What is a Biological Species?

We see different types of organisms all around us, and there are many, many more that are too small for us to see at all. What is it that makes one species distinct from another? Where do scientists draw the line?

Sexually Reproducing Organisms and Species Distinction

For organisms that reproduce by doing the wild thing, the distinction between one species and the next is quite clear. Sexually reproducing species include many of the living things that we see around us on a daily basis; human, mammals, birds, insects, plants, etc. For sexual reproducers, a species is a population of organisms that can mate and produce fertile offspring.

Ligers, Tigons and Mules, Oh My!

There are a few types of 'dead end' animals out there; novel organisms that produce sterile offspring, and therefore are not considered to be new species. The cross between a male donkey and a female horse is a mule, and mules are sterile. For other species, which have been shown to interbreed in captivity and very rarely produce fertile offspring, these offspring are still not necessarily considered as distinct species.

The captive breeding of lions with tigers results in sterile ligers and tigons that are almost always sterile. Although the offspring are occasionally fertile, in nature, lions and tigers exist in separate geographic locations and would never encounter each other in the wild. Oddball cases aside, the distinction between sexually reproducing species is, for the most part, easy to define.

Asexually Reproducing Organisms and Species Distinction

While it is relatively easy to distinguish different species of sexually reproducing organisms, it is much trickier to determine what a species is when attempting to classify asexual reproducers, such as bacteria. Scientists can't use the definition, "organisms that can breed and produce fertile offspring", since bacteria, and other asexual reproducers, multiply by cloning themselves. Barring mutation, and bizarre types of horizontal gene transfer, the offspring are genetically identical to the parent.

Classifying Genetically Promiscuous Bacteria

To make the classification of bacteria even more difficult, these little microbes have multiple ways of getting hold of genes that are not their own. Bacteria will share genes with each other or even pick up naked DNA from the environment. With all this gene transfer, how in the world do microbiologists define different species of microbes?

The answer is that scientists must resort to using clues that the bacteria provide, such as waste products of bacterial metabolism (the 'poop' that bacteria generate based on what they eat) or specific chemicals that are part of the structure of the bacterial cell. Defining species of bacteria really does come down to a whole lot of 'Sherlocking', and the boundaries between these microbes are shifting all the time.

Sources

Bauman, R. (2003) Microbiology. Pearson Benjamin Cummings.

Campbell, N. and Reece, J. (2002) Biology. Benjamin Cummings