Category: biogeography

New finding: Iguanas rafted more than 8000 km from North America to Fiji

March 26, 2025 • 10:40 am

Sometimes oceanic islands—islands formed de novo from beneath the sea, as with volcanic and coral islands—harbor endemic species that don’t seem like their ancestors could have gotten there. Birds, insects, and plants can easily disperse to distant islands from continents, but reptiles, amphibians, and mammals have a harder time, for they have no easy way to cross big expanses of salt water. The absence of the last three groups on oceanic islands, as compared to continental islands like Britain and Sri Lanka, was first noticed by Darwin, who used it as evidence for evolution in The Origin.

But sometimes you do find reptiles, amphibians, and mammals on isolated oceanic islands. The Galápagos Islands, for example, are famous for their marine and land iguanas, as well as other lizards that are found nowhere else. And although Madagascar was once connected to Africa, primates got there long after this separation had occurred, crossing the expanse of sea between the continent and the island. The geographic split occurred about 160 million years ago.  But after that, about 50 mya, a primate made it to the island and radiated into the many species of lemurs found nowhere else. How did this primate (and it must have been either one pregnant female or two or more individuals of different sex) get there? The likely explanation is “rafting”, explained in Wikipedia:

Once part of the supercontinent Gondwana, the island of Madagascar has been isolated since it broke away from eastern Africa (~160 mya), Antarctica (~80–130 mya), and India (~80–90 mya).  Since ancestral lemurs are thought to have originated in Africa around 62 to 65 mya, they must have crossed the Mozambique Channel, a deep channel between Africa and Madagascar with a minimum width of about 560 km (350 mi). In 1915, paleontologist William Diller Matthew noted that the mammalian biodiversity on Madagascar (including lemurs) can only be accounted for by random rafting events, where very small populations rafted from nearby Africa on tangled mats of vegetation, which get flushed out to sea from major rivers.

There can also be smaller rafts, like individual trees or small masses of plant material, and these can carry things like small amphibians or invertebrates.  But the new PNAS paper below documents what is now the longest known rafting event among all terrestrial vertebrates: the dispersal of a land iguana from North America to Fiji. That’s a distance of over 8,000 km, or about 5,000 miles. Click the screenshot below to read the paper, and you can find the pdf here.

There are four species of the large iguana Brachylophus on the Pacific islands composing Fiji, where they’re endemic (Tonga also had a giant iguana that’s now extinct).  Here is one of the species studied in this paper, Brachylophus bulabula (this is a male):

JSutton93, CC BY-SA 4.0, via Wikimedia Commons

How did these reptiles get there and where did they come from? And when did this dispersal event take place? The first thing we need to know to answer this is what is the closest living (or fossil) relative to the Fijian species. It turns out that using DNA to gauge relationships also gives us an estimate of dispersal time using the calibrated “molecular clock,” in which DNA divergence, often calibrated with fossil data, can give us both genealogical relationships and divergence times.

The authors used more than 4,000 genes in each of 14 species of iguanas from eight of the nine known genera. It turned out, as the iguana family tree shows below, that the closest related genus to Brachylophus is the genus Dipsosaurus, which contains two living species, both found in North America:

Here’s the Desert Iguana:

Wilson44691, CC0, via Wikimedia Commons

And here’s the DNA-based family tree. The two genera at the top) are clearly more closely related than Brachylophus is to any other species, and they branched off from other genera of iguanas early during the divergence of the entire group (click all figures to enlarge them):

(from the paper): Phylogenomic timetree of iguanas based on StarBeast3 analysis of 150 loci (50 AHE, UCE, RELEC) and three fossil calibrations (for brevity, only two calibrations labeled and outgroups removed), and time-stratified DEC+J analysis from BioGeoBEARS using areas allowed and manual dispersal matrices, and additional areas added to accommodate all alternative hypotheses for the origin of Fijian iguanas. Pie charts indicate the relative probability of the possible ancestral geographic ranges at nodes and at splits immediately after the corresponding cladogenetic event, and tip boxes indicate extant species ranges. Stars at nodes indicate fossil calibrations. The globe inset shows a representation of the transoceanic dispersal of iguanas from North America to Fiji that occurred at the divergence between Dipsosaurus and Brachylophus or along the Brachylophus branch.

Note that both of the North American regions are dry and these iguanas are adapted to a hot, low-water ecosystem.  The relationship between these two genera as sister taxa is very strong, and the divergence time between the two genera is estimated at about 34 million years. That fits nicely with the time that Fiji was created by volcanic activity—about 39 million years ago.  It is likely, given this tree, that the Fiji iguanas came from a North American ancestor, and that would mean rafting 8000 km.

Could it have come from somewhere else? Other hypotheses are possible.  Early biogeographers posited huge land bridges between Pacific islands and the continents, but there is no evidence that such bridges existed. They could have island-hopped from SE Asia or traveled from Gondwana before it broke up.  Other models are possible, but these can be tested using various models, and also looking to see if there are fossil iguanas in other places that are more related to Brachylophus.  Here’s a figure showing some of the models tested, but only one, with the lizard icon on it, was supported by the data. That’s a long trip, and given the size of these animals, it must have involved a fairly substantial raft.

 

But could an iguana really survive floating on a raft of vegetation over that immense distance? Well, for one thing there are currents that go that way, which would speed up the voyage, estimated by the authors to have taken between 80-120 days.

Can an iguana live that long without fresh water (there may have been food on the “raft”)?  The answer is “probably,” because during cold weather many lizards undergo a period of metabolic and activity dormancy called brumation, during which they do not eat (though they need water). Here’s what the authors say:

 Herbivorous iguanids forgo food for months at a time during brumation, and extant Dipsosaurus brumate from October–March. However, floating vegetation mats are a known substrate for oceanic dispersal, so iguanas rafting from North America to Fiji could have had a food source during their journey. Additionally, some iguanas have other traits that may augment their capacity to survive overwater dispersal, including resistance to heat and dehydration. For example, Dipsosaurus have the highest voluntary thermal maximum temperature among lizards and largely inhabit areas without permanent freshwater.

The only thing that concerns me with this hypothesis is this: where did the rafting iguanas get fresh water? The authors don’t really address this, but do mention iguanas’ resistance to dehydration. Also, there’s rain in the ocean, and any rain falling on a raft could be sucked up by the lizards aboard.

The best hypothesis, then, seems to be rafting, and the authors concatenate all the evidence supporting it:

The combination of evidence supporting oceanic rafting from North America to Fiji is 1) phylogenomic analyses that support a sister taxon relationship between Brachylophus and Dipsosaurus, 2) the distribution of fossil iguanids, extant Dipsosaurus, and most other extant iguanids in North America, 3) statistical biogeographic analyses that favor long-distance dispersal from North America over alternative hypotheses, including dispersal via Eurasia, South America, Antarctica, and/or Oceania, and 4) the late Paleogene divergence time between Brachylophus and Dipsosaurus.

Finally, just for fun, here’s are two bar graphs from the paper showing the greatest distances between islands harboring iguanids and the nearest mainland (first graph) and the same graph for diofferent groups of terrestrial vertebrates.  The captions for the two graphs include this: “A) Distances between island and mainland for extant iguanid lizards and (B) distances for other proposed long-distance, overwater dispersal events in terrestrial vertebrates.”

Among iguanas, Brachylophus is The King, by far!:

Looking at all vertebrates, Brachylophuis still the king!

The asterisks in this graph indicate that stepping-stone dispersal is possible, with the distances for that scenario given by the white line across the two bars. The second longest dispersal, leaving out the asterisked animals involve Cadeidae, otherwise known as Cuban keel-headed worm lizards. They are found on Cuba but are said to have dispersed some 6000 km. This genus comprises two Cuban species and is enigmatic, but is thought to have rafted from the Mediterranean!

And so we have many instances of “founder-event speciation”: ancestors making it to distant islands and forming new species (in this case, four) after they land on islands or archipelagos.  Note that this differs from the old and largely discredited theory of “founder-EFFECT speciation,” which posited that weird genetic stuff happens on small founding populations that speeds up formation of new groups. That theory was promoted by, among others, Stephen Jay Gould.

Biogeography of the Canary Islands

April 29, 2022 • 2:00 pm

by Greg Mayer

Jerry has long insisted–quite rightly, to my mind– that biogeographic evidence was critical in leading Darwin to accepting descent with modification, and that to this day it is among the most pedagogically effective tools in teaching about evolution. Darwin’s visit to the Galapagos Islands is the most heralded of his encounters with island biota, and I would argue that his visit to the Falkland Islands was also quite formative of his views. It is less well known that HMS Beagle also called at Tenerife in the Canary islands but that, much to Darwin’s disappointment– for he had long wished to visit the island– the crew was not allowed to land, due to a cholera scare. Despite Darwin’s disappointment, many of the phenomena of island life that so impressed him can be observed in the biota of the Canaries; and we can look forward to Jerry’s return, when he’ll be able to share more photographs of the islands and their biota.

Frontispiece of Bannerman and Bannerman, volume 2, on the birds of the Madeiran archipelago. There are two similar species in the Canaries.

As Jerry has already noted, these are volcanic islands which emerged from the sea and have never been connected to the African mainland to the east. They are thus, biogeographically speaking, oceanic islands: they have received their biota via what Darwin called occasional means of transport— floating, flying, or swimming. The vertebrate fauna of the islands thus consists of birds, bats, lizards, a few now extinct rats/mice, and a single species of shrew. There are no native large land mammals, no snakes, and no amphibians. Endemism (species found only in the Canaries) is quite high, especially among the lizards and land mammals.

The lizards include skinks (Chalcides), geckos (Tarentola), and “true lizards” (Gallotia; they are in the family Lacertidae, lacerta meaning lizard in Latin, and thus are the “true” lizards). All of these have their closest relatives in North Africa and the Mediterranean region, from whence they came. The most interesting lizards by far are those of the genus Gallotia.

The giant lizard of Roque Salmor, from Boulenger (1891).

This genus is endemic to the Canaries, and is found throughout the archipelago. Some species/populations are endangered or already extinct. There can be up to three species on an island– small, medium, and large– thus coming closest of any to an adaptive radiation; most endemism in the Canaries is of representative forms on each island, rather than a genuine splitting of lineages leading to multiple related species sharing the same island (as Darwin’s finches in the Galapagos do).

The Canaries fall in the Palearctic zoogeographic region, the affinities of most of their fauna being with North Africa and Europe. The Mediterranean Sea is only a partial zoogeographic barrier, and North Africa lies in the Palearctic region (which extends to Japan and Siberia), rather than the Ethiopian region, which comprises sub-Saharan Africa. Within the Canaries, there’s quite a bit of ecological and climatic variation. The eastern islands are dry and desert-like, while the western islands are wetter, with a vegetational zonation of scrub on the coasts with pine forest and the distinctive laurel forest at higher elevations, with an alpine scrub at the very highest elevations.

The following is an annotated bibliography for those who might wish to learn more about the Canaries. It is somewhat idiosyncratic, reflecting my own interests and what I happen to have in my own library, rather than any comprehensive review of the literature.

Books for a broad audience

Bannerman, D.A. and  W.M. Bannerman. 1963-1968. Birds of the Atlantic Islands. 4 vols. Oliver and Boyd, Edinburgh. It’s been a while since I’ve looked at a copy, but this is a classic, with some beautiful plates and a comprehensive account of the avifauna as known at the time. The Canaries are volume  1.

Bowler, J. 2018. Wildlife of Madeira and the Canary Islands. Princeton University Press, Princeton, NJ. The only general guide to the Canaries I’ve seen, but fairly disappointing. The field guide illustrations are photographs, many of which seem inadequate for identification; scientific names are relegated to an appendix; the author misunderstands concepts of island biology; and there is no overall appreciation of the islands’ biodiversity and its origins. Nonetheless, I think it is a must have if you ever make a visit.

Garcia-del-Rey, E. 2018. Birds of the Canary Islands. Christopher Helm, London. I have not seen a copy of this, but the available extracts online show it to be a well-done modern bird guide by a Canarian with fine illustrations. Although general European field guides sometimes include the Canaries, you’d want to have this book with you if visiting.

More technical articles

Most of these, depending on your library access, will be paywalled. Discreet inquiry may yield a copy.

Böhme, W., and R. Hutterer, eds. 1985. Ergebnisse des 1. Symposiums “Herpetologia Canariensis.” Bonner Zoologische Beitrage 36(3-4): 233-606. Zoologischen Forschunginstitut und Museum Alexander Koenig, Bonn. (Biodiversity Heritage Library) A collection of papers in German, Spanish, and English that focuses on herps, but has more general articles as well. This was published shortly after some of the exciting discoveries of living giant lizards; see Maca-Meyer et al. (2003) for some more recent work.

Boulenger, G.A. 1891. On Simonyi’s Lizard, Lacerta simonyi. Proceedings of the Zoological Society of London 1891(1):201-202. (Biodiversity Heritage Library) A technical description, with illustrations, of a specimen that had been in the London Zoo.

Grant, P.R. 1979a. Ecological and morphological variation of Canary Island blue tits, Parus caeruleus (Aves: Paridae). Biological Journal of the Linnean Society 11:103-129. On some of the Canaries, blue tits, in the absence of the pine-dwelling coal tit, have shifted into pine forest, and their beaks have become adapted to this foraging substrate.

Grant, P.R. 1979b. Evolution of the chaffinch, Fringilla coelebs, on the Atlantic Islands. Biological Journal of the Linnean Society 11:301-332. Presents morphological evidence for ecological character displacement between common and blue chaffinches in the Canary Islands, and that Azores common chaffinches have evolved a more generalized, intermediate, morphology.

Illera, J.C., J.C. Rando, D.S. Richardson, and B.C. Emerson. 2012. Age, origins and extinctions of the avifauna of Macaronesia: a synthesis of phylogenetic and fossil information. Quaternary Science Reviews 50:14-22. Reviews recent molecular phylogenetic work on the birds of these islands, finding that most extant birds are recent (<4 Mya) colonists, even though the islands are much older (ca. 30 Mya).

Lack, D. and H.N. Southern. 1949. Birds on Tenerife. Ibis 91:607-626. A classic paper pointing out many interesting problems of ecomorphological adaptation and distribution, particularly in chaffinches and tits, which have been studied by later workers, e.g. Grant (1979a,b). Less technical and quite readable.

Maca-Meyer, N., S. Carranza, J.C. Rando, E.N. Arnold, and V.M. Cabrera. 2003. Status and relationships of the extinct giant Canary Island lizard Gallotia goliath (Reptilia: Lacertidae), assessed using ancient mtDNA from its mummified remains. Biological Journal of the Linnean Society 80: 659-670. Uses ancient DNA to investigate the relationships of the Canary-endemic genus Gallotia.