Optimistically, this immature phase may finally be ending; historical biogeography may at last be developing a full-fledged paradigm. In this view, several steps within the past half-century have been critical to the science’s maturation. The first and probably the most obvious one was the acceptance of plate tectonics, which produced a dynamic view of Earth history that, at least in rough outline, almost everyone interested in biogeography agrees upon. In the face of all the evidence for tectonics, returning to a belief in fixed continents and ocean basins would be absurd, like an astronomer regressing to a geocentric view of the universe, or a physicist trying to study subatomic particles using Newtonian mechanics. Even if one doesn’t believe that “Earth and life evolve together” in the extreme sense envisioned by Croizat, one has to place life’s history within the context of the changing configurations of continents and oceans.
The second step, following closely on the heels of the first, was the rise of cladistic thinking, the replacement of the muddy “river delta” approach to evolutionary relationships with methods for building those precise, unambiguous branching diagrams called cladograms. Like plate tectonics, cladistics was a major step forward; knowledge of relationships is critical to interpreting where lineages came from, and precise knowledge is better than muddiness. Beginning in the 1960s, there have been heated debates over exactly how to figure out such relationships, with statistical methods largely taking over from the nonstatistical approaches of the die-hard cladists (a few of whom are still unhappy about this change). However, for biogeography, these debates are just a sidelight, and almost everyone agrees that building evolutionary trees is a necessary part of inferring the geographic history of life. After reading the summary of a paper on historical biogeography, what is the first thing that most biogeographers do? More than likely, they look for the tree diagrams.
As we have seen, though, the melding of plate tectonics and cladistics did not by itself produce a movement that took over the field. The vicariance biogeography that came out of this marriage of ideas was influential and was adopted by many, but, as noted above, it never came close to being universally embraced. In that sense, the vicariance “revolution” wasn’t like the rise of Copernican heliocentrism or Einsteinian relativity, or even, within geology, plate tectonics itself. It was not, in the Kuhnian sense, the rise of a paradigm.
The problem was that something was missing, something fundamental to understanding history. The opponents of vicariance biogeography saw this and, as a result, could not be fully converted to the new view, while the proponents of vicariance biogeography tried to ignore or gloss over the omission. The missing element was the river in which all of history flows. The missing element was time.
The addition of time, in the form of molecular dating results, is, in my view, the final step that might finally produce a paradigm in historical biogeography. Of course, timelines for some evolutionary events have existed since before publication of The Origin of Species, but, because these were based entirely on fossils, the information was missing for many groups and could be easily dismissed for others. In the 1940s, it was possible for a late-lingering land-bridger, Carl Skottsberg, to argue for the great age of many Hawaiian lineages without sounding like a complete lunatic. Today it is much harder to make that kind of claim (although a few have done so) because molecular evidence indicates that nearly all Hawaiian lineages studied thus far are relatively young. Basically, molecular dating has greatly extended the reach and precision of information on the age of evolutionary events. This cumulative evidence of timelines—what I called the “forest” in Chapter Six—has for many evolutionary biologists now passed a tipping point, such that it strains credibility to simply dismiss it as some sort of misguided intellectual fashion.
The question of the moment is whether molecular dating will progress to the point where virtually everyone who studies historical biogeography accepts this approach as valid (even if they don’t believe every single age estimate). There are signs that the field is heading in that direction. Certainly, many mainstream scientists now see the rejection of molecular dating as irrational and view the extreme vicariance school as intellectually stagnant. For instance, Anne Yoder, the lemur biologist who has focused on the biogeography of Madagascar, refers to vicariance scientists who refuse to accept the dispersal origins of Madagascan mammals as being “impervious to evidence.” Along the same lines, when I asked Steve Trewick why the panbiogeographers, once prominent in New Zealand, had been “exiled,” he replied, “They were seen for what they are, a group of fundamentalists who have refused to engage with other thinkers or other evidence.” And here’s how the botanist Michael Donoghue, in typically informal fashion, described his reactions to a research talk by a cladistic vicariance biogeographer: “I thought, man, I’m having some really weird flashback to some era that I can even barely remember. . . . And she’s livin’ that, she’s livin’ it. It’s bizarre and, like, no movement forward as far as I can tell.”
It is true that some biogeographers continue to argue vehemently against the timetree approach, referring to the dispersalist conclusions drawn from it as “artefactual,” “reactionary,” and “ignoring basic biogeographic realities.” Increasingly, however, those criticisms sound like the desperate salvos of an endangered group. The extent to which the field is shifting away from these vicariance scientists can be seen in a simple graph showing the parallel increases, over the past twenty years, of studies that use molecular dating and studies that discuss long-distance dispersal (see Figure 11.2). Those rapidly ascending curves reflect real shifts in the methods biogeographers are using, and in what they believe about the history of life. The curves illustrate how timetrees have shifted thought away from vicariance and toward long-distance dispersal.
11.2 Graphing an intellectual sea change: scientific studies using or discussing molecular clock analyses have taken off in the past twenty years, almost certainly driving the parallel rise in studies that use the term “long-distance dispersal.” Note the log scale. Redrawn and modified from an unpublished graph made by Susanne Renner from a search of the Web of Science database on November 21, 2012.
None of this is to imply that vicariance as a process is insignificant; in fact, molecular dating studies support the role of Gondwanan breakup and other fragmentation events in quite a few cases. However, the graph, and the strong opinions of many mainstream scientists, suggest that the vicariance worldview may be on its way out. Despite the past contributions of vicariance biogeography—especially the clear use of evolutionary trees and the search for patterns common to many different taxonomic groups—that school of thought now appears to be an intellectual branch that is drying up and may eventually fall off the tree for lack of sustenance. If and when the vicariance approach, in its extreme form, fades away, and the evidence of time is at the core of all historical biogeography, the discipline might at last achieve its elusive paradigm.
55Some arboreal bristletails have directed aerial descent, meaning they glide, and it has been suggested that this represents an early stage in the evolution of insect flight. Apparently the long tail bristles are important for gliding; if they are removed, the animal can’t maneuver in air nearly as well (Yanoviak et al. 2009).
56Molokai, Lanai, and Maui have been connected by land at various times, together forming the prehistoric island of Maui Nui; thus, the presence of bristletails on those three islands could have been the result of a single overwater colonization (Ziegler 2002).
57Panbiogeographers tended to complicate matters by also emphasizing the conglomeration of formerly separated landmasses, but the simpler view that focused primarily on fragmentation was more widespread.
58Physicists, in particular, often have chosen theories on the basis of beauty or elegance. For instance, Paul Dirac, known, among other things, for predicting the existence of antimatter, wrote of the general theory of relativity: “One has a gre
at confidence in the theory arising from its great beauty, quite independent of its detailed successes” (McAllister 1998, 174). Similarly, some of Brian Greene’s arguments for string theory—currently the most popular “theory of everything” but notoriously difficult to test—are based on the theory’s apparent elegance (Greene 1999).
59Retrograde motion was just one of many anomalies that had accumulated by the time Copernicus revolutionized astronomy.
60On a more general level, Darwin and Wallace established a paradigm for historical biogeography based on the ideas of common descent and deep time, as described in Chapter One. However, this evolutionary paradigm left open the specific explanations for disjunct distributions; the fact that Darwin and Wallace favored long-distance dispersal as an explanation did not make that explanation or the assumptions behind it part of the accepted paradigm. From the beginning of the Darwinian Revolution, some evolutionists preferred the land-bridge explanation, and by the time Wallace died in 1913, the land-bridge school was well established.
On or around October 12, 1988, fishermen near the island of Trinidad, near Venezuela, came across thousands of dead locusts floating on the sea. Soon reports of live locusts were cropping up from scattered sites in the West Indies and northern South America—from Grenada, Guadeloupe, Jamaica, Antigua, Guyana, and Suriname, among other places. The locusts were memorable for their large size, pinkish color, and, in some places, their vast numbers; a supertanker nine hundred miles southeast of St. Lucia became a landing spot for so many of these insects that the entire deck was awash in pink, while on Dominica a single swarm (one of six reported there) was estimated to contain some 10 million to 20 million individuals. The locusts appeared to be weather-beaten and starving, as if they had endured a long and harrowing journey, which indeed they had: they were desert locusts, Schistocerca gregaria, an Old World species, and they had just flown across the Atlantic Ocean.
In North Africa, plagues of locusts have devastated crops throughout history—swarms are referred to as the “teeth of the wind”—and farmers and government officials in the Caribbean imagined such a disastrous outcome from the 1988 invasion. Fortunately, that scenario never materialized. The damage caused by the desert locusts was minimal, and the species quickly disappeared and has not been reported again in the Americas. However, recent DNA studies indicate that desert locust ancestors or their close relatives were more successful in the deeper past; within the past several million years, they successfully dispersed across the Atlantic from Africa, perhaps several times, and very rapidly gave rise to some fifty New World species.
Locust swarm in North Africa. Photo by Eugenio Morales Agacino.
Chapter Twelve
A WORLD SHAPED BY MIRACLES
POTATOES AND THE HOMOGENOCENE
This book began with the great conundrum of biogeography, the observation that certain closely related lineages occur on land areas separated by wide stretches of ocean, that is, by barriers that seem insurmountable for many organisms. By now it should be clear that, for a large number of these cases, the primary explanation of the vicariance biogeographers—that is, that such distributions came about through the movement of drifting tectonic plates—was the wrong explanation. Instead, many different kinds of plants and animals have crossed such ocean barriers and then have successfully established populations in their new lands. Chance, long-distance voyages of this sort have taken place many times across every sea and ocean in the world. Furthermore, plants and animals that seem to have no business making such long ocean journeys have done so, from monkeys and southern beech trees to frogs, freshwater snails, and probably even dinosaurs.
These examples have a kind of parlor game interest. What flightless arthropods have been extraordinarily successful at crossing ocean barriers? Spiders, especially ones that can “fly” by ballooning. Where can one find dinosaur fossils—Gondwanan relics—alongside a modern biota descended only from overwater colonists? The Chatham Islands, east of New Zealand. What do crocodiles, geckos, skinks, amphisbaenians (worm lizards), and thread snakes have in common? All of them are reptiles that crossed the Atlantic.
However, these cases of long-distance dispersal, collectively, have deeper meanings as well. Specifically, they help to answer a fundamental question that people have been asking for millennia, namely, “Why is our world the way that it is?” Of course, there is a multitude of ways to answer that question—an infinite number, actually—even if we leave Divine purposes out of it. I suggest, however, that the answers that emerge from the biogeographic evidence are at once exceptionally broad and deep in their application. They are answers that can change the way we look at the world.
In this final chapter, I first address this fundamental question in a relatively straightforward way, arguing that the biota of the Earth—specifically, the identities and locations of living species—has been profoundly influenced by natural ocean crossings. This influence has been so great, I will suggest, that if one could somehow go back in time and eliminate all of these overwater colonizations, and then jump back to the present, the living world as we know it would be transformed into an alien one. This conclusion that ocean crossings have had an enormous impact then leads to another, more conceptual, kind of answer to the question, an answer that reveals a general property of the history of life. As will be discussed below, some (probably most) biologists believe that evolution is unpredictable; they recognize that even small events can have unforeseen and profound consequences. Take out one species in the Cambrian, so the story goes, and the subsequent course of life on Earth would be radically altered. However, the exact nature of these history-changing events has typically been an intangible, a subject only of speculation. The evidence for ocean crossings, I will argue, makes the nature of such events far more concrete and indicates that the diversification of life has frequently been deflected by purely chance occurrences. This deduction, in turn, implies a fundamental unpredictability to the large-scale history of living things, to the way in which evolution has unfolded through deep time.
As a launching point for thinking about the general importance of natural ocean crossings, we will consider first an unnatural one, an organism transported by Europeans from the Americas to the Old World. The point of this detour is to introduce the subject of historical consequences, of paths taken and not taken, by considering a species that is both familiar and has had an undeniably large impact on humanity. From this recent and readily comprehended history of dissemination, we will then segue to the deeper and less apparent past that encompasses the multitude of natural ocean crossings.
Let us consider the potato, Solanum tuberosum. Native people in what is now Peru first cultivated potato plants from wild progenitors more than 4,000 years ago, an event that would change the history first of South America and then of the entire world. Potatoes became the staple crop in the Andean altiplano region and, rendered into a freeze-dried form that could be kept in centralized storehouses, were the fuel for the labor gangs and armies of the Inca Empire. When the Spaniard Francisco Pizarro met the Inca in 1526, he was encountering a civilization built upon the potato.
As with cases of natural dispersal, no one knows exactly when the Spanish first brought potatoes to the Old World, but it probably happened in the 1550s, on ships coming from the Pacific side of South America, and the potatoes likely were on board as provisions for the voyage rather than as an intended import. Rumors dogged the plant at first. The tubers were thought to be poisonous. They were said to cause leprosy. Ultimately, however, need and convenience won out. People found that potatoes were not only edible and highly nutritious, but also could be grown on land that was marginal for most other crops. As a result, by 1800, potato farming had spread all across northern Europe and into Asia.
In Ireland, the impetus for growing potatoes was especially strong, because the climate was too soggy for productive wheat farming, and the new crop was adopted early on, i
n the mid-1600s. For poor Irish, in particular, that changed everything, first for apparent good, then great ill. Families that had barely been able to grow enough food to sustain themselves found they could produce an excess of nutritious spuds; thus, a region that had been held back by strong limitations on food production was released from restraint. Young Irish men and women could start families at an earlier age; children who would have succumbed to starvation or associated diseases survived. The predictable result was a population explosion. In the early 1600s, pre-potato, there were about 1.5 million people in Ireland; two centuries later, the number had jumped to over 8 million. Other factors probably were involved, but in large part the increase seems to have been driven by potato farming.
After that dramatic, exponential rise came the crash. In 1844, near the border of France and Belgium, a botanist noticed dark spots on the leaves of some potato plants. This was the potato blight, a disease that started in the leaves and then moved down the stem to the tubers and roots, quickly causing the whole plant to disintegrate into a putrid, black mess. In the summer of 1845, the blight spread rapidly outward from its epicenter in Belgium, reaching Ireland by mid-September. The culprit, discovered in 1861, was a microorganism called an oomycete, fungus-like in many of its properties but actually more closely related to brown algae. This particular kind of oomycete, Phytophthora infestans, was, not surprisingly, an import from the New World, just like the potato itself. It had evolved with S. tuberosum in the Americas, and perhaps had been carried across the Atlantic to Belgium in a shipment of seed potatoes from the United States.
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