by Greg Mayer
The evidence from biogeography is arguably the most important evidence for evolution. P.J. Darlington, perhaps the greatest zoogeographer of the 20th century, said that zoogeography showed Darwin evolution. And Jerry has long insisted that biogeography is at least among, if not the, most persuasive evidence for evolution.
It was thus with great pleasure that I read a recent paper by Jeremy Austin and colleagues (ref below, news piece here) on the Falkland Islands fox or wolf (Dusicyon australis), a species which had intrigued Charles Darwin, and which he wrote about in the Zoology of the Beagle, the Voyage of the Beagle, and The Origin.
But first, let’s recap the features of island faunas that Darwin thought cried out for an evolutionary explanation. In examining island faunas, Darwin distinguished between continental islands, which had had a connection to a mainland in the recent past (e.g. Great Britain, which was connected to France by Ice Age sea-level lowering as recently as about 12,000 years ago), and oceanic islands, which had never had a connection to the main (e.g. mid-ocean volcanic islands like the Galapagos).
Darwin identified four characteristics of oceanic islands, which I like to call the “four D’s”. First, island faunas are depauperate— they hold fewer species than did comparable areas of mainland habitat. Second, they are disharmonious— they are inhabited by an unusual concatenation of taxa, rather than the usual combinations of predators, herbivores, and omnivores. Instead of cattle and deer, the large herbivores of islands were things like giant tortoises (as in the Galapagos) or giant geese (as in Hawaii). And large predators, such as cats and dogs, were usually lacking altogether (although some islands had very large birds of prey). Third, island faunas showed signs of dispersal— the animals that were there showed the ability to cross salt water. So birds and bats were usually present, but large land mammals and amphibians were usually absent. And finally, there was a strong effect of distance on the character of the fauna– the Galapagos fauna, for example shows clear affinity to the Americas, while the fauna of the similarly situated but Atlantic archipelago of Cape Verde shows affinity to Africa.
Darwin argued that all of these features can be explained if the inhabitants of oceanic islands are the modified descendants of animals that had been able to disperse there. These animals would need be susceptible to occasional means of transport (dispersal), come from the most accessible mainland (distance), and would be a small, non-representative sample of what occurred on the mainland (disharmonious, depauperate). Darwin contrasted this with what we might expect under an hypothesis of special creation. Why were the island faunas “undercreated” relative to the mainland, and why would they bear the plain stamp of affinity to the nearest mainland, rather than being related to the faunas of other similar islands?
Although we all associate Darwin with the Galapagos, Darwin also visited the Falklands, and they supplied, I believe, an important bit of evidence in his thinking about islands. Darwin was a bit perplexed about the Falklands. In many ways they seemed like oceanic islands. There was only a single species of land mammal, the Falkland Islands fox, which was clearly related to South American foxes (South America has a modest radiation of canids, which are variously called dogs, foxes, or wolves in English). The mammal fauna thus shows 3 of the four D’s: depauperate, disharmonious, and distance.
An alternative explanation for island faunas being depauperate and disharmonious is that the ecological conditions on the islands are unsuitable, despite seemingly appropriate physical environmental conditions. This alternative explanation is easily tested by introducing exotic species to the island, and seeing how they fare. If they become established, then the cause of their absence is a failure of dispersal, not a failure of environmental suitability. This is where the Falklands helped Darwin, I think. The Galapagos in the 1830s were still nearly pristine, but the Falklands showed him the fauna of an island with little direct habitat disturbance and a small human population, but whose population had brought their animals with them. At the time of his visit, Darwin recorded wild populations of cattle, horses, pigs, rabbits, rats, and mice, with feral cats and at least domestic dogs and sheep coming later. The Falklands were thus quite capable of supporting a diverse and harmonious mammalian fauna; the mammals just needed help getting there. (The increasing human population, and consequent increased disturbance and hunting, led to the extinction of the Falklands fox by the late 1800s.)
But how did the fox get there? Carnivores, in general, are not known to be good at dispersing across sea barriers, and a fox is unlikely to have been able to cross several hundred kilometers of open sea. This is what puzzled Darwin, and led him to suggest that the islands had been connected to the continent, despite the lack of all other animals that might have been expected to cross over on a land bridge. In later years, it was even suggested that the fox was semi-domesticated, and had been brought to the islands by Indians. This is where the latest paper by Austin et al. comes in.
As I noted above, South America is home to a modest radiation of canids, but the closest living relative of the Falklands fox, the maned wolf (Chrysocyon brachyurus), is not very close, with an estimated divergence time of 7 million years ago. What Austin and colleagues have done is extract DNA from fossils of Dusicyon avus, a very recently extinct canid (ca. 3ooo years ago) that was widespread in southern South America. Comparing their DNA to that from skins of the even more recently extinct Falklands fox, they found that the two are very closely related. Indeed, in their best phylogenetic estimate, the mainland avus is paraphyletic with respect to australis, and this is exactly what we would expect if the Falklands had been colonized by mainland foxes from the part of South America nearest to the islands. Furthermore they were able to date the divergence to 16,000 years ago– the height of the last glacial maximum. As is well known, the glaciers withdrew massive amounts of water from the sea, lowering sea levels by about 120 m. Looking at the seabed between the Falklands and the main, we can see that a 120 m lowering would substantially reduce the distance between them. Austin and colleagues favor an even greater lowering, further reducing the distance, but either lowering would reduce the distance to the order of tens, rather than hundreds, of kilometers.
But could a fox cross even tens of kilometers of sea? Yes– on ice floes or sea ice. How do we know? The arctic fox (Vulpes lagopus) ventures way out on to the sea ice, and can float out on ice floes, having been recorded as turning up occasionally in eastern Canada, far to the south of its native range. It is also the only native land mammal of Iceland, an island which has never had a continental land connection, and which it must have reached on ice floes. Darwin himself noted the possibility of ice transport, writing in the Origin “icebergs formerly brought boulders to its [Falklands] western shores, and they may have formerly transported foxes, as so frequently now happens in the arctic regions.”
So Darwin’s dilemma is solved. Glacial sea-level lowering and sea ice provide an “occasional means of transport”, and the fossil record and DNA analysis lead to an identification of the ancestor, and dating of the event to the precise time when such means were most available.
Austin, J.J. et al. 2013. The origins of the enigmatic Falkland Islands wolf. Nature Communications 4(1552). (pdf, subscription required)
Darwin, C.R. 1859. On the Origin of Species. London: John Murray. (DOL)
Darwin, C.R. 1860. Journal of Researches into the Natural History and Geology of the Countries Visited During the Voyage of H.M.S. Beagle Round the World. Last revised edition. London: John Murray. (DOL)
Matias, R. and P. Catry. 2008. The diet of feral cats at New Island, Falkland Islands, and impact on breeding seabirds. Polar Biology 31:609-616. (pdf)