One of the big controversies in the study of speciation involves the spatial scale of the process. Can an ancestral species split into two descendants within a single small area (“sympatric speciation”), or do populations have to be geographically isolated before they can evolve into new species (“allopatric speciation”)? Clearly the formation of new species is easier if gene flow between the incipient species is prevented, for that gene flow impedes the evolutionary divergence that creates new species. (My—and most biologists’—notion of species are groups that cannot exchange genes because of evolved, genetically-based barriers to gene flow. Those “reproductive isolating barriers” evolve more easily in populations that are physically prevented from exchanging genes, such as those isolated by rivers or mountain ranges, or those that invade a remote island.)
In the book on speciation I wrote with Allen Orr in 2004, we concluded that sympatric speciation, while theoretically possible, was rare. There were a few putative examples of species that seem to have arisen in small areas (the tiny lakes in volcanic craters of Cameroon, for example, contain more than a half-dozen species that descended from a common ancestor), but these examples were not common.
However, even if sympatric speciation were common, it would be hard to find, for you have to identify closely related species that you know were never geographically isolated. Given the history of climatic change and the movement of species’ ranges even in recent times (glaciation moves species around willy-nilly), that’s a tall order.
In our book, Allen and I suggested a way around this problem: look for closely related species confined to very small areas like isolated island and volcanic crater lakes. If in those places you can find “monophyletic” species—that is, species more closely related to each other than to other species—you could reasonably conclude that those species had formed in those small areas—that is, that sympatric speciation had occurred. Such studies don’t require one to know the biogeographical history of the species, since, living in small confined areas, they could never have been geographically isolated.
My colleague Trevor Price and I did the first systematic study of this problem, looking at bird species on isolated “oceanic islands” (that is, those islands that arose from beneath the sea, bereft of life). Surveying 46 of these islands, we found not a single example of two avian “sister species” (i.e., each other’s closest relatives) on an oceanic island. Our conclusion: birds did not undergo sympatric speciation. The same conclusion came from a survey of island lizards by Jon Losos and Dolph Schluter in 2000. In these groups, then, geographic isolation seems necessary for the origin of new species.
In a new paper in The Proceedings of the National Academy of Sciences, Alex Papadopulos and six other authors continue this strategy, but looking at plants instead of vertebrates. They chose to survey the flora of an isolated oceanic island, Lord Howe. Lord Howe is small (about 16 square kilometers), was formed as a volcano about 6.9 million years ago, and is located between Australia and New Zealand:
Although I’ve never been there, it’s a gorgeous place. Here’s a aerial photo sent to me by one of the paper’s authors (click for a splendid enlargement). You can see that while a bit of of the island is settled, a lot of it is native forest, and thus affords a good chance to see if an ancestral species can split into two or more descendants in this small space. An earlier paper by Savolinen et al. had shown that two species of palm trees on the island were each other’s closest relatives, and this result spurred scientists to do a more extensive survey.
So did they find speciation events occurring in Lord Howe plants? The short answer is yes. I’ve written a two-page commentary for the journal, explaining the results and why they are important. Although the paper itself and my commentary are behind a paywall, I’ll be glad to send pdf files to anyone who requests them by email.
A survey of the plants, combined with a phylogenetic study of the genera of those plants (such a study is required to show that two closely related species on Lord Howe really are each other’s closest relatives) identified at least nine other species—and perhaps as many as 18—that may have descended from common ancestors on the island. Adding the two species of palms that were already identified, this brings the proportion of total species on the island that arose via sympatric speciation to between 4 and 8%. That figure is larger than most biologists would have guessed.
As I note in my commentary, this is not only an important finding, but one that can be extended to many other islands and groups. After all, the oceans are full of isolated islands, and many of them have endemic species just begging to be studied systematically. It’s through that kind of work that we’ll eventually learn how common is the process of sympatric speciation. If it seems to occur often on these islands, then it probably also occurs often on the continents, where it’s much less likely to be identified.
Oh, and here are the two sister species of palms that formed on Lord Howe: Howea forsteriana (l.) and H. belmoreana (r.). The former, also known as the Kentia Palm, is grown throughout the world as an ornamental plant.