How is it determined how many asexual species there are, both plants and microbes, and which specific species each individual belongs to? With plants it seems obvious that looking at it would suffice, but is there anything more rigorous than that? And how does it work for single-celled critters that reproduce by fission? Can getting an episome (or other plasmid) change an individual’s species?
To make it clear: From what little I know, it seems that two sexual individuals are in the same species if they can successfully produce fertile offspring together. Asexuality seems to screw with that notion quite comprehensively, and promiscuous sharing of plasmids takes it and bends it over the desk.
Even with sexual creatures, the notion isn’t entirely well-formed. For example, no two men are going to be able to successfully produce fertile offspring together, despite being in the same species (as a particular pedantic special case, I can’t produce fertile offspring with myself, despite being of the same species). Humans with various disorders might be infertile, yet still be considered part of the human species. More subtly and perhaps troublingly, it’s not hard to imagine a chain of creatures, A, B, C, …, Z, such that A can produce fertile offspring with B, B can produce fertile offspring with C, etc., all the way down but A cannot produce fertile offspring with Z. Thus, using this definition, the property of species-sharing would not be transitive, which is strongly counter-intuitive. But, actually, that’s pretty reasonable. (If you think about it, such soritical species-sharing chains are pretty much guaranteed to exist between any two animals with a common ancestor, on a reasonable understanding of species-sharing; there’s no particular node along the way where a child is suddenly of different species from its parent).
The truth is that species is really a more fuzzy notion than it’s often presented as; at any rate, what’s really important for most purposes is the evolutionary branch on which an organism sits, and this is what we try to capture with the notion of species. We could say two organisms have distinct species if their evolutionary branches have diverged “significantly”, where “significantly” means something like “with little chance of merging back together any time soon”. But really pinning it down with a single concrete definition is something of a thorny problem (and, I would say, not particularly important, as long as one has a not-perfectly-concrete-but-generally-well-motivated-and-usable idea of what one intends the term to capture).
Now, I’ll sit back and let the actual experts tear me to shreds, or at least tell you what’s done in practice for classifying the species of asexual creatures/etc.
That’s nothing to do with species definition, really - and inability of specific individuals to reproduce (which you go on to mention) doesn’t really have anything to do with it either.
Real-world examples of this phenomenon have been observed - they’re called Ring Species.
I think the problem is not that the notion is fuzzy, but rather that the domain it tries to define is fuzzy. If divergence events all happened simultaneously across the whole world, and genetic changes happened at a constant global rate, then we would be able to put everything in very neat boxes, but we can’t, because it doesn’t.
Yeah, sorry, I didn’t mean to imply those were significant difficulties with the very concept of species-sharing; they were just minor but immediate problems with accepting “can have fertile offspring” as the end-all, be-all definition of such.
I suppose I said “the notion isn’t well-formed” when I meant to go back and edit it at some point to say “the often stated ‘definition’ isn’t coherent” or something like that. As you correctly note, nothing I mentioned should be taken as significantly troubling for the very notion of species until the transitivity problem.
In bacteria, there’s a somewhat (OK, entirely) arbitrary standard that if two cells share 80% or more of their DNA, they’re in the same species. There are, of course, exceptions to the rule.
Asexual species throw a good many wrenches into species concepts. Basically, what we wind up with is two “types” of species concepts: the “reproductive unit” and the “taxonomic unit”.
Within the “reproductive unit” category, we have 4 concepts, each of which rely on different criteria to establish a definition:
[ul]
[li] Reproductive: the Biological Species Concept[/li][li] Ecological: the Ecological Species Concept[/li][li] Process: the Evolutionary Species Concept[/li][li] Historical: the Paleontological Species Concept[/li][/ul]
Gahh…my above got posted before I was done, and the edit time ran out before I could modify it.
Anyway, the primary “taxonomic unit” species concept is the Morphological Species Concept, wherein species are grouped together if they look similar (this is what Linneaeus used, essentially).
Asexual organisms can fall under the pervue of either the Ecological Species Concept, Evolutionary Species Concept, or the Morphological Species Concept, depending on who’s doing the classifying. Secondarily asexual lineages can be grouped using the Biological Species Concept.
