How are asexual animals assigned a species?

Question

Organisms that reproduce sexually have a few different definitions of a species (whether they can produce viable offspring, behavioral/niche compatability, whatever). But how do scientists/taxonomists decide which organisms belong to a specific species when they reproduce soley by asexual means, like bacteria? And especially since they are constantly changing and adapting to their environment, how do they know that any two bacteria belong to the same/different species?

Answer 1

They used to classify bacteria by visible and environmental characteristics.   Gram positive, gram negative, the presence of absence of cilia or flagella, bacilli, cocci, etc.  They would also classify them by what type of environment they lived in.  

Now that we have more advanced genetic testing,  scientists still do some of that, but it's proven to be inaccurate in some cases.  Bacteria that look a lot alike may not be related.  They instead use genetic similarity to determine evolutionary relationships.  A chart of this is called a cladogram.  In order to determine how closely the two species are related, the rRNA sequences are aligned and the percentage of similarity is calculated.  

Most bacteria classification and names are still determined by visible and environmental characteristics.  In cases where this places bacteria in a group they are not closely related to, the bacteria are renamed and placed in a different group.

The definition of a species is determined by the organisms within it having a certain percentage of rRNA identical.  A species must also share the same cell structure and metabolism.  Groups of bacteria that differ from a species but do not differ enough to be considered a separate species are considered a sub species.

Answer 2

The genetic species concept seems to be the one that is taught to innocent young students who don't know any better. However, it is useful for less than one percent of species. It is useless for asexual and extinct organisms.

Before that concept became popular, we used a morphological specied concept. If two animals were more different than two litter-mates could be, they were considered different species.

For my money, the best species concept was stated by C. Tate Regan, an ichthyologist. He said, "A species is whatever a competent zoologist working with the group says is a species." It sounds flippant but it works quite well.