darwin and a hapless marine iguana is reported in the Journal. Having been rudely tossed, the iguana swam straight back to where Darwin stood, like an apache dancer returning to her partner. Darwin threw it again. Again it returned. “It swam near the bottom, with a very graceful and rapid movement, and occasionally aided itself over the uneven ground with its feet. As soon as it arrived near the margin, but still being under water, it either tried to conceal itself in the tufts of sea-weed, or it entered some crevice. As soon as it thought the danger was past, it crawled out on the dry rocks, and shuffled away as quickly as it could.” But the hapless iguana wasn’t dealing with some idle yahoo, some sadistic schoolboy with a short span of attention; it was dealing with Charles Darwin. “I several times caught this same lizard, by driving it down to a point, and though possessed of such perfect powers of diving and swimming, nothing would induce it to enter the water; and as often as I threw it in, it returned in the manner above described.” Despite his occasional moments of character lapse (especially twenty-three years later, in the matter of Wallace), Darwin seems generally to have been an admirable, unpretentious man as well as a great scientist, and I take this iguana encounter as exemplary. He was inexhaustibly curious. He was observant. He was gentle but firm. He was smart. And he wasn’t afraid to look like a lunatic.
