I.2 Two possible explanations for the piecemeal distribution of Thamnophis validus. Gray shading shows the range of the species. Upper: fragmentation of the range through the rifting that created the Sea of Cortés. Lower: dispersal across the sea (shown by arrow). Modified from de Queiroz and Lawson (2008).
The best explanation of this extreme genetic similarity is that snakes on the Mexican mainland crossed the 120-mile width of the Sea of Cortés very recently (“very recently” meaning within the past few hundred thousand years) and established a population in southern Baja California (see Figure I.3). They didn’t ride with the drifting peninsula, but instead jumped the gap long after the sea had formed. If there was any kind of raft involved, it was probably a literal one, a log or a clump of vegetation driven by an easterly wind and carrying a few snakes (or even just one pregnant female) across the sea.
I.3 Part of a DNA-based “timetree” for garter snakes. The tree suggests that Baja California Thamnophis validus separated from mainland snakes only within the past few hundred thousand years, much more recently than the physical separation of the peninsula from the mainland (indicated by shading). Modified from de Queiroz and Lawson (2008).
The landmasses-as-life-rafts hypothesis is part of a school of thought with the somewhat imposing name of vicariance biogeography. Although I will try to avoid the use of scientific jargon in this book, “vicariance” is a word that I cannot get around and need to define. (For more definitions, see Box.) It refers to the fragmentation of the range of a species or larger group into isolated parts by the formation of some sort of barrier, as with the Sea of Cortés. As another obvious example, consider the effects of rising sea levels after the most recent ice age. At the peak of glaciation, about 18,000 years ago, a vast layer of ice extended from the Arctic past the Great Lakes in North America and as far south as Germany and Poland in Europe. Because so much of the world’s water was tied up in this ice, sea levels were much lower than they are today, which meant that many areas that are now underwater were exposed as land. As the ice melted, seas rose by more than 300 feet, and some places that had been parts of continents were transformed into islands, like sand castles surrounded by a rising tide. For example, much of what had been continental Southeast Asia was inundated, leaving the higher regions as the islands of Sumatra, Java, and Borneo, among others. With the fragmentation of land areas, terrestrial species that had been spread out across the region during the glacial period inevitably had their ranges broken up as well. Today, populations of the same species of frogs, snakes, monkeys, and other organisms can be found on Sumatra, Java, Borneo, and the Southeast Asian mainland. Many of them probably were in all those places before the rise in sea level; they achieved their piecemeal distributions simply by staying put while the waters rose around them, isolating their populations on the various islands and on the continent. The frogs, snakes, monkeys, and other species experienced a vicariance event, a breaking up of their formerly continuous ranges.
A FEW THOUGHTS ON BIOGEOGRAPHIC TERMS AND CONCEPTS
The basic notions of long-distance dispersal and vicariance are fairly straightforward, but a few points about these and related concepts may be helpful. This box also serves as a glossary for the very few technical terms commonly used in this book.
Normal dispersal is the expected movement of organisms either within continuous tracts of suitable habitat or between patches of suitable habitat that are close together. Say the climate is warming at the end of an ice age. As the ice retreats and new habitat slowly opens up, beech trees and squirrels on the edge of the area move into the previously ice-covered region. That’s normal dispersal for the trees and the squirrels. No improbable jump is required to explain it. Long-distance dispersal, in contrast, involves the movement of organisms across an area that is, for those organisms, a substantial barrier to dispersal. Because of the barrier, this kind of movement is both unexpected and unpredictable; long-distance dispersal is thus sometimes referred to as chance or sweepstakes dispersal. Obvious examples include the movements of nonflying vertebrates from continents to islands many miles offshore or of many kinds of lowland organisms across high mountains. In general, a population founded by long-distance dispersal will be genetically isolated from the source population because movement between them is difficult; thus, populations originating in this way will tend to diverge from the source population. This is why, for instance, native land animals on remote islands are almost always classified as distinct species from related mainland forms. Both normal and long-distance dispersal must be defined in light of an organism’s particular dispersal abilities. For example, crossing a mile-wide sea channel would qualify as long-distance dispersal for a frog or a mouse, but would be normal dispersal for many birds.
A disjunct distribution, in the simplest terms, is any discontinuous distribution in which some part of the species or larger group is separated from another part. The cases described in this book always involve disjunctions in which the parts are separated by a substantial barrier (or barriers) to dispersal, usually an expanse of ocean. One way to think of these distributions is that movement between the separated parts today would require long-distance dispersal by the organisms in question (if the movement is even possible).
Vicariance is the splitting of the continuous range of a group into two or more parts by the development of some sort of barrier to dispersal. In its strict sense, vicariance refers to the fragmentation of the range of a species, and is a mechanism whereby one species becomes two or more species. For example, in the case of the ratite birds, vicariance implies that each geologic fragmentation event—the separation of South America from Africa, India from Madagascar, etc.—divided the range of a ratite species. I follow this strict definition, with a major exception. Specifically, when dealing with molecular clock and other dating studies, I take vicariance to mean the fracturing of the distribution of any taxonomic group (whether a species or a higher-level taxon such as a genus or family), a process that might or might not be connected to the birth of new species. As an illustration, suppose that an ancestral ratite species had spread by normal dispersal all over Gondwana, but that, while the supercontinent was still intact, this ancestor evolved into distinct species in the areas that would become Africa, South America, and so on. The breakup of Gondwana would then have left ratites on landmasses separated by oceans, as in the strict case, but, in this alternate scenario, new species would have arisen before the fragmentation of the supercontinent.
This broader definition has been implicitly adopted in many molecular clock studies, probably because it simplifies distinguishing long-distance dispersal from fragmentation. Specifically, vicariance, broadly defined, subsumes all explanations that involve fragmentation of an ancestral range and do not require long-distance dispersal. Thus, if we reject vicariance in this sense, we are necessarily also supporting long-distance dispersal. For molecular clock studies, what this means is that results fall into two categories: if a particular evolutionary branching point is estimated to be as old or older than the fragmentation event in question, that branching age is deemed consistent with vicariance, while, if the branching point is estimated to be younger than the fragmentation event (as in Figure I.3), then long-distance dispersal is supported. In any case, the general message of the book is not affected by these definitional issues.
To produce a disjunct distribution, long-distance dispersal has to be followed by the establishment of a permanent population in the new area. In many cases, establishment in a new environment may be more difficult to achieve than long-distance dispersal per se. I will often use “dispersal” to mean “dispersal and establishment”; the meaning in these instances should be obvious from the context.
A taxon is a taxonomic group and might refer to a species, a genus, a family, or a group at any other level in the taxonomic hierarchy. Homo sapiens is a taxon, as is the genus Homo, and the family Hominidae. The plural of “taxon�
� is taxa.
Sister groups are lineages that are each other’s closest evolutionary relatives. Among living species, for example, the two species of chimpanzees are sister groups to each other, and these two chimp species together form a lineage that is the sister group to humans. The concept can apply to any level in the Tree of Life; marsupial mammals and placental mammals are sister groups, as are green plants and red algae.
A timetree is a representation of an evolutionary tree in which the estimated ages of the evolutionary branching points (for example, the split between the human and chimp lineages) are indicated (see Figure I.3).
A continental island is one that previously was connected to a continent and became an island, either because of submergence of a land bridge (as was the case for Sumatra, Java, and other islands of the Sunda Shelf), or because of tectonic processes (as was the case for pieces of Gondwana, such as Madagascar and New Zealand). An oceanic island is one that emerged de novo from the sea and has never been connected to a continent. All of the oceanic islands discussed in this book were created by volcanoes. Hawaii and the Galápagos are classic examples.
The archetypal vicariance event (actually a series of events) is the one I began with, the fragmentation of the distributions of Gondwanan plants and animals through the breakup of the supercontinent. In that case, as in the example of Southeast Asian islands, the newly formed barriers are seas or oceans, but there are many different kinds of barriers, many different ways that members of a group can be cut off from each other. For instance, the onset of a drier climate can turn wooded lowlands into desert, while leaving woodlands intact at higher elevations; the result might be fragmentation of the ranges of woodland species into isolated populations on separated mountain ranges. In effect, the dry climate turns the mountains into habitat islands. Similarly, the formation of a land connection creates a barrier for aquatic organisms, as when the rise of the Isthmus of Panama some 3 million years ago separated populations of fishes, shrimp, and other ocean species in the Pacific and Caribbean. Ultimately, those barriers generate new species because the separated populations no longer exchange genes and eventually evolve in different directions. Many of the sea creatures that had their distributions divided by the Panamanian Isthmus, for example, are now classified as separate species on the Pacific and Caribbean sides.
Vicariance biogeography emphasizes such fragmentation events as explanations for the distributions of species and higher taxa (genera, families, etc.). In particular, when a biogeographer with this mindset comes across a taxonomic group with a distribution made up of disconnected areas—like the flightless ratite birds spread across the southern continents—his first thought is “What external process (say, climate change or continental drift) broke the distribution into pieces?” He may think it’s conceivable that a piecemeal distribution of that sort could be the result of long-distance ocean crossings, but that possibility will be an afterthought, something almost unworthy of real attention. (In fact, as I will describe later, many biogeographers of this school think that hypotheses invoking long-distance dispersal, whether over land or water, are not only unimportant but unscientific.)
The rise of vicariance biogeography in the 1970s was a big deal within the discipline, to put it mildly. It changed the way biologists thought about the distributions of living things in two fundamental ways. First, as just mentioned, it put the fragmentation of environments at the front of people’s minds. Second, because fragmentation affects many groups in the same way—for instance, rising seas will break up the ranges of multiple terrestrial species at once—it made people think about generalities, about patterns of distribution that are shared by different taxonomic groups. Biogeography has a long history of attempts to generalize across such groups, but the emphasis on vicariance made that kind of generalization almost inescapable. In other words, it forced people to consider, not just the geography of their own favorite genus of legless lizards or snapping shrimp, but how the distributions of whole biotas may have been broken up through time. Vicariance biogeography often has been called a scientific revolution: it dramatically changed many biologists’ views of the history of life, and the way they approached their science. To teach biogeography today without mentioning vicariance—and tectonic-driven vicariance, in particular—would be like teaching physics without quantum mechanics, or molecular biology without the double helix.
At the time of our snake-collecting trip to Baja California, I knew relatively little about biogeography, and what I did know was mostly filtered through the lens of vicariance. For instance, in teaching an evolution course at the University of Colorado, I had devoted a couple of lectures to biogeography and had used, as my key example, distributions fragmented by the breakup of Gondwana. Thus, when I began reading articles as background for writing the paper on garter snakes crossing the Sea of Cortés, I expected to encounter mostly studies supporting landmass-as-life-raft theories, that is, vicariance via continental drift. That is not what I found. Instead, I kept running across recent papers in which the authors expected to find evidence for landmasses as life-rafts, but ended up arguing for a very different kind of explanation for disconnected distributions, namely, dispersal of plants and animals across seas and oceans. In other words, lots of biologists were finding just what we had found for the Baja California garter snakes.
Many of these studies were about the southern continents and continental islands, the pieces of ancient Gondwana. The papers arguing for ocean crossings kept piling up on my desk—tortoises from Africa to Madagascar, some two hundred plant species between Tasmania and New Zealand, southern beeches among several Southern Hemisphere landmasses, baobab trees between Australia and Africa, rodents from Africa to South America. At some point in my frenzied reading of all these articles, I went from thinking that there were some really weird cases of oceanic dispersal out there to thinking that the weird cases might actually be the norm. To put it another way, my mind jumped from the iconic view of Gondwanan landmasses as life-rafts to something resembling an airline map, with the route lines tracing countless ocean crossings between the disconnected and now widely separated fragments of the supercontinent.
This epiphany, which I soon learned was happening to other biologists as well, was dramatic. Obviously, the continents had moved—nobody was claiming that the theory of plate tectonics was wrong—and obviously, they had carried species with them, but somehow, these facts did not explain nearly as much about the modern living world as we had thought. Instead, what accounted for many of the most strikingly discontinuous plant and animal distributions was a process that had previously occupied some sleepy backwater in my mind, that is, seemingly implausible, improbable ocean crossings.
The goal of this book is to tell the story of this recent sea change in biogeography, from a view dominated by vicariance to a more balanced outlook recognizing that the natural dispersal of organisms across oceans and other barriers is also hugely important. In a nutshell, the point is to recount how the field of biogeography flipped from landmasses-as-life-rafts and other fragmentation scenarios to something closer to the airline route map, using Gondwana as the geographic focus. Ultimately, I also want to explain what this dramatic shift in thought tells us about both the nature of scientific discovery and the history of life on a grand scale. It may even tell us, on one level, why we are here.
The book is divided into four sections. The first provides the historical background, setting the table for what will follow. This section begins with Charles Darwin and the birth of evolutionary views about the distributions of living things, describes the rise of vicariance biogeography, and ends with inklings of the sea change among New Zealand scientists. The brief second section deals with a critical but controversial source of evidence in biogeography, namely, molecular clock analyses, which are used to infer the ages of branching points in evolutionary trees (such as the time at which Old World monkeys and New World monkeys separated from each other). The t
hird section is, in an obvious sense, the “meat” of the book; there I set forth the main examples that have turned biogeography on its head. The four chapters of this section can be seen as successive ratcheting steps in an argument for discarding the extreme vicariance position and replacing it with the view of a living world strongly molded by ocean crossings and other chance dispersal events. Finally, in the fourth section, I present the deep implications—the “big picture” messages—of the new worldview with respect to, first, the way in which science progresses (or fails to progress) and, second, the nature of the long history of life on Earth.
In December 2006, a few years after my garter-snake-induced epiphany, I found myself visiting one of the smaller fragments of ancient Gondwana. Tara, her mother, and our friend Jan—all botanists—had signed up for a field course on the ferns of New Zealand, and it had taken Tara about ten seconds to convince me that I should go too. For a naturalist, New Zealand is one of the wonders of the world; the biologist Jared Diamond has called its flora and fauna “the nearest approach to life on another planet.” As pretty as ferns are, I didn’t want to spend two weeks fixated on them while crawling on all fours in the mud, but I figured I could go off on my own and try to find some of Diamond’s alien life forms, then meet up with the others after their course was over. Perhaps I could see a tuatara, a lizard-like reptile in an order that is thought to have died out everywhere else while dinosaurs still roamed the Earth; or imposing kauri trees, as thick as California’s giant sequoias and covered with their own forests of epiphytes; or a Wrybill,3 a shorebird with a beak that bends not up or down but sideways (almost always to the right, as it turns out). So, while Tara and the rest of the “ferniacs” left Wellington in their tour bus, I rented a car, headed north for the kauri forest, and eventually ended up traversing most of the length of the country. (While there I was very careful about driving on the left, looking right when crossing streets, and so on, but on returning to the United States, with my brain still reverse-wired, I promptly turned onto the wrong side of a busy boulevard in Las Vegas. Luckily, Tara yelled loudly before we came close to colliding with the oncoming traffic.)
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