The Monkey's Voyage Page 9

  Another of Nelson’s American Museum converts, a young ornithologist named Joel Cracraft, studied the anatomy of an iconic Gondwanan group, the ratites—large, flightless birds such as ostriches, rheas, and kiwis, among others. As with Brundin’s midges, Cracraft’s cladogram of the ratites seemed to fit the sequence of Gondwanan breakup. Cracraft also combed both the plant and animal literature and came up with other cases showing an apparent match between phylogenetic history and continental separation, some more compelling than others. Among his plant examples was the first application of cladistic vicariance thinking to another iconic Southern Hemisphere group, the southern beech trees of the genus Nothofagus. For both the ratites and the southern beeches, one could also make a more traditional kind of argument that favored vicariance; few people thought that large flightless birds or plants, such as Nothofagus, with seeds ill-suited for long-distance dispersal, could traverse an ocean on their own. Groups like these had always been conundrums for dispersalists, and now, with the acceptance of plate tectonics, they seemed like obvious examples of Gondwanan vicariance.

  2.6 Southern beech trees, genus Nothofagus, in Chile. The occurrence of members of this genus in southern South America, Australia, New Guinea, New Zealand, and New Caledonia has been attributed to persistence through the breakup of Gondwana. Photo by the author.

  The logic of the cladistic approach, its prospects for producing general explanations on a global scale, and a few nice real-world examples were certainly critical to the rise of vicariance biogeography, but there were other forces at work too. For one thing, there was a kind of scientific seduction involved, embodied by Nelson, in his role as persuasive salesman. Michael Donoghue, a systematist and evolutionary biologist at Yale, who, among many other things, studies the biogeography of Northern Hemisphere plants, was a graduate student in the 1970s, and remembers going to a bar near the American Museum with Nelson. As Donoghue tells it, “We were sitting at a place that had a table that had like a glass top. . . . And he [Nelson] said, ‘How do we know that biogeography isn’t like the following: . . . I take a hammer and I hit it right in the middle of the glass and it just splinters the whole thing into different, like, continents. . . . How do we know it wasn’t like that?’” I asked Donoghue what he thought of Nelson’s shattered-glass analogy, and he said, “Well, I thought it was bold. I thought at least here’s a guy who’s thinking outside the box. And that was the attraction of Gary Nelson.”

  Nelson and other vicariance biogeographers could impress potential converts by being bold and personable, but they could also be bold and bullying. There was definitely a sense of “if you’re not with us you’re against us, and if you’re against us you’re a fool.” I remember in graduate school seeing a talk by one of the American Museum cladists, and feeling afterward as if I and everyone else in the audience had been berated. The speaker’s demeanor during his talk and, especially, in the question-­and-answer period that followed, was like a prolonged sneer. That was hardly an isolated case. Both in person and in print, scientists of the cladistic vicariance school often sounded like arrogant, sarcastic (though intellectually rarefied) schoolboys. As another case in point, Nelson began one of his cladistic papers, not with a quote from some scientific luminary like Darwin or Hennig, but with a Stevie Wonder lyric turned into a condescending dig at his opponents: “When you believe in things you don’t understand, then you suffer; superstition ain’t the way.”

  That attitude clearly turned off many people, especially the “establishment” scientists who were being sneered at. For instance, Darlington, in a rebuttal to Brundin, was obviously miffed by the cladists, writing that their “central attitude is one of self-conscious superiority.” John Briggs, a well-known biogeographer and author of several books on the subject, described with displeasure vicariance scientists “shouting at meetings and interrupting presentations,” a common complaint from more traditional scientists offended by the lack of decorum. On the other hand, the sneering, antiestablishment, anything-goes stance may have been like catnip for young scientists—mostly young, male scientists—already leaning in that direction. Although it would be hard to prove it, I have the sense that cladism and cladistic vicariance biogeography were movements heavily fueled not just by intellectual considerations, but also by this radical belligerence that at times seemed like a stratagem. That attitude translated to an effective style of argument based on equal parts shouting and sarcasm, and also may have served as a recruiting tool—Join the irreverent radicals and be irreverent and radical yourself! You too can use Stevie Wonder lyrics to mock your elders! (And, by the way, if you don’t join us, we’ll be mocking YOU.)

  The whole cladism/vicariance biogeography camp seemed a bit like an unruly cult, an intellectual gang, complete with passwords (“synapomorphy,” “area cladogram,” “component analysis”), a deity (Hennig), and gang leaders (Nelson chief among them), but it was a cult that grew quickly. By 1980 the gang had formed an official organization, the Hennig Society, which published the journal Cladistics and had board members, an annual meeting, and all the other usual trappings of an academic society. Because, of course, it is not the goal of most rebels to remain outside of the establishment, but, rather, to become the establishment.

  Michael Donoghue had said that he was attracted to Nelson’s boldness, his ability to think outside of the box. Replacing the muddled, river-delta view of phylogeny with an explicit, cladistic approach was a valuable leap away from the old biogeography, as was the incorporation of continental drift. It wasn’t all good though. As I will describe in the next chapter, Nelson and many other vicariance biogeographers adopted an approach that, in some ways, was even more constraining than the old New York School dispersalism. On the charitable side, one could say they did it all in the search for a kind of Holy Grail, the ability to make grand generalizations about biogeographic history. But, in the process, they tried to bury the old dispersalism, and in that attempt they ended up arguing themselves into a strange intellectual corner where they envisioned an idealized history of life that never was.

  11The traditional evolutionary taxonomy of the time also was focused on an often misguided search for ancestors and the notion of a ladder of life that positioned living organisms on a scale of advancement, with humans at the top, of course. Note, for instance, the arrangement of the groups in Figure 2.1, with the main trunk leading from fishes through amphibians and reptiles to the supposed pinnacles of evolution, birds and mammals. Cladists rightfully rejected these notions, replacing them with a more objective view of the diversity of life.

  12Because of the delays, Hess’s paper appeared after one by R. S. Dietz which presented a very similar theory. Dietz himself gave credit to Hess for coming up with the theory first, although he also claimed that he wasn’t aware of Hess’s work (which had been circulating before it was published) when he developed his ideas. It was Dietz who coined the term “seafloor spreading” (Lawrence 2002).

  13A Canadian geologist named Lawrence Morley, looking at magnetic evidence from the northeast Pacific, had independently come to the same conclusions as Vine and Matthews. However, Morley’s manuscript was rejected by two scientific journals, and he wasn’t able to publish his ideas until 1964, the year after the Vine and Matthews paper appeared. Nonetheless, the connection between the magnetic data and seafloor spreading is now usually referred to as the “Vine-Matthews-Morley” hypothesis (Lawrence 2002).

  14In fact, some later vicariance biogeographers criticized Brundin for being too much of a dispersalist.

  Anoles, the common arboreal lizards of the Neotropics, are found throughout the Caribbean and must have reached many islands by natural overwater dispersal. The brown anole, Norops (formerly Anolis) sagrei, seems to be an especially frequent oceanic voyager, having spread from Cuba to many smaller islands in the vicinity. (This species has also been introduced to the southeastern United States, southern California,
Mexico, Hawaii, Taiwan, and Singapore.) Some of these journeys probably required rafting, but two researchers, Amy and Tom Schoener, wondered whether the lizards might have simply floated in the sea in some cases—for example, to move between islands in the Bahamas. To evaluate that possibility, they conducted what amounted to a version of Darwin’s experiments on the survival of seeds in salt water, only with lizards as the subjects. They caught brown anoles in the Bahamas and put thirty-nine of them, one at a time, into a saltwater tank in which small waves were generated. Then they recorded the animals’ fates.

  All thirty-nine of the anoles were alive and floating after one hour, and ten were still alive and floating after twenty-four hours. (It’s a little unclear whether the Schoeners rescued the twenty-nine lizards that were not floating at the end of the twenty-four hours. Incidentally, this study was done in the 1980s, when rules about experimentation on animals, especially non-fuzzy animals like lizards, were a bit looser than they are now. It’s doubtful whether this kind of experiment would be approved by an institutional animal-use committee today.) The lizards usually floated with their forelegs extended downward and their heads held well out of the water. These results marked the anoles as champion floaters compared to small mammals such as mice, gophers, and moles, which, as previous experiments had shown, could only float for a few minutes, on average. The Schoeners suggested that brown anoles could indeed have dispersed among islands by floating free in the ocean.

  Why these anoles are such good floaters isn’t entirely clear. However, it seems to have something to do with surface tension: when the Schoeners added detergent to the water tank, to break surface tension, all the lizards immediately sank.

  Chapter Three

  OVER THE EDGE OF REASON

  JURASSIC HAWAII

  Twenty miles off the southeastern coast of the Big Island of Hawaii and 3,000 feet below the surface of the ocean, another island is forming. At that spot, lava periodically spews from the summit of an underwater volcano and cools to basaltic rock, building up the mountain in fits and starts. Along with Mauna Loa and Kilauea on the Big Island, this submarine volcano, named Lo‘ihi, is the active manifestation of the Hawaiian Hotspot, where magma welling up from the Earth’s mantle has created a string of islands as the Pacific Plate slides northwest over the stationary plume of magma. Within the next 100,000 years, Lo‘ihi will likely emerge above the waves, the newest island in the Hawaiian chain and a dramatic embodiment of the geologic origins of the entire archipelago.

  That volcanic origin means that the Hawaiian chain is a series of classic oceanic islands, ones that have never been connected by land to a continent. Beyond that, the archipelago is also the remotest of the remote, located some 2,400 miles from North America and even farther from any other continent (unlikely as that may seem when one is lying in the shadow of a giant resort hotel on the crowded beach at Waikiki). Those facts of geologic origin and location suggest an obvious biogeographic conclusion: the rich native biota of Hawaii must be derived from numerous chance, long-distance colonizations. Almost all biogeographers, even many who are otherwise in the vicariance camp, agree that this must be the case.

  But not Michael Heads.

  Heads is a New Zealand botanist and biogeographer who thinks that successful long-distance dispersal is essentially a mythological process; at best, according to Heads, such events happen so rarely that there’s no reason to even consider them as viable explanations for piecemeal distributions. He believes that virtually all successful dispersal is of the “normal” variety, meaning the usual movement of organisms from one piece of suitable habitat to another.

  Heads’s views remain uncompromising even when it comes to Hawaii, despite the obvious volcanic origins and extreme remoteness of that archipelago. “The taxa that colonized Hawaii,” he says, “did so by normal dispersal . . . the same process you see in your garden.” He believes that organisms made these predictable, garden-variety jumps to Hawaii, not over vast expanses of the Pacific but, rather, from nearby islands that no longer exist. Superficially, this doesn’t seem too farfetched because, in fact, the hotspot that created the current islands also produced a long line of former high islands that now exist as low atolls or submerged seamounts, stretching first northwest from Kauai and then making a bend to head nearly due north all the way to the Aleutians. In theory, many organisms could have made a series of easy jumps as each new volcanic island formed along this line from Alaska to the modern Hawaiian Islands.

  Geologically, then, Heads’s idea for the origins of the Hawaiian biota sounds possible. However, when we start thinking about the plants and animals themselves, we run into a big problem. Normal dispersal by island-­hopping, either from north or east of Hawaii, could not have happened recently, because the relevant islands did not exist. Finches or violets or tarweeds taking short, garden-variety hops from Alaska or California toward Hawaii within the recent past would have ended up afloat in the Pacific.15 Thus, if Hawaii was colonized via normal dispersal, we should find that the evolutionary connections of island lineages to mainland ones are not recent, but instead are relatively ancient, on the order of 70 million years old or older (the age of the earliest islands in the hotspot chain). The problem is that hardly any Hawaiian lineages seem to be that old. Molecular evidence indicates that almost all of the island taxa, including the famous honeycreepers, the Nene Goose, and the silversword plants, separated from continental relatives within the past 20 million years, and most of them did so within the past 5 million years. If those dates are roughly correct, the ancestors of all those organisms must have crossed a wide expanse of the Pacific to reach Hawaii. They could not have colonized the archipelago by normal dispersal, but instead must have done so by more unusual means in each case—birds blown on storm winds or beetle eggs attached to a natural raft of vegetation or something of that sort. In other words, they must have arrived by chance, long-­distance dispersal.16

  Even if one doesn’t believe the molecular dating evidence, there’s a similar, commonsense argument against Heads’s view. This argument is based on the fact that quite a few taxa in Hawaii are classified as subspecies of more widespread species. For instance, the pueo is considered a subspecies of the Short-Eared Owl, the ae‘o a subspecies of the Black-Necked Stilt, the ope‘ape‘a a subspecies of the hoary bat, the o‘helo papa a subspecies of the beach strawberry, the kupukupu a subspecies of the Boston swordfern, and the pa‘u o Hi‘iaka a subspecies of the oval-leaf clustervine. None of these forms were introduced to Hawaii by humans in recorded history, and prehistoric introduction by Polynesians is extremely implausible for most of them; bats, nondomesticated birds, and plants such as the strawberry and swordfern that are not found in Polynesia were not likely passengers or stowaways on the prehistoric boats that reached Hawaii. Thus, the ancestors of these subspecies apparently arrived on their own and afterward evolved slight differences from their relatives elsewhere. If we make the reasonable assumption that these Hawaiian forms only separated from their close relatives, which are still considered part of the same species, within the past few million years (and probably within the past few hundred thousand years in some cases), then we must also conclude that they could not have come from nearby land, because, again, there was no nearby land. They must have reached the islands by natural long-distance dispersal.

  Heads is recalcitrant in the face of these well-known observations about the Hawaiian biota. The evidence for long-distance colonization of the archipelago doesn’t budge him at all. He doesn’t believe the divergence date estimates or the commonsense argument about subspecies. He apparently thinks that most of the Hawaiian biota has been in the mid-Pacific region, on various ancient islands or larger landmasses, since the Jurassic, which would push their origins back to well before the beginnings of the hotspot chain. And his antidispersal ideas don’t stop with Hawaii. He believes the same thing about all oceanic islands—the Galápagos, Easter Island and the Marquesas, Ascension, Tr
istan da Cunha and the Azores, Mauritius, Rodrigues, and Île Amsterdam, to name just a few. Together, the volcanic islands of the world are home to thousands of endemic species, and, according to Heads, the ancestors of all of them reached those islands by normal, everyday dispersal. The same peculiar story must then be repeated over and over again: An island may seem to have been extremely isolated for its entire existence, but that is never really the case. There was always some ancient connection to other land: either a direct bridge or a series of closely spaced, stepping-stone islands over which every instance of colonization could have taken place. There is always reason to reject the evidence for recent evolutionary connections of island species to continental ones and, therefore, the idea of chance, long-distance colonization. Despite what their DNA and morphology indicate, the origins of the island owls, stilts, iguanas, silversword plants, ferns, damselflies, crickets, lava lizards, tortoises, mockingbirds, pigeons, prickly pears, skinks, bristletail insects, and butterflies are all ancient, long predating the current islands. That whipsnake on Isla Clarión, four hundred miles off the west coast of Mexico, may look as if it only branched off from mainland whipsnakes within the past couple of million years, but its origins actually stretch back ten or twenty or thirty times further into the past.