Re: What is the definition of a species for an asexually reproducing organism?

Hi Sandra,

I once heard someone define biology as "the science where every rule has an exception". You are quite right that biology textbooks define species using the concept of "reproductive isolation". Tigers and lions for instance can interbreed, but produce infertile offspring. However, as this excellent article points out, this definition is rather idealised as it assumes sexual reproduction from the outset.

It is most certainly true that bacteria of different genera can exchange genetic information - this can occur because many types of bacteria contain the genes required to build a tube (called a pilus) through which DNA can be threaded and inserted into another cell. These genes are themselves mobile between species. So perhaps the bacteria are a good starting point to answer your question.

Microbiologists have traditionally classified bacteria according to their physical characteristics and metabolic capabilities. For instance, a procedure called Gram staining will differentiate bacteria into 2 types, depending on what kind of cell wall they possess. Under the microscope, bacteria appear to have different shapes - for instance spherical (called coccoid), elongated, spiral and so on, which provide another level of differentiation. Metabolic capabilities are then examined - does it require oxygen? Does it produce gas and if so which one? Does it grow on various types of sugar, or starch, or other carbohydrates? What trace elements does it require? By applying a large battery of such tests, we can start to assemble groups of microorganisms with similar characteristics.

In more recent times, we use the techniques of molecular biology to define species of bacteria (and indeed all organisms). All cells contain machinery for synthesising proteins, named ribosomes. One component of the ribosome is a nucleic acid called ribosomal RNA. It turns out that the sequence of this molecule is very highly conserved between all organisms (because it has to perform a similar task in all cells), but there are regions of the molecule that are more varied. If you examine these sequences from several organisms, you can calculate the differences among them and use that to classify the organisms into groups. More recently still, we have the complete genome sequences of many organisms, so you might also define a species as "a group of organisms with the same genome sequence."

If this interests you and you want more technical details, have a look at the Ribosomal Database Project, where ribosomal RNA sequences from many bacterial species are stored and analysed. You may also want to search the Web for "Carl Woese" (start with this link). He is the man who pioneered ribosomal RNA analysis and discovered a whole new domain of life (the Archaea) in the process.