I have wondered for a while about how species is defined for a purely asexual organism such as a bacteria.

 

But I particularly wonder how such organisms remain grouped into different species. Surely if they are simply dividing and random mutations are happening all the time, then you would end up with a continuous range of types with no sharp divisions between them. Is there always some limited genetic exchange? Is it possible that early in life history there were no divisions between different types/species?

With prokaryotes there is extensive horizontal gene transfer, which complicates matter quite a bit. Generally the traditional species concept (as coined in the 40s) is not really useful. Instead other parameters based on DNA similarities are used. Traditional a 70% DNA-DNA hybridization has been used to classify prokaryotes, but nowadays phylogenetic trees based on 16srRNA sequences (or other conserved regions) are more common. With more and more whole-genome sequences it is probably only a matter of time until more global approaches are being used.

The usual comment from my side on this issue is that species are always somewhat arbitrary delimiters that are useful in certain (many) situations, but are not a perfect reflection of nature under all circumstances.

Thanks for the reply.

 

With prokaryotes there is extensive horizontal gene transfer, which complicates matter quite a bit.

 

What does this mean?

Horizontal gene transfer allows organisms to exchange genetic information, in some cases quite sizable amounts. If you happen to use these genes to infer phylogeny you would group them together, although they may be quite distant, for example.