For sexually reproducing organisms, the scope of a species is somewhat fixed by reproductive compatibility. However, this all collapses for organisms that exclusively reproduce asexually. Here, my impression is that historical momentum is the primary driving factor of species scopes, i.e.: Clades continue to stay on the species level to maintain continuity, while they would be categorised with a higher or lower rank if clades had to be taxonomically ranked from scratch without respect to history.
For example, the species of Escherichia coli and Pseudomonas aeruginosa have been discovered (and thus have been made species) very early and now comprise a diversity of subspecies. In particular, the genus of Shigella appears to considerably overlap with E. coli or is even thought to be a subspecies of it.
I am here looking for references supporting (or refuting) my impression, for example:
- Has this ever been prominently noted?
- Are there any instances of species being split on account of being “too large” (as opposed to other miscategorisations)?
Background: Why I care
As long as the taxonomy is phyletic, what does it matter at what level clades are ranked? Right now, this affects me in several ways (assuming that my impression is correct):
The common way to present the composition of an ecological community is showing the abundances on a fixed taxonomic level. For example: “Our community contained 80 % Pseudomonas, 15 % Escherichia, and 5 % Bacillus.” and analogous graphical representations. Of course, I can deviate from this (e.g., by listing lower ranks for diverse clades), but then I have to justify it – and here the desired reference might help.
If differing subspecies of a “big” species are present in an ecological community, this may lead to a superficial underestimation of the community’s diversity. Mind superficial: I am aware that this does not affect any good quantification of diversity. However, it does affect superficial diversity as estimated from representations such as that mentioned in the previous point, and the resulting discrepancies between objective and subjective diversity often demand an explanation.
Biosafety classifications are often determined on the species level, in particular, if the subspecies is unknown. For example, even though P. aeruginosa lives on our skins, it automatically gets classified as risky on account of some black-sheep subspecies.