February 12, 2010

For Darwin Day: The Biogeography of Darwin's Gourd

ResearchBlogging.orgIn September of 1835, Charles Darwin was visiting an island of Floreana, one of the smaller islands in the Galapagos archipelago where he came across crawling beds of Sicyos villosus, a fairly typical member of the squashes and cucumbers (Cucurbitaceae). Darwin noted that the cucurbit was "injurious" to the surrounding vegetation, referring to its prolific takeover of the landscape nearby.


Darwin sent a sample of S. villosus (pictured above) back to Great Britain along with 209 other plants from the Galapagos, where Joseph Hooker described it and since then, it's been sitting in one receptacle or another for the past century and a half. The desiccated cucurbit was recently pulled out of storage at the University of Cambridge for a bit of molecular analysis. Why this plant in particular? S. villosus hasn't been found in the Galapagos islands or anywhere else since Darwin plucked it from those infectious beds nearly 175 years ago.

The analysis of the cucurbit's DNA, extracted from the seed samples taken by Darwin, revealed that S. villosus is closest in relation to cucurbits in North America and Mexico. The species probably diverged roughly 4 mya, when the Galapagos were still geologically young. Dispersal was not human in origin, meaning long distance from the mainland, potentially from its spiny fruits stuck to birds, the authors suggest.

As far as why it disappeared and so quickly, there's no way of knowing for sure, but it's likely that the settlement of the island was to blame. Cucurbit-specific viruses could have been carried from the mainland and transferred to the natives, or the plant could have been grazed to extinction. When Darwin was on the island in 1835, there were already 2,000 head of cattle and later in the 19th century, it was reported that agriculture had wrecked much of the ecology.

These exact processes - divergence via evolution, dispersal and extinction - were described and linked in The Origin of Species, bolstered by experiments that Darwin himself carried out; they also happen to be the central tenets of modern biogeography.

He called it Geographical Distribution in Origin, and there are two chapters toward the end that delve into his thoughts regarding these tenets, particularly dispersal:

If the difficulties be not insuperable in admitting that in the long course of time the individuals of the same species, and likewise of allied species, have proceeded from some one source; then I think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. We can thus understand the localisation of sub-genera, genera, and families; and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a manner linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent.

John C. Briggs wrote a paper last year on Darwin's contribution to biogeography, and how his ideas on dispersal - a cornerstone of biogeography and ecology - were challenged in the 1970's.

Darwin was necessarily focused on dispersal in his exploration of species. The ocean, in particular seemed impassable, and yet somehow animals and plants were able to transverse these inhospitable environments and establish themselves and their descendants on far away land masses like islands. He immersed seeds in seawater and found that 14% of them were still viable after 28 days of exposure. He found seeds in bird pellets and on muddy bird feet. In the soil caked to the leg of partridge long dead he found the seeds of 82 different plants. They clung to anything they could, even turning up in the dung of flying insects like locusts. Darwin was curious about how land snails turned up on islands so far away from continental land masses, so he tested how resilient their hibernation techniques would be on an oceanic voyage, surmising that their operculum could be a sufficient barrier to immersion for up to a week. It was clear to him that species originated from central points and found ways to transverse environmental barriers via their own dispersal abilities and subsequently diversify. For the next century or so, this was plainly accepted.

Following the development of plate tectonic theory in the late 1960's, there was a push to bring vicariance, the idea that geological boundaries have caused allopatric speciation, to the forefront of biogeography. Some even went so far as to discount dispersal as the main shaper of diversity and distribution:

From Croizat et al. in 1974:

We reject the Darwinian concept of the center of origin and its corollary, the dispersal of species, as a conceptual model of general applicability in historical biogeography.

Now, vicariance was always acknowledged as a potential factor, but it was always considered a relatively minor one. The idea had its proponents and its proponents had - what else - a prioi assumptions. According to Briggs:

Thus, vicarianists came to rely on a model that did not permit them to consider all aspects of the distribution puzzle, and their work was therefore handicapped. As time went on and more works indicating dispersal were published, this handicap became more obvious and contributed to a gradual loss of confidence in the method.

The ease of molecular analysis in the 1990's vindicated Darwin's initial ideas regarding evolution and dispersal, according to Briggs. S. villosus tells the story of its origins, its journey and its life through its DNA and that story shows us that it ended up on Floreana as a stowaway on an unsuspecting seabird, not as splintered remnant from the remains of supercontinents. So it was a quiet battle for footing, virtually unknown outside of this specific scientific community, but proof again of Darwin's extraordinary problem solving abilities and intuition.

Sebastian, P., Schaefer, H., & Renner, S. (2010). Darwin’s Galapagos gourd: providing new insights 175 years after his visit Journal of Biogeography DOI: 10.1111/j.1365-2699.2010.02270.x

Briggs, J. (2009). Darwin’s biogeography Journal of Biogeography, 36 (6), 1011-1017 DOI: 10.1111/j.1365-2699.2008.02076.x

Briggs, John C. 1987 Biogeography and plate tectonics / J.C. Briggs Elsevier ; Distributors for the U.S. and Canada, Elsevier Science Pub. Co., Amsterdam ; New York : New York, NY, U.S.A.

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