Defining Species

Biologists talk about species all the time. (Incidentally, that word is both the singular and the plural. There is no such word as "specie.") Strange as it may seem, however, that word is very difficult to define in a precise and consistently useful way.

It's not that we don't have an official definition for species. We do. The textbook definition of species is that it is a group of organisms which can interbreed and produce fertile offspring. So humans of all races are the same species, but horses and donkeys aren't. Any normal, healthy male and female human can mate and produce offspring which are not only perfectly healthy, but are also perfectly capable of producing eggs or sperm and interbreeding with the rest of the species. But while a horse and donkey can mate, and they produce a very healthy offspring (which we call a mule) that mule is sterile.

Easy, right? Well…no. It's not always that straightforward. There are instances in the world where the lines between species are very fuzzy. There are groups which can breed and produce fertile offspring, but don't do so unless you force them to. There are examples where a widespread organism type has continuous fertile interbreeding between neighbor populations throughout its range, but in which populations which are distant from each other are unable to mate, or if they mate, are unable to produce fertile offspring. Is this all one species or not? The answer is pretty arbitrary.

Then there are plants, which are much more tolerant of reproductive shenanigans than animals are. Plants do all sorts of creative things like hybridize and reproduce asexually until an accidental chromosome doubling enables them to breed normally again. Bingo. Instant new species. This is where wheat came from. Our wheat is actually the product of natural hybridizations among three different grass species. These ancestral species are still around, so we can do genetic studies and establish without doubt that this is the correct history for wheat.

There is a very important lesson to be learned from the problem of defining the word "species." Humans are driven to categorize and classify everything. We do it with everything in our lives. Unfortunately for our peace of mind, nature doesn't always go along with the game. The reality is that there are many "in between" situations when it comes to the distinctions between species.

As if this problem weren't difficult enough with living species, it becomes truly overwhelming when we try to divide similar fossils into distinct species because, of course, we don't have any information about who could breed with who else. Paleontologists are forced to make species decisions based simply on the physical features of the fossil creatures they find. Naturally, since paleontologists are human beings, this leads to all kinds of disagreements. All species have diversity, and you have to decide just how much difference is enough to place two fossils into two different species. Paleontologists even have special terms for different approaches to species decisions. A "splitter" tends to take any little difference as enough to place a fossil into a brand new species, so they tend to want to name many, many species. A "lumper" tends to ignore a lot of differences, and to put many fossils together into the same species, so they tend to name relatively few species. Most agree that the middle path is probably the best approach.

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Updated 25 September 2004