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Question: Why can't they split up the animal kingdom into more groups?
As it happens, they can. If a very unusual and different kind of animal were found - say, in the ocean abyss - then the scientists who organize living things, who are called taxonomists, might need to make a new branch on the animal kingdom's limb of the tree of life, to accommodate a new named group, called a taxon. This does not happen very often as most places on Earth have been fairly well studied. New fossils more often call for this kind of major alteration, requiring a new genus or family to be added to the system invented by the Swedish botanist Carolus Linnaeus. Taxa (named groups) are rank ordered as follows:
At the other end, at the tips of the branches of the tree of life, specialists studying groups of animals, often in remote rarely explored areas, sometimes add new twigs. This would generally only add a new species or subspecies to our register of animals. The subspecies is the least officially recognized taxon, even though races and varieties are often used informally to simplify discussion. If it could be shown in further studies that this new kind is capable of successfully interbreeding with members of a recognized species, then its specific rank would be denied or removed, but it might still count as a subspecies. Kinds that can interbreed to make fertile offspring are reckoned to be of the same species. So, since mules are infertile, we know that asses and horses are really different species. For this reason all of today's people are considered to make up one species, 'sapiens' in only one living genus, 'Homo'.
A factor that comes in here as well is the philosophical viewpoint of the taxonomist. Some taxonomists are 'splitters', who enjoy naming new kinds on the slightest evidence, while others are 'lumpers', who try their best not to have to make up a new taxon. These are the radicals and conservatives of biological systematics (the field that studies taxonomy). If the breeding test is not possible, then, in these times of high technology, a splitter might even be able to look at the details of the genetic material (DNA) in order to find evidence of difference to pose against the arguments of a lumper. This is because the genes are considered to be the place where informational differences between kinds are stored, to be passed on to offspring. Of course, breeding experiments would settle the argument definitively if the new proposal were to be a species. So, as in all human occupations, we find that judgment comes into play in biological taxonomy as well.
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