Beans, of course, are enormously important food plants, but, from a biodiversity perspective, they are much more than that. The bean family, Fabaceae, is a cosmopolitan group and the third largest family of flowering plants, with nearly 20,000 described species. (For comparison, there are about 5,400 known species of mammals.) In temperate parts of the world, the family is mostly represented by herbs—lupines, vetches, locoweeds, and the like—but in the dry tropics and subtropics, woody legumes such as acacias, locusts, and mesquites are often the most abundant large plants. If one thinks of the Sonoran Desert, the first plant that comes to mind is the saguaro cactus, but in fact that desert is dominated by bean trees and shrubs of various kinds. The same goes for many parts of the world that have warm, dry climates punctuated by a distinct rainy season.
Matt Lavin had done his PhD dissertation in the 1980s on evolutionary relationships within a genus of woody legumes called Coursetia, traveling to Mexico, the Greater Antilles, Argentina, and Venezuela to collect specimens and, in the process, becoming interested in bean trees and shrubs in general. In the mid- to late 1990s, he was contemplating taxa of woody legumes that contain species on both sides of the Atlantic. In particular, he was wondering whether these distributions had been made possible by Cenozoic land bridges that at various times linked northeastern North America with Europe. The idea here was that when climates were relatively warm, tropical and subtropical plants could have spread northward and then moved, via normal, “garden-variety” dispersal, over these bridges from the Old to the New World or vice versa. When the climates cooled, forcing warmth-loving plants south again, the Old and New World lineages would have become widely separated, a classic case of vicariance.32 In a paper published in 2000, Lavin and several colleagues argued for this scenario for a group called the dalbergioid legumes. In the same paper, they also claimed to have found “relatively few instances where over-water [that is, transatlantic] dispersal can be invoked.”
I asked Lavin if his whole focus at that time was on vicariance, and he said, “Oh definitely, definitely,” which means he was thinking like a lot of other people (though by no means everyone) at the time. Like many scientists with a vicariance mindset, he was interested in searching for common patterns, so he extended his work by “gleaning” the legume family tree as a whole for groups that he thought were especially old, old enough to have had their ranges pushed southward and, thus, fragmented by the cooling climate of the mid-Cenozoic. He turned up some good candidates, ones that, for instance, contained large subgroups in which all the species were entirely restricted to Madagascar, Mexico, or the Caribbean. That kind of geographic confinement seemed to indicate that these plants weren’t moving around very much, which, in turn, implied that chance, long-distance dispersal was especially unlikely for them. The large numbers of species in these taxa also suggested to Lavin that they had been around for a long time. Pondering these groups, he thought, “Man, these things have to be old. If anything’s old, it has to be these.”
Although Lavin was focused on vicariance, he didn’t share the notion of some of his colleagues that cladograms, representing the evolutionary branching relationships among groups, were the only definitive kind of evidence in historical biogeography—the only things that could be used to reject or support vicariance. Specifically, he thought that molecular dating could be useful, especially now that relaxed clock methods—ones that did not assume a constant molecular clock—had been developed. So he took DNA data from the legumes and ran them in one of the new relaxed clock programs to get estimates of the ages of separation of Old World and New World lineages. And something odd came out.
The ages were all too young. “We tried to bias ’em and make them old,” Lavin said, “ . . . by putting the fossil [used to calibrate the clock] on the crown instead of the stem node . . . or taking the oldest possible minimum age that the fossil could be, if that makes any sense.” In other words, in trying to make the data support vicariance scenarios, Lavin and his colleagues played with the fossil calibration points to make the ages come out as old as possible, probably making them unrealistically old. But it didn’t matter. From the vicariance point of view, nothing worked. Lavin continued, “We could hardly make them old, we could not make them older than 20 million years or something,” which meant that the evolutionary splits weren’t old enough to have been caused by the “land bridge plus cooling climate” hypothesis.
In 2004, Lavin, along with eight other botanists studying legumes, published an extensive molecular dating study with more or less the diametrically opposite conclusion from Lavin’s earlier paper on the dalbergioids. From DNA data on fifty-nine groups in the bean family that had distributions broken up by oceans or seas—occurring, for instance, in Africa and Middle America—they found only eight that were old enough to have spread via land bridges (never mind spreading within an unfragmented Gondwana, which was irrelevant for all the groups). The other fifty-one likely dispersed from one area to the other by crossing seas or oceans—the Atlantic, the Caribbean, or the Mozambique Channel between Africa and Madagascar. They were fifty-one strands in the green web. In just a few years, Lavin had gone from thinking that for woody legumes there were “relatively few instances where over-water dispersal can be invoked” to concluding that most cases of distributions broken up by oceans had to be explained in that way. It was a major shift in thinking for him about a group of plants he’d been studying for years.
7.2 Matt Lavin. Bean plants and molecular clocks changed his view of biogeographic history.
More generally, Lavin’s whole view of biogeography had changed, essentially because of the molecular clock. Where before he had “definitely” been thinking in the vicariance mode, he now sees things as fundamentally about rates of dispersal, with those rates often being much higher than people had previously imagined. For vicariance biogeography, he predicts “a long slow death,” and he clearly sees his new dispersal-oriented view as one in which the pieces have fallen into place. After he had finished the big study on legumes, for instance, he was at the Royal Botanic Garden in Edinburgh and went to an informal talk on the Araucariaceae, a group of conifers that occurs on several Southern Hemisphere landmasses and is considered a prime example of Gondwanan vicariance. The speaker was showing an evolutionary tree for the group, and it struck Lavin that some of the branches were very short, meaning there were hardly any genetic changes in those parts of the tree. The rest of the audience seemed to want to interpret this as a drastic slowdown in the rate of molecular evolution; after all, that was what was required to push those evolutionary branching points deep enough into the past to have been affected by continental drift. Lavin, however, was thinking of a completely different explanation. If you weren’t wedded to the notion of Gondwanan vicariance, you didn’t need to imagine a precipitous decline in the rate of genetic change. You could instead infer that some of the branching points in this evolutionary tree were relatively young. And, in that case, what the data were saying was that Araucariacean conifers, like Lavin’s woody legumes, hadn’t just stayed put while the Earth and its environments fragmented, but had sometimes found their way across ocean barriers.
Those fifty-one ocean-crossing legumes that Lavin and his colleagues uncovered turned out to be just one wave in a massive flood of evidence—mostly from molecular studies—addressing that big question in biogeography, “What explains distributions broken up by oceans?” Since the early 1990s, there have been studies supporting oceanic dispersal by plants of grasslands, deserts, dry shrublands, temperate forests, and tropical jungles; by grasses, herbs, shrubs, and tall forest trees; by plants in dozens of different families and with seeds of all sizes, dispersed by wind, water, or animals. An especially prolific German botanist named Susanne Renner, on her own and with a variety of colleagues, has carried out molecular dating studies that reveal many dozens of ocean crossings by plants.33 In a particularly striking 2009 study, Hanno Schaefer, Christoph Heibl, and Renner
(all then at the University of Munich) found evidence for some forty ocean crossings in the single family Cucurbitaceae (cucumbers, squash, and relatives) (see Figure 7.3). Many cucurbits form gourds that will float, an obvious means of ocean transport, and others have barbed fruits that could latch onto bird feathers, so it isn’t surprising that they are proficient at crossing ocean barriers. Still, before this study, few botanists would have guessed that members of this family had made forty successful long-distance ocean journeys.
The collection of recent studies still represents only a fraction of the world’s plant groups, but a coherent picture is nonetheless emerging from it. To see that picture more clearly, it’s appropriate to start with New Zealand, a hub of biogeographic studies, and then move outward to encompass all of Gondwana.
“EXPLAIN NEW ZEALAND . . . ”
We left off the story of New Zealand in 1994, with Mike Pole having bucked the tide of vicariance thinking by claiming that the country’s native plants generally are not Gondwanan relicts. Pole had argued instead that most, maybe even all, New Zealand plant species are descended from ancestors that arrived from over the ocean, most of them long after Zealandia broke away from Antarctica/Australia. Plants could also, of course, have dispersed from New Zealand to other landmasses. Applied to the general issue of ranges broken up by oceans, Pole’s view meant that plant taxa found on New Zealand and elsewhere had these piecemeal distributions because they had traveled over water either to or from New Zealand (or both to and from New Zealand). As we have seen, his conclusions were based mostly on the turnover of plant groups in the fossil record and on the existence of living “Gondwanan” plants on oceanic islands like Norfolk and Lord Howe, places that could only have been colonized by overwater dispersal. As I mentioned in Chapter Four, Pole did refer to molecular dating work that supported his case, but at the time such studies were few and far between. However, since then, as described in Chapters Five and Six, molecular dating has exploded, and a disproportionate number of these studies have dealt with New Zealand, probably in part because the belief persists that if one can explain New Zealand, the biogeography of the world will fall into place around it.
7.3 Some of the many overwater colonizations indicated by a timetree of the cucumber family. The land configuration is for the Mid-Miocene, about 14 million years ago, and all the colonizations shown are estimated to have happened in the Miocene or later; “11x” and “12x” represent 11 and 12 independent dispersal events. Redrawn and modified from Schaefer et al. (2009).
To evaluate the importance of dispersal versus vicariance for the New Zealand flora, we basically need two lists: one for cases in which the split between a New Zealand lineage and its nearest relatives on another landmass is estimated as too recent to be explained by continental drift, and the second for cases in which the split could have occurred at or before the time of fragmentation. One can think of the first list as the set of ocean voyagers—at least one ocean journey to or from New Zealand is required to explain the distribution of each of these groups—and of the second list as the set of Gondwanan relicts. Graham Wallis and Steve Trewick, biogeographers working in New Zealand, recently compiled those exact lists based on published molecular studies. Their numbers show a striking pattern.
On the first list, there are forty-one cases, representing, minimally, forty-one ocean journeys either to or from New Zealand.34 (The number is actually considerably larger because some of these cases represent multiple dispersal events.) In most instances, the age estimates strongly favor overwater dispersal, because these estimates are not just a little too young to be explained by Gondwanan vicariance, they are far, far too young; for the majority of them, the ages fall within the past 30 million years, some 50 million years after Zealandia separated from Antarctica/Australia. Additionally, almost all of them represent dispersal to rather than from New Zealand, and the majority involve colonizations by Australian species.
On this list of overwater colonists are many of New Zealand’s most abundant and conspicuous plant taxa, ones I remember seeing there even though I was mostly watching birds. They include diverse shrubs and small trees in the genus Pittosporum; two lineages of podocarp conifers; the world’s largest buttercup species (the one Tara and I saw at Arthur’s Pass on the South Island), along with all the other New Zealand buttercups; the many species of Celmisia daisies; the ubiquitous, scaly-leaved Hebe shrubs; Sophora bean trees with showy yellow flowers; and two lineages of southern beeches (Nothofagus). Basically, anyplace in New Zealand with lots of native plants is home to many taxa on this list of long-distance colonists.
In addition, more general molecular dating studies for flowering plants imply that the catalog of ocean voyagers must contain many other groups as well. We know this because these dating studies indicate that many entire plant families that include New Zealand species are too young to have been around at the birth of Zealandia, and this means that these groups had to cross the Tasman Sea or some other part of the Pacific to get there. Conservatively, there are about three dozen such families, probably encompassing hundreds of cases of oceanic dispersal. One of these young groups, the Asteraceae (sunflowers), is easily the most diverse plant family in New Zealand and by itself represents perhaps several dozen overwater colonizations, only a few of which were included in Wallis and Trewick’s list of ocean voyagers. To think of that particular case in another way, consider that there are 24 native genera and some 240 native species of sunflowers in New Zealand, and that not one of them is a Gondwanan holdover. They are all simply too young. The same goes for all the species in the mustard family, the pink family, the gentian family, the mint family, the mallow family, and many others.
The ocean voyager list even includes some members of two groups that have been viewed as “obviously Gondwanan,” the podocarp conifers and the southern beeches. If even these iconic Gondwanan trees are not relicts, one may well wonder what New Zealand plant lineages are on the second list, the one for piecemeal distributions caused by continental drift. It turns out that in Wallis and Trewick’s compilation, there are exactly four, including one podocarp lineage and that other emblem of Gondwana, the giant kauri tree (the lone New Zealand member of the Araucariaceae), although the ages for the latter vary widely and are only consistent with vicariance when certain debatable fossil calibration points are used. In any case, even if the kauri is counted as a relict species, the studies, to date, suggest there are about ten times as many overwater colonist lineages as Gondwanan holdover lineages. And that ratio is likely to become even more skewed in the future, because the old lineages have likely attracted a disproportionate amount of study up to this point. In short, the evidence from molecular dating clearly supports Mike Pole’s contention that the flora of New Zealand overwhelmingly derives from long-distance colonists.
That conclusion is startling, given the earlier belief that New Zealand is a prime example—maybe the prime example—of the dominance of vicariance, a so-called “Moa’s Ark” on which Gondwanan lineages have been transported by the engine of seafloor spreading. That thought takes me back to our 2006–2007 visit to New Zealand, to an evening boat trip from the town of Oban on Stewart Island to an isolated peninsula, where we would see a kiwi picking sandhoppers (amphipod crustaceans) off a sandy beach. On the boat, about ten of us were crammed together in the dark on wooden benches just behind the captain, and among the other passengers was a couple, both fisheries scientists at a university in Wellington, both obviously sharp people. At some point, they were remarking on the primordial nature of New Zealand forests, the old story of Gondwana. As an outsider, I felt uncomfortable about correcting them and kept my mouth shut. Also, at the time maybe I wasn’t quite sure myself. Now I am. That view of the New Zealand biota as ancient and Gondwanan is clearly wrong, at least for plants. Instead, it makes more sense to think of New Zealand as an obvious long-distance destination (and, to a lesser extent, a starting point) in the web of relatively recent ocea
n crossings.
“ . . . AND THE WORLD FALLS INTO PLACE AROUND It”
Most scientists spend a good deal of time reading published papers (and manuscripts before they are published), and at some point, for many, this becomes a chore as much as a pleasure. There are various symptoms—you read fewer and fewer entire papers, and instead merely skim the summaries; you turn down requests to review manuscripts; if you have an anti-institutional streak, as I do, you cut off your subscriptions to journals (they’re almost all online now anyway, but somehow their greater availability makes you read less rather than more). You find, above all, that even the “take-home message” of most papers gets lost somewhere in the dim neuronal passageway between short-term and long-term memory.
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