In viruses and one-celled organisms, how do we tell the difference between a strain and a species?

At that level, where reproduction is asexual, species are generally distinguished by a specific characteristic or a set of characteristics. Any organism that does do have such characteristics would be excluded as a member of that species. Characteristics relevant in establishing the species for one-celled organisms are generally phylogenic in nature (those that demonstrate some evolutionary link with others in its genus). Viruses species, for which such links are very rare, are most often established by strictly phenetic (similar) characteristics, and as such, do not necessarily reflect any shared evolutionary history within a genus.

Strains are almost always artificially created variants of a particular species, which means that while they retain their standing as a member of a particular species, they vary in some way from that archetypal of the species, but not enough to be considered a separate species.

In one sense, strains are not unlike the various forms in which humans occur: Asians, Blacks, Caucasians, etc, all of which belong to the same species, Homo sapien. So, to ask how one would tell the difference between a strain and a species is similar to asking how you would tell the difference between an Asian and a H. sapien, although it is a good question, it is not a meaningful one.

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Originally posted by Minnesota:
So, to ask how one would tell the difference between a strain and a species is similar to asking how you would tell the difference between an Asian and a _H. sapien,_ although it is a good question, it is not a meaningful one.

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could you clarify that? it doesn't make sense to me. firstly, how to tell the difference between a strain and a species sounds like a perfectly meaningful question to me. secondly, asking what the difference between strain and species is not at all like asking what the difference is between asian and h. sapien, though you might say it is like asking what the difference is between a race and a species.

to answer the question...
there is no universally accepted definition of species simply because there is no definition that works in all cases. one of the most common definitions is that 2 organisms are members of the same species if they can interbreed. this definition obviously fails in the case of organisms that reproduce asexually, such as viruses.
earlier definitions were based on physical characteristics, but that sometimes fails because members of the same species can look very different (think of domestic dogs) especially if they aren't the same sex (think of many birds). this also causes problems, especially with certain plants, when the organism has several life-stages, like a butterfly.
one definition which works very well in most cases is the degree of genetic divergence. if two organisms have nearly identical DNA then they are considered the same species. as far as strains go, this definition would mean that diferent stains have specific differences in DNA, but not enough difference to go so far as calling them different species. This definition requires analyzing DNA, but i would imagine it would be the best to deal with viruses.

methos

Before I elaborate on my answer, I would like to reemphasize your point that while the ability to interbreed and produce viable off spring was, at one time, a fairly well accepted definition of a species, it has since been pretty well discarded in favor of more particular definitions, each constructed to best serve the discipline to which they are applied. One of the principle deviations in this regard was the abandonment of phenetic classification (where possible) and the adoption of phylogenetic analysis (usually expressed in cladistic terms). When you start talking about DNA characteristics as a means of establishing species groupings one must be VERY careful not to over simplify its use or overstate its value. So, when you say, "if two organisms have nearly identical DNA then they are considered the same species," you have immediately raised the question/problem of "Just How Nearly Identical" is near enough? It is true that all members of a species share near identical DNA, but such identities by themselves are rarely used to establish a species' identity.

On to my statement. The crux of it is that, as you, yourself, have alluded to in your example with DNA, "different stains have specific differences in DNA, but not enough difference to go so far as calling them different species." Whether it is DNA or some or mark that identifies a strain, those characteristics that identify it as a specific species must not be compromised. So the strains of a specific species, no matter how much they may vary among themselves, all retain their identities as members of that species. Just like humans, some of which are identifiable by their ethnic characteristics and therefore categorized into ethnic groups, all share those characteristics that identify them as H. sapiens. (And please, let's not open up the "race" can of worms. )

Therefore, when I said that it is not meaningful to ask what is the difference between a strain and a species, I was alluding to the fact that because strains are a category of species, as Asians are a category of humans, there is no "difference" between the the two, only a relationship of hierarchal inclusion.

i still differ with you on that last point. you cannot compare "asian" to "strain" or "H. Sapien" to "species". In both cases, the first is a specific example of something and the second is a category. and it is perfectly "meaningful" to ask what the difference between two categories in a hierarchy are, just as it is meaningful to ask what the difference between "species" "genus" "family" etc. is.

Methos

Please excuse my inadvertent misquoting of my own post, which may be the cause of this misunderstanding. In my first post I said,

So, to ask how one would tell the difference between a strain and a species is similar to asking how you would tell the difference between an Asian and a H. sapien, although it is a good question, it is not a meaningful one.

The point of bab's question was how does one tell the difference. This is what I was speaking to: not what is the difference between a species and a strain, but how does one tell the difference between the two. As I say, unfortunately in my last post I misspoke and said, "Therefore, when I said that it is not meaningful to ask what is the difference between a strain and a species. . . . This, of course changes the whole focus of my point, and even led me to offer up my irrelevant conclusion. So, taking me up on it, you are absolutely right in saying that "it is perfectly "meaningful" to ask what the difference between two categories in a hierarchy are, just as it is meaningful to ask what the difference between "species" "genus" "family" etc. is. Sorry I slipped up and caused the confusion.