Unambiguous, general, statistical, and repeatable doesn’t sound all that bad, of course. However, in my experience, evolutionary biology (of which historical biogeography is a part) is not a science that lends itself to the kind of narrow methodological approach or, especially, the extreme generalization that came out of the vicariance movement. The impulse to make grand generalizations has a long and fairly unhappy history in this field. For instance, some evolutionists have pushed the view that almost all features of organisms are adaptive (Wallace, among others, was an early proponent of such “panselectionism”), while others, in complete opposition, have claimed that virtually all features arise automatically because of the way development happens (and has to happen), with little input from selection. Similarly, some have consistently attributed the success of especially large evolutionary groups, such as insects, to special traits (“key innovations”), while others see the success of such groups entirely as a product of chance. None of these extreme and extremely simplistic views stand up to scrutiny; the complexity, the plurality of nature always intrudes. I’m reminded of something a friend of mine, an evolutionary biologist named John Gatesy, likes to say when he senses that some biologist is missing the trees for the forest: “The only good generalization in biology,” says John, “is that there are no good generalizations in biology.”
Of course, it could be that vicariance scientists have hit upon the exception to the rule that there are no rules. In theory, the living world could be as they imagine, with just about all disjunct distributions caused by the fragmentation of ancestral ranges. In that world, Croizat’s phrase “Earth and life evolve together” would be an accurate general description of biogeographic history. It could all be as Gary Nelson envisioned when he talked to Michael Donoghue in that bar in New York, the hammer smashing the glass table into pieces.
It could be like that, but it isn’t.
15In a 2011 paper, Heads also mentioned other possible sources of the Hawaiian biota, such as former high islands that now exist as the submerged Musicians Seamounts north of Hawaii and the low-lying Line Islands to the south. The same argument about the age of connections applies to these sources as well. He also mentioned former lands east of Hawaii that are now accreted to or subducted beneath western North America, but these would not have formed closely spaced stepping stones allowing normal dispersal.
16A recent study suggests that there were no islands in the chain emerging above water between 29 and 33 million years ago. If this is true, the island-hopping route from Alaska could not have populated the current islands (Clague et al. 2010).
17The one book he did not publish on his own, Manual of Phytogeography, adds another 700 pages to his body of work.
18More specifically, Gould argued that the modern evolutionary synthesis (i.e., the integration of evolutionary theory with the genetics of populations) “hardened” from a relatively pluralistic form into one that exaggerated the scope of natural selection.
In the early 1900s, professor G. E. Beyer of Tulane University in New Orleans collected many Upland Sandpipers (Bartramia longicauda) on the Gulf Coast as the birds passed through during their spring migration. (More than likely, he killed them with a shotgun, a standard method for collecting birds in those days.) These birds invariably had small freshwater snails of the genus Physa attached to the feathers on the undersides of their wings. Beyer wrote: “I used to count the number of snails regularly; at one time I found as many as forty-one, often between twenty and thirty, never less than ten or twelve.” He thought the sandpipers might have placed the snails in their wing feathers on purpose as a source of food, as if they were carrying provisions in a backpack.
Although Beyer couldn’t tell whether the snails were the same as the local Physa species, he noted that, at the time of year the birds were collected, the local Physa snails were not often seen—they only became abundant later in the spring. Furthermore, only sandpipers collected soon after their arrival from the south carried snails. These observations indicated that the birds were not finding the snails locally, but instead had carried them on their journey across the Gulf of Mexico, either from islands in the Caribbean or from points farther south.
Chapter Four
NEW ZEALAND STIRRINGS
HENRY OF GONDWANA
On that trip to New Zealand in the winter of 2006–2007, I traveled to Invercargill, one of the southernmost cities in the world, to rendezvous with Tara, her mother, and our friend Jan after their fern class. I arrived a day before they did, so I had some time to wander, more or less aimlessly, around the city. It was early summer in the Southern Hemisphere, but in this sub-Antarctic location, a cold sea wind was blowing, funneled through streets lined with ornate Victorian and Art Deco buildings dulled by an overcast sky.
From the expansive gardens of Queens Park—the Central Park of Invercargill—I ducked out of the weather into the Southland Museum to find an odd and oddly entrancing mix of displays: a film on the stark and perpetually windswept islands south of New Zealand (I asked at the desk about the next showing and, instead of giving the time, a staff member just walked me over to the auditorium and turned on the projector); an exhibit telling the story of the wreck of the General Grant in 1866 and how nine men and a woman survived on one of those desolate archipelagoes for over a year, eating seals and feral pigs; intricate Maori wood carvings and household artifacts; the leg bones of one of the giant, prehistoric, flightless moa birds (sort of like an emu or a cassowary, but much bigger). Oddest of all was the zoo section of the museum, a zoo dedicated entirely to one kind of animal, the lizard-like tuatara, one of the many species peculiar to New Zealand.
Through a glass partition, I watched a few tuatara doing what tuatara do most of the time, namely, not much. One of them was larger and—was it possible?—more stately looking than the others. He was an attractive pale green shading toward olive, maybe a foot and a half long counting his tail, and, with elongated scales running along his spine, he somewhat resembled an iguana, only stouter. Rows of bumps ran along his sides and tail, like lines of hills in miniature. He looked like a survivor.
That tuatara’s name is Henry, and people who should know think he must be more than 110 years old, which is old for a tuatara, but not record-setting. When I saw him, he had shown no interest in sex since being captured almost fifty years earlier, but a couple of years later he gained some notoriety by suddenly turning amorous and mating with a female tuatara named Mildred, who then laid a clutch of eggs that hatched out into eleven tiny tuatara. Assuming his estimated age is correct, when Henry himself hatched out, New Zealand was a colony of Great Britain under Queen Victoria, the Wright brothers were making plans to build an airplane, and Alfred Russel Wallace was in his seventies and still actively writing, although he was by then thinking about the evils of vaccination more than biogeography. You could think of Henry as a relict from a different age.
4.1 A relict from the Victorian era and perhaps from Gondwana: Henry the tuatara in the Southland Museum, Invercargill, New Zealand. Photo by the author.
According to the usual story about the biogeography of New Zealand, Henry is also a relict from the much more distant past—from the Mesozoic Era, when what is now New Zealand was part of Gondwana. He is a member of a group called the sphenodontids, which contains just the one species of tuatara, Sphenodon punctatus, found naturally only on thirty-two small islands off of New Zealand. The closest living relatives of tuatara are lizards (including snakes, which, cladistically speaking, are a group of lizards), but the two lineages are not very close, having split from each other some 250 million years ago, somewhat before the origin of the dinosaurs. (For comparison, a chicken and a turtle show about the same degree of separation.) Tuatara, despite their lizard-like appearance, are anatomically distinct; for instance, unlike lizards, they have chisel-like downgrowths at the front of the upper jaw that look like teeth but are actual
ly part of the skull, and male Sphenodon do not have a penis (lizards have two). As Henry has demonstrated, tuatara also can live a long time, longer than any lizard, and they’re active at colder temperatures than any lizard. At least some of these differences are an indication of the deep evolutionary separation of the tuatara and lizard branches.
The sphenodontids were never a large group—even at their peak they didn’t approach the diversity of living lizards—but in the Mesozoic there were several genera, and they crop up in widely separated regions, such as Mexico, England, Argentina, South Africa, Morocco, India, and China. In one sense, then, it has been clear for a long time that living tuatara are relicts, a single species confined to a minuscule fraction of the globe, the remnants of a larger and far more widespread group. However, with the emergence of vicariance biogeography and plate tectonics, tuatara have become known not just as taxonomic relicts but also as relicts of the Gondwanan biota.
The story of the tuatara—and, I should say from the start, it may even be the correct one—goes something like this: By about 83 million years ago, in the Late Cretaceous, a rift in the crust of eastern Gondwana had become a spreading ridge, like the one that made and is widening the Red Sea, like the one that formed earlier in the Cretaceous in western Gondwana and became the Atlantic Ocean. The magma rising up at the ridge spread out to form the floor of a sea, the Tasman Sea. On the western side of the sea lay Antarctica and Australia, which were still joined, and on the eastern side a continent about the size of India that most of us would not recognize. That continent encompassed what we now call New Zealand, New Caledonia, Campbell Island, the Chatham Islands, Lord Howe Island, and a few smaller bits of land. Geologists have dubbed it Zealandia.19 On a map that shows ocean depths, one can see an area of relatively shallow water that looks vaguely like the head of an antelope, with its downward-pointing snout several hundred miles south of New Zealand and two horns stretching northward, one of them containing New Caledonia near its tip (see Figure 4.2). To a reasonable approximation, that antelope head is the old continent of Zealandia.
4.2 The now mostly submerged continent of Zealandia (roughly outlined in white) indicated on a relief map of the ocean floor. New Zealand is shown in black near the center of the continent. Modified from an image from the National Oceanic and Atmospheric Administration.
As Zealandia moved eastward, it slowly sank, probably because the crust it was made of was becoming both cooler and thinner. By the Late Oligocene, some 25 million years ago, it may have sunken far enough that almost all of the original continent was underwater; most geologists think that, at that time, the part that we now think of as New Zealand had an area considerably smaller than it has today. In any case, the current extent and mountainous landscape of New Zealand is a product of more recent collisions along a plate boundary that formed within Zealandia, the boundary between the Australian and Pacific Plates.
The ancestors of the tuatara, so the story goes, were living on the far eastern side of Gondwana when the spreading ridge began forming in the Cretaceous. Thus, when Zealandia separated from Antarctica/Australia, it carried tuatara ancestors with it. It is usually assumed that the Antarctic side also held some of these animals for a time, although there isn’t any clear evidence of that; no sphenodontid fossils have ever been found in Antarctica or Australia, but, then again, sphenodontid fossil sites in the Southern Hemisphere are few and far between in general. In any case, the tuatara ancestors persisted on Zealandia as it drifted east and subsided, and they survived the time of greatest marine transgression in the Late Oligocene and Early Miocene, when the land area might have been quite small. With the uplift that created modern New Zealand, tuatara populations perhaps had something of a renaissance; their remains have been found in widely scattered locations on both the North and South Islands. Then, some seven hundred years ago, the ancestors of the Maori reached New Zealand, bringing Pacific rats (kiore) as stowaways, and the rats ate tuatara eggs and probably devastated their populations. When Europeans arrived, they brought cats, pigs, and both Norway and ship rats, and those animals, particularly the rats, likely finished off the tuatara on the two main islands, leaving them only on much smaller offshore islands that remained rat-free. Those small islands had been connected to the main islands of New Zealand during the most recent ice age and were probably part of the contiguous range of tuatara at that time, before the climate warmed, melted much of the ice, raised the sea, and left some Sphenodon isolated on the remaining bits of land. Living tuatara are thus relicts twice over, having first survived on Zealandia, while sphenodontids everywhere else went extinct, and then persisting on those offshore islands while the plague of rats wiped out their brethren on the North and South Islands. Today they pass a rather strange existence, by reptilian standards, often sharing burrows with seabirds, and coming out mostly in the cool, maritime night. On Stephens Island, in the strait that separates the North and South Islands, if one walks in the forest at night, one can hear crunching noises in the darkness, the sounds of tuatara eating the giant, cricket-like weta. If the biogeographic story is right, they are the sounds of one Gondwanan relict feasting on another.
WHAT THE FOSSIL PLANTS SAID
“With regard to general problems of biogeography,” Gary Nelson wrote in 1975, “the biota of New Zealand has been, perhaps, the most important of any in the world. It has figured prominently in all discussions of austral biogeography, and notable authorities have felt obliged to explain its history: explain New Zealand and the world falls into place around it.” In fact, New Zealand had been a focal point of biogeography since before The Origin of Species. Darwin’s friend Joseph Hooker placed it among the group of southern lands that he believed must all have been connected by land bridges, because of the many plants they had in common. Not surprisingly, Darwin and later dispersalists, such as Matthew and Darlington, had argued instead that New Zealand was colonized by chance ocean crossings; that would explain, among other things, why its biota didn’t match expectations for a continent, why, for instance, the place didn’t have any snakes or land mammals. Now, as the vicariance revolution took root, Nelson was trying to turn New Zealand into an archetype of the new worldview. New Zealand’s isolation from the other major landmasses of Gondwana since the Late Cretaceous meant that it could not have been colonized by normal, overland dispersal in the recent past. That, in turn, meant that New Zealand provided an ideal arena for judging the importance of long-distance dispersal versus vicariance. Could it be that the ancient connection to Gondwana explained the existence of virtually all New Zealand taxa? To Nelson, the answer seemed clear. Waxing poetic, he saw “stranded upon the shores of New Zealand not the waif beginnings of its modern biota, but only bits and pieces of the center of origin/dispersal paradigm.” New Zealand, in Nelson’s view, held a relict biota, the product not of chance dispersal but of continental drift, and the sooner people accepted that notion, the better.
The vicariance worldview, it must be said, never came to thoroughly dominate historical biogeography in the way that, say, plate tectonics took over geology. However, in New Zealand, it came fairly close, not in the consistent use of cladograms or Croizat’s tracks, but in the belief that an ancient vicariance event was the key to understanding the biota. New Zealand had all these weird, seemingly relict creatures—the tuatara and the kiwi and the nocturnal, flightless Kakapo Parrot, to name a few—and it had been part of Gondwana, thus part of the iconic story in vicariance biogeography (see Figure 4.3). The pull to fit New Zealand into this new worldview, maybe even make it the centerpiece example of that view, was apparently strong. Explain New Zealand and the world falls into place around it. I asked several New Zealand biologists what people there were thinking at the time, roughly from the mid-1970s to the early 1990s, and they all had the same memory: vicariance had taken over as the paradigm for biogeography. In 1982, two New Zealand biologists wrote, typically for the times, “By the middle of the Cretaceous period the charact
er of New Zealand’s modern forests was ensured.” In other words, it was all about Gondwana. The story of the tuatara was also the story of the moa, the kiwi, and the Kakapo, the geckos and frogs, the southern beeches, the massive kauri trees, and countless other lineages.
4.3 Another possible Gondwanan relict in New Zealand: the Kakapo (Strigops habroptilus), a nocturnal, ground-dwelling, flightless parrot. Painting by John Gerrard Keulemans.
On the face of it, the fossil record of the weird vertebrate lineages seems to present a problem for this Gondwanan relict idea. That hypothesis requires the continuous presence of those lineages in New Zealand and Zealandia for the past 75 million years or so (by which time Zealandia was fully isolated by the Tasman Sea), but the fossil record doesn’t show anything like that. For instance, fossils indicate that the various species of moa birds were dominant herbivores in the forests and shrublands of New Zealand before human settlement, but the moa record peters out quickly as one goes back in time, with the earliest remains only about 16 to 19 million years old. The record for kiwi is even shallower, with the oldest definite fossils less than a million years old, although there are 10-million-year-old footprints that might be a kiwi’s. Tuatara, perhaps the most iconic of all the supposed relict lineages, first show up in the same Miocene fossil beds as the earliest moa, not even close to the 75-million-year mark. The same goes for all the other possible living vertebrate relicts of New Zealand; none of them have fossil histories that suggest they were on Zealandia when it drifted off on its own.
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