Jerry’s back from his southern sojourn now, and may have made some posts from Chicago by the time you see this, but as he settles back in I thought it would be good to recall the lessons that another famous evolutionary biologist learned in the Falklands. Although we all associate Darwin with the Galapagos, his visit to the Falklands (also during the Beagle voyage) supplied an important bit of evidence in his thinking about islands, and the phenomena of island life were crucially important components in his argument for evolution.
Darwin was a bit perplexed by the Falklands. In many ways they seemed like oceanic islands—islands never connected to a continent, which had received their biota from across the seas by what Darwin called “occasional means of transport”. There was only a single native species of land mammal on the Falklands: 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 was thus depauperate (few species); disharmonious (lacking major ecological or taxonomic groups); and showed affinity to the fauna of the nearest continent (the effect of distance)—all of these are characteristics of oceanic islands.
How the fox got to the Falklands is an issue that concerned Darwin, but that’s not what I think was most important. (The issue of how they arrived, and when, was largely solved a few years ago, and we discussed it here at WEIT: the short answer is that lowered sea levels during the last glacial maximum greatly shortened the distance to the continent, and the fox came across, perhaps floating on ice floes, as Arctic foxes do.) The problems that the Falklands helped Darwin with most was why oceanic faunas were depauperate and disharmonious. Darwin’s evolutionary hypothesis was that it was difficulties of dispersal that led to oceanic faunas being apparently “undercreated”.
But there was an obvious alternative explanation: the ecological conditions on the islands are unsuitable for a species-rich, harmonious, fauna, despite seemingly appropriate physical environmental conditions. The oceanic faunas were not “undercreated”, but inhabited by the ecologically appropriate species.
These competing explanations are 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 the people who had settled the Falklands brought their animals with them. At the time of his visit, Darwin recorded wild populations of cattle, horses, pigs, rabbits, rats, and mice, with cats, 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!
So we can see that exotic mammals of all sorts do quite well in the Falklands, and that Darwin’s evolutionary hypothesis is thus supported. Although I’ve read up on the Falklands, Jerry’s visit there is the first by anyone I know, and he has provided firsthand reports on, and pictures of, the exotic mammals. So here again is a bovine, the Belted Galloway:
and a dog:Jerry tells me that sheep are “all over the place there!”, but, unfortunately, he didn’t get any pix of them. So here’s one that I found on the Internet, by Jeremy Richards, who has also sagely captioned it:
My Okinawan correspondent sends the following photograph of an apparently window-killed bird.
I thought immediately, “a thrush”, noting the similarity in bill, body and leg shape to that of the familiar North American Robin (Turdus migratorius). I was also immediately reminded of the justly famous opening passage in Alfred Russel Wallace’s Island Life, in which, comparing the birds of Britain and Japan, he finds them remarkably similar:
WHEN an Englishman travels by the nearest sea-route from Great Britain to Northern Japan he passes by countries very unlike his own, both in aspect and natural productions. The sunny isles of the Mediterranean, the sands and date-palms of Egypt, the arid rocks of Aden, the cocoa groves of Ceylon, the tiger-haunted jungles of Malacca and Singapore, the fertile plains and volcanic peaks of Luzon, the forest-clad mountains of Formosa, and the bare hills of China, pass successively in review; till after a circuitous voyage of thirteen thousand miles he finds himself at Hakodadi in Japan. He is now separated from his starting-point by the whole width of Europe and Northern Asia, by an almost endless succession of plains and mountains, arid deserts or icy plateaux, yet when he visits the interior of the country he sees so many familiar natural objects that he can hardly help fancying he is close to his home. He finds the woods and fields tenanted by tits, hedge-sparrows, wrens, wagtails, larks, redbreasts, thrushes, buntings, and house-sparrows, some absolutely identical with our own feathered friends, others so closely resembling them that it requires a practised ornithologist to tell the difference. If he is fond of insects he notices many butterflies and a host of beetles which, though on close examination they are found to be distinct from ours, are yet of the same general aspect, and seem just what might be expected in any part of Europe. There are also of course many birds and insects which are quite new and peculiar, but these are by no means so numerous or conspicuous as to remove the general impression of a wonderful resemblance between the productions of such remote islands as Britain and Yesso.
(Perhaps inspired by Wallace, the Japanese ornithologist Masa Hachisuka once published a comparative list of the birds of Britain and Japan.) Wallace went on to contrast the remarkable similarities between the birds of these two distant archipelagos with the differences one finds when crossing the narrow strait between Bali and Lombok:
In the Malay Archipelago there are two islands, named Bali and Lombok, each about as large as Corsica, and separated by a strait only fifteen miles wide at its narrowest part. Yet these islands differ far more from each other in their birds and quadrupeds than do England and Japan. The birds of the one are extremely unlike those of the other, the difference being such as to strike even the most ordinary observer. Bali has red and green woodpeckers, barbets, weaver-birds, and black-and-white magpie-robins, none of which are found in Lombok, where, however, we find screaming cockatoos and friar-birds, and the strange mound-building megapodes, which are all equally unknown in Bali. Many of the kingfishers, crowshrikes, and other birds, though of the same general form, are of very distinct species; and though a considerable number of birds are the same in both islands the difference is none the less remarkable—as proving that mere distance is one of the least important of the causes which have determined the likeness or unlikeness in the animals of different countries.
Wallace, of course—in this and many other works—went on to explicate what the important causes of these disparities were, not the least of which are the evolutionary and geological histories of the organisms and land masses. (In the case of Bali and Lombok, the key factor has turned out to be that Bali is on the Asian continental shelf, and thus has been in frequent dry-land contact with the continental fauna, while Lombok is off the shelf, and has received its fauna over water by occasional means of transport.)
Like Wallace’s traveling Englishman, I too was struck by the great familiarity to me of this bird from the opposite side of the world. But while it was certainly a thrush, and almost certainly in the genus Turdus, I could not identify the species. I don’t have a Japanese or East Asian bird field guide, but checking some pictures on the internet, it seems most similar to T. pallidus, a winter visitor to Okinawa. Our deceased friend seems too white below, so I leave its species undetermined. Perhaps some reader will be able to identify it.
In addition to being a familiarly thrush-like bird, it was also, sadly, in a familiar posture: dead outside a glass door. Window-killed Swainson’s thrushes (Catharus ustulatus) are an all too familiar sight here in southeastern Wisconsin. My correspondent added about this bird, “Such a shame to see a dead bird, because they’re actually kind of rare to see. I blame the cats and Habu.” Habu are any of various pit vipers found in the Ryukyu Islands, which I thought were not common. I’ve queried my correspondent as to the relative abundance of cats and habu.
Hachisuka, M.U. 1925. A Comparative Hand List of the Birds of Japan and the British Isles. Cambridge University Press, Cambridge. (paperback published 2015)
Australia is a zoogeographer’s dream world—it’s the most spectacularly distinctive place on Earth, and we know why. Around 250 million years ago, most of the world’s continental plates amalgamated into a single super-continent—Pangaea. During the Mesozoic (the “Age of Reptiles”), Pangaea began breaking up, with many of today’s southern continents (South America, Africa, Antarctica, Australia, and India) pulling away to form the somewhat smaller super-continent of Gondwanaland.
The continental breakup continued, with the various parts of Gondwana separating from one another (hence the traditional rallying cry of irredentist geologists, “Reunite Gondwanaland!”). Africa, India, and, most recently, South America eventually bumped into the northern continental masses, making for interesting geology, and—of the greatest importance for zoogeography—allowing large land animals to move between the major land masses. Such animals are not, in general, susceptible to “occasional means of transport”, as Darwin called them, that allow birds, bats, and smaller animals to traverse greater or lesser expanses of the sea, and thus large land animals require a dryshod path to disperse.
But, unlike those other Gondwanan remnants, Australia has not—yet, anyway—bumped into the northern land masses, and thus has undergone a long and and continuing period of splendiferous isolation, during which time many unique endemic forms have arisen, and radiated into the great variety of ecological niches occupied by different lineages in the rest of the world. Most famous of these are the Australian marsupials, which have undergone a continental-scale adaptive radiation, which Jerry highlighted in chapter 4 of WEIT.
This radiation has brought marsupials into most of the ecological niches inhabited by placental mammals in the rest of the world—predators, herbivores, gnawers, burrowers, insectivores, gliders, etc. Two things are evident in the radiation of Australian marsupials. First, that convergence can lead to remarkable similarity when distantly related lineages adapt to similar environments—Jerry highlights this in the figure above; but, second, that sometimes forms inhabiting the same ecological niches can be quite different.
We can see both of these by thinking about which animals are the big, dominant, mammalian carnivores and herbivores. Everywhere on Earth but Australia, these animals are cats, dogs, cattle, and deer (taking cattle in the sense of the family Bovidae, including antelope, goats, etc.). In Australia, the dominant carnivores are the thylacine (or Tasmanian wolf or tiger), native cats (hence, placental cats in the post title, to make clear who I meant), and devils. The skull of the thylacine is remarkably wolf-like, showing a close convergence in shape and dentition to the placental wolf. Native cats have the name, but are less similar to cats; and devils are pretty much sui generis. The dominant big herbivores in Australia are kangaroos: instead of plains full of buffalo and antelope, Australia is full of kangaroos. Although they eat similar types of plants, the modes of locomotion are startlingly distinct, showing that close convergence is not inevitable. (There were some more ungulate-like marsupials in the past, but they are now extinct.)
All this was brought to mind by a new article in BMC Evolutionary Biology by Katrin Koch and colleagues on the placental cats of Australia. For some millennia now, in addition to Darwin’s occasional means of transport, another factor has allowed animals to cross the seas—human transport. And for thousands of years, man has broken Australia’s tens of millions of years of isolation by bringing in a diversity of placental mammals. Most famous is the dingo, the feral descendants of dogs brought from southeast Asia about 4,500 years ago. Since European settlement, the number of mammalian imports—both wild and domestic—has increased dramatically.
In a careful review of historical records (which includes this memorable sentence in his methods section: “Incidentally, I discovered that indexers of books rarely index ‘cat’.”), Ian Abbott (2002) showed that, despite the potential for cats to have been brought to Australia earlier by Aborigines, Malay trepangers (sea cucumber fishermen—it’s great that there’s a word for that!), or shipwrecks, the first cats seem to have been brought in by the earliest European settlers in the late 18th century. The map below shows places where cats were presumed to have been introduced (arrows) and dated records of cat appearance (dots):
Koch and colleagues looked at microsatellite and mitochondrial DNA—both rapidly evolving parts of the genome, and thus good for studying infraspecific phylogeny—in over 200 feral cats from Western Australia and a number of islands round Australia, including the outlying territories of the Cocos (Keeling) Islands (an island group where cat control is an issue, due to their depredations on wildlife: Algar et al., 2003) and Christmas Island.
What they found was that the cats of mainland Western Australia, and the big island of Tasmania, were all genetically fairly similar (the red areas in the figure), while the smaller islands were more distinct genetically, including an island very close to the mainland (Dirk Hartog Island, DHI–green), as well as the two outlying islands of Cocos (yellow) and Christmas (blue). Curiously, two smaller islands off southeastern Australia grouped with Christmas. Small island populations can diverge due to random genetic effects (founder effect and genetic drift), historic phenomena (founded from different sources), and selective differences (distinct environments on islands). But it’s hard to tease these apart from these data alone.
This paper has made a minor splash in the media (see Jerry’s mention here about NY Times coverage), with most places proclaiming that it shows that Australian cats came from Europe as opposed to southeast Asia. Now, the paper does show that of the 63 mitochondrial haplotypes that they found, 25 are also present in Europe (the European data are from a paper by another group). But to distinguish sources of colonization, you need to have samples from all the potential sources, find out if the source populations have any diagnostic or characteristic alleles or mutations, and then see if these are found in the colonized (i.e., Australian) populations. But Koch et al. had only three non-Australian cats—apparently one from Sulawesi, and two from Borneo. (Curiously, they refer to these as “Malaysian”, and also use that term for 17th century Malay trepangers. But Malaysia is a 20th century political construct, and Sulawesi is in Indonesia, not Malaysia, so Malay would be a better term that covers the cultural/linguistic/geographic region.)
The three Malay cats group with Australian mainland cats. But on such a slim basis, we can make no conclusions about the relative importance of the two potential source areas. While concluding their data “indicate a mainly European origin of feral cats in Australia”, the authors do allow that, “However, caution is needed in inferring the involvement of Asian cats in the history of cat colonization in Australia due to the small number of Asian samples.” We can, in fact, be sure that a significant, if not the greatest, part of Australian cat ancestry is European, but that is because of the historical researches of Abbott (2002). The genetic work of Koch et al. lays a basis for further studies of genetic variation in Australian cats and their relation to cats from other regions, but it does not, on its own, really speak to the latter question.
Abbott, I. 2002. Origin and spread of the cat, Felis catus, on mainland Australia, with a discussion of the magnitude of its early impact on native fauna. Wildlife Research 29:51-74. abstract
Algar, D., G. J. Angus, R.I. Brazell, C. Gilbert and D.J. Tonkin. 2003. Feral cats in paradise: focus on Cocos. Atoll Research Bulletin 505. (Actually published in 2004.) pdf
K. Koch, K., D. Algar, J. B. Searle, M. Pfenninger and K. Schwenk. 2015. A voyage to Terra Australis: human-mediated dispersal of cats. BMC Evolutionary Biology 15 (262), 10 pp. pdf
In the paper, they report an Aldabra tortoise that came ashore on a beach in Kimbiji, Tanzania in 2004. After considering several possibilities, they conclude that the tortoise had floated in from Aldabra, over 700 km away across the Indian Ocean. The copious growth of large barnacles on the limbs and lower parts of the carapace certainly suggested that the tortoise had spent a considerable amount of time in the sea.
Dennis’s video shows that the tortoises enter water, and how they move about in the shallows. The Kimbiji tortoise, despite the ability to swim, could not have swum to the main, but was rather mostly carried by the currents, and presumably spent its time at sea keeping its head, and most of the dome of its carapace, above water. It was, however, walking ashore, apparently intentionally, when found.
So why is this important? It has long been supposed that animals and plants get to oceanic islands by what Darwin called “occasional means of transport“: carried along on logs, masses of vegetation, ice floes, attached to birds, or floating by themselves in the water. (Darwin carried out a series of experiments on the ability of various plant seeds to float in sea water, and their ability to germinate after varying periods of immersion.) Tortoises have usually been thought to float by themselves because of the difficulty they would have in clinging to vegetation, and also because the practice of mariners of earlier centuries of putting giant tortoises in the holds of their ships as a living food supply had shown that tortoises could live for many months without food or water.
Although Darwin and many subsequent zoogeographers (e.g., P.J. Darlington) invoked such occasional means of transport, there has always been a school of thought arguing that such crossings of the ocean by land animals were nigh impossible, and that the presence of non-flying land animals on an island implied a past land connection. In the first half of the 20th century, this school constructed speculative land bridges crisscrossing the oceans, in order for every island animal to have had a dry-shod passage to the island from its home of origin. In the later 20th century, with the development of plate tectonics, the land bridge builders were succeeded by drift enthusiasts, who thought drifting crustal plates could serve to bring oceanic islands into juxtaposition with continents, so that, again, animals might get to islands without having to cross water (or at least not much). (The drift enthusiasts had the advantage that continental drift actually does occur, even if not in the exact plate configurations they hoped for, whereas the land bridge builders’ long, thin isthmuses crossing abyssal oceans have not been borne out by geology.) So, the more or less direct observation of transoceanic crossing by an occasional means of transport provides a crucial link—a vera causa—in the argument for the occurrence and importance of such means in the colonization of islands.
Some younger biologists, raised (and properly so!) on plate tectonics, and perhaps lacking acquaintance with older literature and island organisms in the field, had taken the drift enthusiasts’ claims too much to heart, and seemed to be unaware of the importance of transoceanic dispersal. Now that molecular phylogenies often bolster the argument for the importance of occasional means of transport, they seem a bit surprised to find out that there indeed has been a lot of ocean crossing, not just to oceanic islands, but between continents and continental islands as well. We discussed one such case here on WEIT, the ratite birds, where what had seemed to actually be a good case for continental drift seems to actually involve a fair amount of oceanic crossing. The zoologist Alan de Queiroz has written a popular book on the biology of oceanic dispersal, and the sociology of its rediscovery by some biologists.
de Queiroz, A. 2014. The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life. Basic Books, New York.
Gerlach, J, C. Muir and M.D. Richmond. 2006 The first substantiated case of trans-oceanic tortoise dispersal. Journal of Natural History 40(41–43): 2403–2408. pdf
My Minnesota correspondent found this fellow along Caribou Trail (a road) and Jonvick Creek near Lutsen, Cook County, Minnesota, about a half mile from the north shore of Lake Superior, on 10 June 2014. The region is mixed spruce and maple forest; the frog was in a “mapley” area. The great herpetologist Robert C. Stebbins thought the species’ distribution tracked, for much of its range, pretty closely to the distribution of spruce.
The distribution of wood frogs is interesting for at least two reasons. First, they are the most northerly distributed of any North American amphibian (or reptile, for that matter), and extremes are always interesting. They can survive for weeks at temperatures below freezing, in part through elevated levels of blood glucose acting as an “anti-freeze”.
They’re not immune to freezing though—I once found, during an early spring field trip near Northampton, Massachusetts, a dead female who had laid her eggs in a small pond. She was perfectly intact, and I suspected she had frozen, as contact with ice crystals (from the pond) makes them more vulnerable to freezing.
Second, there are a number of outlying populations to the south of the main range (which, as shown above, crosses northern North America from the Bering Strait to the North Atlantic, descending into eastern North America along the Appalachians). In particular, note the outliers in Colorado and Wyoming. These are almost certainly relicts from cooler glacial times when the frog occurred further south in the Rockies; it moved northward as the glaciers retreated, leaving behind populations in some favorable southern localities. The isolated Colorado-Wyoming population was named as a distinct species (maslini), but currently it is not recognized, not even as a subspecies.
Wood frogs are are also famous for another “non-subspecies”: cantabrigensis, a short-legged form from the northwestern part of the range, versus the longer legged ones to the east. While the variation in leg size is real, there is a gradual cross-continental gradient (a cline, in technical terminology), with no break in leg size, and most systematists do not distinguish such clinal patterns of geographic variation with nomenclatural recognition. So cantabrigensis is not recognized either, and the wood frog has become a classic case of clinal variation.
Bagdonas, K.R. and D. Pettus. 1976. Genetic compatibility in wood frogs (Amphibia, Anura, Ranidae). Journal of Herpetology 10:105-112 (jstor)
Costanzo, J.P., M.C.F. do Amaral, A.J. Rosendale and R.E. Lee. 2013. Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog. Journal of Experimental Biology 216:3461-3473. (pdf)
Dodd, C.K. 2013. Frogs of the United States and Canada. Johns Hopkins University Press, Baltimore (publisher) (Google books)
Porter, K.R. 1969. Evolutionary status of the Rocky Mountain population of wood frogs. Evolution 23:163-170. (jstor)
Stebbins, R.C. 2003. A Field Guide to Western Reptiles and Amphibians. 3rd ed. Houghton Mifflin, Boston. (publisher)
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 distanceon 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)
Waterhouse, G.R. 1838. Mammalia. The Zoology of the Voyage of H.M.S. Beagle, under the Command of Captain Fitzroy, During the Years 1832 to 1836. Part 2, No. 1. (BHL, DOL)
BBC reporter Rebecca Morelle has been hot on the trail of the nearly extinct Hispaniolan solenodon in the mountains of the Dominican Republic. (There is another living species of solenodon in Cuba.) Here’s what they look like.
Solenodons are among my favorite mammals, not just because they are so bizarre in their own right, but for the zoogeographic lessons we can learn from them and other island animals. In WEIT (and earlier writings), Jerry, despite being a geneticist, emphasized the biogeographic evidence for evolution. In doing so, he was following a long tradition, dating back to Darwin and Wallace themselves. P.J. Darlington (from Jerry’s and my alma mater, the MCZ), perhaps the greatest zoogeographer of the last century, said that zoogeography showed Darwin evolution.
Island faunas, especially of oceanic islands (i.e. islands never connected to a mainland, such as the Galapagos), exhibit what I like to call the three “D’s”: they are depauperate (having fewer species than equivalent pieces of mainland), they are disharmonious in their taxonomic composition (major groups from the nearest mainlands are missing, while others are curiously diversified), and show evidence of dispersal(the taxa in the fauna are able to travel over water).
The terrestrial mammals of the West Indies are definitely depauperate (there are many fewer species on Jamaica than in an equivalent piece of, say, Honduras). And they are disharmonious: the only major terrestrial mammal groups in the West Indies are insectivores (solenodons, and another, recently extinct, genus Nesophontes, called island-shrews), ground sloths (also recently extinct), monkeys (again, recently extinct), and a couple of groups of rodents (some recently extinct, but a modest number of survivors). They are completely lacking the most diverse groups of the surrounding mainlands: carnivores, both large (cougar, jaguar, bear) and small (raccoon, skunk, coati), hoofed mammals (deer, tapir, peccaries), opossums, and others.
The signs of dispersal are less clear in West Indian mammals. Rodents disperse over water well (by what Darwin called “occasional means of transport”), as, oddly, do sloths (they crossed from South to North America before the Panama land bridge formed); but insectivores and monkeys are not known for water-crossing. If we add in bats, we do see clear evidence of dispersal in the many species shared between islands and mainlands. But overall, some zoogeographers have seen the insectivores and monkeys as perhaps relicts of a former land connection, and the argument over whether the West Indies are oceanic or old continental islands (islands once connected to continents, but so long ago as to obscure the faunal characteristics: Madagascar is a classic example) is an old one in zoogeography. Indeed, the West Indies contain oceanic islands (the Lesser Antilles) and even recent continental islands (Trinidad), so that Cuba or Hispaniola being old continental islands would just complete the range of possible faunal conditions.
Island faunas also show the two “E’s”: endemism (species being found on the islands and nowhere else) and extinction (island species go readily extinct when confronted by man, his habitat changes, and his many introduced species from the mainland). West Indian terrestrial mammals show high endemism (the solenodons and island-shrews are each endemic families, and there are others; no species is shared between islands and mainland), and also, regrettably, extinction: two of the four known solenodon species are extinct, and the living two are in trouble; about 80% of the 80 or so species of West Indian terrestrial mammals have become extinct in the last few thousand years.
Darwin (and Wallace) argued that all five of these “D” and “E” phenomena could be explained by descent with modification, creating a consilience of inductions: if organisms had to cross water barriers by occasional means of transport (floating, swimming, flying, etc.), rather than being created in situ, then some species and groups wouldn’t get there, those that did would diversify under different biotic conditions than those on the mainland, thus filling different places in the economy of nature, and thus not being adapted to the presence of mainland forms.
The plates here are from G. M. Allen’s classic 1910 monograph, published as one of the Memoirs of the Museum of Comparative Zooogy, on the anatomy of the Hispaniolan solenodon. Most have probably never been posted to the web, so I do so here. Here’s a Hispaniolan solenodon at the Museum of Comparative Zoology, from an unsourced photo I found on the web; it is perhaps one of the specimens studied by Allen a century ago.
Allen, G.M. 1910. Solenodon paradoxus. Memoirs of the Museum of Comparative Zoology 40:1-54.
Allen, G.M. 1942.Extinct and Vanishing Mammals of the Western Hemisphere with the Marine Species of All the Oceans. Special Publication No. 11 American Committee for International Wild Life Preservation.
Coyne, J. A. 2005. The faith that dare not speak its name: the case against intelligent design. The New Republic Aug. 22, pp. 21-33.
Darlington, P.J. 1959. Darwin and zoogeography. Proceedings of the American Philosophical Society 103:307-319.
Darwin, C. 1859.On the Origin of Species. John Murray, London. Chap. 12, “Geographical Distribution”.
Although far from the longest chapter in WEIT, I find the chapter on biogeography the single most persuasive one for showing why evolution is true. I think Jerry finds it compelling as well. This might seem surprising since he’s a geneticist: one might think he would find some of the genetic evidence most compelling. But I don’t think it is surprising, given that it was the biogeographic evidence, that, as the great zoogeographer P.J. Darlington put it, showed Darwin evolution.
The first thing you might think needs explication is the disjunct distribution. But before tackling this, a mis-impression must be corrected: although we tend to think of tapirs as typically South American, from a historical perspective, they are recent interlopers. Along with many other animals we consider typically South American (jaguars, llamas, peccaries), they entered South America from the north about 3 million years ago when the Panamanian portal became the Panamanian isthmus during the Great American Interchange.
What, then about the disjunction: how did they get from Central America to Malaya? They didn’t. Tapirs are a northern group. They and their relatives date back to the lower Eocene (ca. 50 mya). The modern genus, Tapirus, dates back to the Oligocene (ca. 30 mya), and was found in Europe, Asia, and North America. They have gone extinct in Europe, most of Asia, and most of North America. Tapirs thus have a relict distribution, being still found at two endpoints of their historical distribution. Geology, paleontology, and systematics thus combine to give a most satisfying account.