Every once in a while, though, you read something that rises above the gray miasma, that shakes you out of slumber, something that seems to make the ground shudder a little beneath your feet. Instead of glancing at the summary and forgetting what it said as soon as you turn the page, you read the entire paper and then find yourself going back to it again and again, picking up nuances you missed before. In the course of researching the subject of disjunct distributions, I’ve been lucky enough to read several papers or passages like that. The long fourth chapter of Lars Brundin’s midge monograph, for instance. A paper by George Gaylord Simpson on the mechanics of dispersal. Mike Pole’s article on the origins of the New Zealand flora.
Another was written by a young Spanish biogeographer named Isabel Sanmartín and a Swedish scientist, Fred Ronquist, who are both known for devising quantitative biogeographic methods. Out of all the biogeographic papers written in the past twenty years, this one, published in 2004, may have produced the biggest shudder of all.
The details of Sanmartín and Ronquist’s approach are complex, but basically they were trying to see if the order of branching in evolutionary trees for Southern Hemisphere taxa matched the sequence of breakup of Gondwanan landmasses.35 In that sense, they were doing what Lars Brundin had done in the 1960s with his chironomid midges, and what Gary Nelson and others had advocated in the 1970s and 1980s as the way to approach the geographic history of life. By including as many groups as they could, Sanmartín and Ronquist also were following in the footsteps of Léon Croizat, who, for all his looniness, had admirably pushed the idea of looking for generalities rather than focusing on the histories of single taxa. But Sanmartín and Ronquist were (and are), in a significant way, not at all like Nelson or Croizat. In particular, they were open to the idea that long-distance dispersal might be important. They wanted to find out what evolutionary trees of Southern Hemisphere groups really had to say.
For plants, what Sanmartín and Ronquist found is that, by and large, those evolutionary trees do not match the order of Gondwanan fragmentation. For instance, the closest relatives of plants in Australia tend to be found in New Zealand, whereas continental breakup predicts that Australian plants should be closest to those in southern South America. Similarly, Madagascan plants tend to have their closest relatives in Africa, yet those two landmasses actually separated very early and, thus, the vicariance explanation predicts only distant relationships between the groups that live on them; Madagascan plants ought to be closer to Indian ones, and African plants closer to those in South America. In short, the results do not support a Gondwanan vicariance explanation as a good generalization for plants; the expected evolutionary links, for the most part, are not there. Sanmartín and Ronquist concluded instead that oceanic dispersal has been frequent, especially over the Tasman Sea between Australia and New Zealand and across the Mozambique Channel between Africa and Madagascar, and even, in some cases, over much longer distances, such as the entire Indian or Pacific Oceans. And, if anything, their results were somewhat biased against finding dispersal, because they left out plant groups found widely in both hemispheres, that is, the kinds of plants most likely to move long distances. In other words, the deck was probably stacked to find support for vicariance, and yet that hypothesis still did not hold up.
Sanmartín and Ronquist’s study was significant for its generality, reminiscent of Croizat, and for the fact that it had essentially used the approach of the vicariance scientists to refute vicariance. Like the cladists, Sanmartín and Ronquist had used only the branching order in evolutionary trees, not the ages of branches, and that was important, because it meant that even people who weren’t sold on molecular dating could believe the results. In short, it was a broad study about the biogeography of the Gondwanan landmasses that almost everyone had to take seriously. If there was one paper that marked the shift in thinking in the field, the watershed moment, this was it. An Australian botanist named Michael Crisp, who has done some of the key biogeographic studies on southern beeches and other Southern Hemisphere plants, described Sanmartín and Ronquist’s study as signaling “the last great gasp of the vicariance paradigm.”
Crisp and his colleague Lyn Cook provided some complementary evidence, from molecular dating studies, on the origins of Southern Hemisphere plants in a 2013 review of the flora of Australia. Interestingly, for one subset of Australian plants, those that have their closest relatives in South America, they found a strong signal of Gondwanan vicariance. Specifically, in fourteen out of twenty-one such cases, the branching points were old enough to suggest that the plants in question had persisted in South America and Australia since before their connection through Antarctica was severed some 30 million years ago. (At that point in my immersion in the botanical literature, I was actually shocked to see such a clear indication of Gondwanan relicts.) However, the rest of the Australian flora reflected a story like New Zealand’s, or like Matt Lavin’s bean plants: there were twenty-eight Australian plant taxa with sister groups on other Gondwanan fragments (six in New Caledonia, eight in New Zealand, and fourteen in Africa), and all twenty-eight had divergence ages too young to be explained by the breakup of Gondwana.
To sum up, evolutionary branching patterns, as in the Sanmartín and Ronquist study, and a large and rapidly growing number of molecular dating studies indicate that Gondwanan fragmentation and vicariance in general cannot explain most plant distributions broken up by oceans. Instead, it has become increasingly clear that over tens of millions of years, the collective flora of the Earth has jumped ocean barriers willy nilly, probably often in the form of seeds—blown on the wind, on the feathers or feet or inside the guts of birds, on rafts of vegetation, or simply floating on the water. Modest barriers like the Tasman Sea have apparently been surmounted hundreds, if not thousands, of times, and even enormous ones like the Atlantic and Pacific have been successfully crossed by numerous plant lineages.
7.4 Isabel Sanmartín. Her study, with Fred Ronquist, on the biogeography of the Southern Hemisphere opened a large crack in the vicariance worldview.
In Matt Lavin’s eyes, many plants, given some thousands or millions of years, can move about the Earth rather easily, undeterred by apparent geographic barriers. “I don’t see the formation of a mountain range or a river or the separation of continents . . . I don’t see those sorts of historical events as imposing any sort of barrier to migration for plants,” he says, expressing a view that would have been foreign to him earlier in his career. “Plants seem to get around. It’s a question of whether they’re going to find opportunity in the new area they land in.” Quite often, the opportunity has been there, as it was for many of the woody legumes that, after crossing a sea or ocean, managed to find a hot, dry region with a pronounced rainy season, that is, an environment similar to the one from which they had come. The ultimate result of such colonizations is the world of the green web. If one began with a vicariance perspective, it’s a world turned upside-down.
THE PLANT DISPERSAL THREAD
A conversion from vicariance to dispersal, like the one Matt Lavin experienced, was a common outcome for botanists as the molecular dating results accumulated. However, it would be misleading to suggest that transformations like his were close to universal. There are, for instance, some botanists who remain unconvinced by the new results, and are steadfastly committed to the vicariance view, people like Michael Heads. In addition, a fairly large contingent of botanists didn’t need to be convinced of the great importance of long-distance dispersal, because they already believed in it. This latter group formed part of an intellectual tradition that can be traced back to the origins of an evolutionary view of biogeography, back to Darwin himself.
“I must now say a few words on what are called accidental means, but which more properly might be called occasional means of distribution.” That’s Darwin, in The Origin of Species, introducing the section on what we would now call chance, long-distance dispersal. The reader expec
ting tales of ballooning spiders, rafting iguanas, or swimming elephants, however, will be disappointed. Darwin probably figured that, if he was going to run through just a few examples, it would be best to stick with ones that his skeptical readers wouldn’t find far-fetched, ones that he himself found convincing. And so, he continued, “I shall here confine myself to plants.” In fact, he pretty much confined himself to seeds. He went on to describe the likelihood of small seeds being transported on the feet of birds, observations of seeds lodged behind stones embedded in the roots of floating trees, the germination of seeds collected from bird excrement, the possible oceanic dispersal of seeds on icebergs, and, of course, his famous seeds-in-seawater experiments.
Darwin’s emphasis on plants for his examples of long-distance dispersal is part of an intellectual thread that runs without any complete break to the present. Basically, since Darwin, there have always been people who believed that plants have often made successful ocean crossings and other long-distance journeys. It was a belief that persisted even when intellectual fashions went against it. Perhaps it did so because it was derived in fairly straightforward ways from botanical knowledge.
Part of the belief came from simply thinking about the properties of plants and, especially, of seeds. Those properties ought to make plants proficient long-distance dispersers, much better, for instance, than nonflying vertebrates. Most flightless vertebrates, with their need for fresh water and vulnerability to overexposure and drowning, almost require a substantial raft to cross a wide stretch of ocean; the tortoise that apparently floated from Aldabra to the African coast is a major exception, not the rule. Many seeds, in contrast, have been molded by natural selection to be borne on the wind, by birds, or in water itself, as in the case of gourds and other plants with air-filled fruits or seed capsules. Also, and crucially, even seeds that are not particularly adapted for wide dispersal can lie dormant for long periods, “awakening” to germinate when they encounter the right conditions; seeds are essentially in suspended animation, enabling successful long-distance journeys to occur in a multitude of ways. To an extent, arthropod eggs—many of which are laid in the fall and lie dormant over the winter—have the same advantage, but they are generally more susceptible to being killed by drying out or by exposure to heat or cold than are plant seeds. Arthropod eggs that are well designed for prolonged dormancy—like the ones from “sea-monkeys” (a kind of brine shrimp) that you can leave in an envelope for years and then revive in salt water—are fairly unusual. Among plants, that kind of dormancy is very common, which is why it seems totally unremarkable that you can buy many kinds of seeds in packets at the local nursery and then let them sit around for months or years before planting them. The results of the seed survival experiments carried out by Darwin and later researchers were striking demonstrations of how this property could facilitate dispersal.
Apart from the obvious properties of seeds, there were other observations that convinced Darwin and his intellectual descendants that chance, long-distance dispersal by plants is especially common. For one thing, plants have populated oceanic islands more readily than animals. For instance, the proportion of the world’s plant lineages that have colonized the remote Hawaiian Islands is far greater than the proportion for any group of land animals, even insects. (The number of insect lineages that have reached Hawaii is somewhat larger than the number of plant lineages, but worldwide there are far more insect than plant species to draw from as potential colonists.) In addition, the fossil record implies that many flowering plant groups with distributions broken up by oceans are too young to have gone through ancient events such as Gondwanan breakup, thus implicating recent ocean crossings. This observation of groups being too young, now based on molecular data, is obviously critical today, but fossil evidence was used to make the same argument long before the molecular dating explosion.
The perceived importance of long-distance dispersal by plants waned (but did not disappear completely) with the rise of land-bridge explanations, and waxed with the general spread of the dispersalist thinking of the New York School. Then came the revolution—the validation of continental drift and the rise of vicariance biogeography—whereupon many botanists were still arguing for the great significance of ocean crossings and other dispersal events. For instance, in the early 1990s, with both plate tectonics and vicariance biogeography well established (but before the flood of molecular dating studies), plant experts contributing to a book called Biological Relationships Between Africa and South America frequently invoked ocean crossings between those continents. None of these botanists denied the fact of plate movements, but they apparently hadn’t absorbed the message that continental drift plus cladograms had to change their whole worldview.
In short, since Darwin’s time, knowledge of the properties of plants, their distributions, and the fossil record has been sufficient to convince many botanists that long-distance dispersal by plants is frequent and important. In the past fifteen to twenty years, these were the people who did not require conversion; they didn’t need it, because they were already there. Susanne Renner, who has probably done more than anyone else in recent years to convince people of the ubiquity of plant dispersal, was one of them. Her graduate school adviser, Klaus Kubitzki, had a strong belief in the importance of long-distance dispersal, and from him she picked up that conviction and never let it go. During a stint as a postdoctoral researcher at the Smithsonian Institution in the 1980s, she was in contact with the other side, the vicariance biogeographers, but she says she always found their extreme views “dogmatic and a bit silly.” For Renner and others, molecular dating simply provided the final corroboration of what they already strongly suspected was true. When it came to plant dispersal, Darwin’s long shadow had never come close to being erased, even if it had at times grown faint.
ON TO ANIMALS
When Isabel Sanmartín and Fred Ronquist examined plant studies, they found that the branching order of evolutionary trees for Southern Hemisphere groups generally did not match the sequence of Gondwanan fragmentation. However, they also examined animal groups—mostly vertebrates and insects—and for these, the story was very different. Evolutionary trees of animals did tend to match Gondwanan breakup; for instance, when a group was found on New Zealand, Australia, and southern South America, the New Zealand branch tended to be outside of a lineage that included both the Australian and South American branches, presumably reflecting the more recent land connection between those two areas. (That particular pattern was the one found by Lars Brundin for the chironomid midges, way back in the 1960s [see Figure 2.5]). In fact, Brundin’s study was included in Sanmartín and Ronquist’s compilation.)
7.5 Susanne Renner. She didn’t have to be converted to a belief in the importance of dispersal; she was already there. Photo by Pierre Taberlet.
There are problems with jumping from these results to the conclusion that Gondwanan vicariance is a good general explanation for piecemeal distributions of Southern Hemisphere animals. Recall, for starters, that Sanmartín and Ronquist’s study excluded groups found widely in both hemispheres, thus perhaps skewing the sample away from animals that are good dispersers, that is, exactly the kinds of animals that might cross oceans. There are other methodological issues as well, such as that the particular algorithm they used may be biased to support vicariance over dispersal, and that, since the ages of branching points were not used, some cases that seem consistent with vicariance might turn out not to be. Also, there are conspicuous exceptions to the pattern. For instance, animal lineages on New Caledonia and New Zealand typically are not closely linked, despite the fact that they were recently connected as parts of Zealandia.
Nonetheless, the results do suggest that animal distributions reflect Gondwanan vicariance more so than do plant distributions. Obviously, both plants and animals must have been carried as passengers on the moving fragments of Gondwana. The difference seems to be that the signature of continental drift for plants ha
s been obscured by subsequent ocean crossings (and by extinction), whereas the drift signal has remained more nearly intact for animals. In short, the green web has overwritten the ancient pattern of continental breakup. Subsequent work indicates that this distinction holds for the Northern Hemisphere as well, with movements between Eurasia and North America being much more common for plants than for animals (although some of these intercontinental movements may represent normal dispersal over land bridges).
The conclusion that animals don’t get around nearly as easily as plants do is a fundamental, if unsurprising, inference.36 However, this doesn’t mean that animals have an insignificant role in the story of chance dispersal. While plants show the importance of long-distance journeys through sheer weight of numbers, recent studies of animals also have something to teach. As I will show in the next two chapters, what they reveal is that, in the long course of evolutionary time, very strange things happen, stranger than fifty-one ocean-crossing legumes, stranger even than an Australian sundew colonizing a South American tepui. Ultimately, what animals show in a particularly clear way is that the history of life is extremely serendipitous and unpredictable. A frog, for instance, might make a journey that no frog seems to have any business making.
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