Humans like rules as a way of ordering the world into familiar and comfortable patterns. For naturalists, one of the basic rules is the concept of biological species, which forms the basis of modern biodiversity and conservation studies.

An iguana unlike any other, being a Galapagos island hybrid that's found a tree-climbing niche for itself

There are all sorts of definitions of this concept, but the simplest ones say, in effect, that organisms fall into recognizably similar groups, called species, and that organisms of one species cannot belong to another. Put simply, a species is a group of individuals that can breed together, while those that cannot breed together belong to different species.

For most of us, that is as far as we need to go, unless we are visiting one of those weird animal menageries that prides itself on the unusual. Confronted with a crazy creature that is a cross between a lion and a tiger, or between a horse and a zebra, simple species definition runs into trouble. If they can breed together, then they must be the same species, right? Wrong.

In light of this, the most widely used definition of a species is that it is a group of organisms that can breed together and produce fertile offspring. Thus, although they can interbreed, lions and tigers, or horses and zebras are different species because their offspring are infertile.

However, the rules that have been made to describe the world are imposed on a natural biodiversity that hasn't read the rule books. In the unnatural pressures imposed by captivity, animals that are clearly recognizable as separate species may attempt to, and even succeed in, mating together.

In the wild, these animals may live in different parts of the world, in different habitats, or have different behavioral cues and communication that another species would be unable to read. In captivity, however, those natural barriers are removed and mating becomes possible. From an environmental perspective, the results of such matings (typically infertile) are a distraction; they are unnatural, artificially bred individuals that should never be allowed to occupy space that could be better used to house breeding pairs of endangered species.

That would seem to imply that such crosses never happen in nature, but in fact, that is not true either.

Hybridization in the wild is very rare, but it does happen, particularly among certain groups of plants. In fact, plant breeders looking for higher crop yields often take advantage of the vigor exhibited by such plants.

Hybridization among mammals in the wild is especially rare, but among birds -- well, certain types of birds -- it occurs somewhat more commonly than among others. Waterfowl seem more prone than most, but even then the results are still very unusual, perhaps one in 100,000, or one in a million. Crosses between mallard and other common duck species are among the most frequently seen, but others happen too. Last winter, in Hokkaido, while searching through a flock of more than 30,000 white-fronted geese, I noticed an apparent hybrid between a white-fronted goose and an Aleutian Canada goose. Similarly, visitors to the famed crane gathering at Arasaki in Kyushu may, if they search really hard, find a hybrid between a common crane and a hooded crane among more than 10,000 other birds.

I recently came across a photograph of a "chessboard swan," a hybrid between a black swan and a mute swan that appeared in Germany in 1982, and was literally covered in black-and-white patches. That swan is a stark reminder that hybridization is more common among waterfowl than among all other groups of birds -- but that hybridization among the larger species in the wild is very rare, making that goose I saw in Hokkaido a very unusual individual indeed.

In captive collections, whooper swans have been known to hybridize with black, mute, trumpeter and whistling swans, and even with greylag and Canada geese. In the wild, though, they are only known to have hybridized a handful of times with mute swans in Europe, and once with a whistling swan in North America.

All this goes to show that if you look really hard for hybridization, you might find examples of it, but among wild creatures it is exceptional, and then only between closely related species. So don't go around looking for hybrids between foxes and badgers, or bulbuls and crows; you just won't find them.

I was unaware of there being hybrids between other animal groups until last spring, when I was watching reptiles on South Plaza Island in the Galapagos Islands. South Plaza is small, with many wildlife attractions, but the fact that it has both marine and land iguanas makes it very special.

Though clearly distantly related, the two iguanas are very different. The marine iguana is black with a short, squat face, long toes and sharp claws. It also has a very long tail, the same length as its body and flattened from side to side, making it an ideal oar for sculling with as it swims and dives for its favourite food -- marine algae. The land iguana is larger, heavier, and has a more massive head, and it ranges in color from pale yellowish-sand to orange. It has a shorter, rounded tail, shorter toes, and is stockier and more massive.

This iguana likes the shade of the Opuntia cactus trees. The males seem to stake out their territories beneath these trees, and there they do their slow reptile thing. They wait for fruit to fall and for females to wander by. They are not particularly fussy. If a fleshy cactus pad or branch falls instead of a fruit, they just eat that instead.

Equally, it seems they are not too fussy about females. After the most recent El Nino event, when marine iguana numbers were severely depressed, they perhaps wandered farther than normal from their shoreline habitat, and perhaps the females had less energy to avoid the advances of male land iguanas, whose approach to sex is both simplistic and aggressive. There is little or no courtship. Rival males outdo each other in size, and by being bigger they are better able to chase and hold down any hapless female that cannot flee quickly enough.

While the marine iguana is at home in the ocean, and the land iguana is at home in the drier areas of certain islands, the accompanying picture shows an unusual sight: an iguana up an Opuntia tree. Tree-climbing is most definitely out of the question for a marine iguana, and while land iguanas will prop themselves up vertically to reach pods and fruit, climbing is not for them. The arboreal creature in question was dark slate-grey, with pale whitish bars on its neck and throat.

In coloration it was more like a marine iguana; in shape it was more like a land iguana, but with the marine iguana's longer toes and claws. Clearly a mix-up both anatomically and behaviourally, and I count myself amazingly fortunate to have met this real oddity of nature. Biologists working in the Galapagos National Park speculate that a typically aggressive male land iguana caught and mated with a female marine iguana, and of the many eggs she may have produced, only this one survived.

Not appropriately adapted for a marine life and no doubt unable to compete either for food or a mate with the larger land iguanas, this hybrid has taken a remarkable step -- it has found a niche for itself up in the trees. Lots of "ifs" here, but: If it could find sufficient food in that habitat, if others of its kind were produced, and if they were themselves fertile and able to breed, then here is just one way in which a new species might appear, as a seemingly doomed hybrid finds a unique ecological niche for itself.

Don't hold your breath, though. The next hybrid of this type might not be born during the existing one's lifetime, and even if it were, it might not be of the compatible sex. Then again, they might simply fail to meet. That, however, is the nature of nature: chance heaped on chance over very long periods of time. I won't be around long enough to know, but it's fun to speculate that one day there might be a third iguana species in the Galapagos -- the tree iguana.