Why aren't there monotypic subspecies?

Nature Talk
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Or, what is a subspecies anyway?

I heard from this thread that it’s not normal practice (in the scientific community?) to have monotypic subspecies. Why is that?

I’d thought that subspecies were defined as a genetically distinct population that was still close enough to other populations (e.g. defined by free ability to interbreed when ranges overlap) that they could be considered the same species. By that definition there’s no reason why there couldn’t be one such population, with all other populations not being distinct from it or one another. So, clearly my definition’s wrong.

I know nothing and welcome corrections.

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A different subspecies is defined in contrast to the original species. And then the original species is termed the “nominate subspecies” like this:

If we discover that Homo sapiens has a subspecies called “inatensis” which all live on the moon, not on Earth,

we would call the original Earth species Homo sapiens sapiens, and the moon subspecies Homo sapiens inatensis.

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Maybe part of your confusion is that if a species has subspecies, all populations must belong to a subspecies (e.g., you can’t have populations that aren’t in one). So you couldn’t have one distinctive subspecies, and then a bunch of populations not in any subspecies. By definition the assorted other populations with have to comprise at least one additional subspecies.

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A different subspecies is defined in contrast to the original species

if a species has subspecies, all populations must belong to a subspecies (e.g., you can’t have populations that aren’t in one)

Yep, this was my confusion exactly. Thank you!

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There are unusual cases where some populations within a species have not been assigned to one of two or more subspecies that have been previously described. Presumably these populations could represent an undescribed subspecies but for the moment are defined at only the species level. (Some populations of the chipmunk Neotamias canipes are an example.)

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I think I got my misunderstanding from trying to figure out what the subspecies of Cervus canadensis were a while back. I think somewhere along the way I read something about the Roosevelt and Tule that indicated they were the only “true” North American subspecies and the rest were ecotypes, and mistakenly assumed that meant that the others were just not subspecies at all. But it sounds like what actually happened is that they were all placed in the nominate subspecies together. (Which explains the taxonomy on iNat, too!)

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The North American elk, Cervus canadensis, has seen some changes in taxonomy including at the subspecies level. For example, C. c. nelsoni has been lumped under C. c. canadensis by some mammalogists but still accepted by others. I’m not sure what iNat is recognizing right now but I’ve been using C. c. canadensis for elk in the U.S. Southwest.

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A monotypic subspecies would be like others said redundant (Homo sapiens sapiens) and unecessary to name as such, and say a second subspecies was discovered it would no longer be monotypic even if the second went extinct.

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