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.

  • $\begingroup$ One issue is that I don't know what you mean by "historical momentum". Do you mean, when humans found the organisms in question? $\endgroup$ Jun 22, 2021 at 13:48
  • $\begingroup$ @MaximilianPress: I mean, e.g., that E. coli continues to be a species only because of its history as opposed to what would be a more balanced taxonomy given our current knowledge. $\endgroup$
    – Wrzlprmft
    Jun 22, 2021 at 15:33
  • $\begingroup$ Right- I talk about that specifically a little, below. I would then ask a different question: are species definitions themselves only still around because of historical momentum? Somewhat related: it might be helpful to read / learn more about classification, here is a video from Joe Felsenstein, old warhorse of computational phylogenetics, discussing this question (starts ~8 minutes): youtube.com/watch?v=PcD15i7yzJ8 $\endgroup$ Jun 22, 2021 at 16:12
  • $\begingroup$ are species definitions themselves only still around because of historical momentum? – To some extent sure, but then there is some merit to having a rough idea of how diverse a clade is by its level. Anyway, this is something I cannot change. $\endgroup$
    – Wrzlprmft
    Jun 23, 2021 at 15:12
  • $\begingroup$ Our exchange has served to confuse me further regarding your question. Is your argument, basically, that E. coli is more diverse than a species first described yesterday because the type strain for E. coli was described first and it has therefore undergone more evolution since then? (That doesn't make any sense to me, but I'm having trouble understanding any other meaning.) $\endgroup$ Jun 23, 2021 at 23:44


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