On this forum, and in biology generally, it is common to see the phrase “natural group” as a synonym for “clade.” Both of these are used to mean “a common ancestor and all of its descendants” with the assumption that every living member of that group is more closely related to each other than they are to any organism outside the group. I’ve studied and taught evolutionary biology, so I fully understand why we focus on monophyletic groups, the limits of that approach, etc. I think it is reasonable that iNat and most taxonomists take this approach. My quibble is with the phrasing “natural group” to mean only this.
Species that photosynthesize are not a monophyletic group, but they are a group defined by a natural characteristic. The same could be said of animals of Australia, RNA viruses, and innumerable other groups that are natural but not clades.
Which raises the question, when did we start using the phrase “natural group” to mean clade (or sometimes the modern or living members of clades), and why? I have an unreasonable dislike of this phrasing, and so had hoped it was a relatively recent invention and would eventually go away.
Alas. A quick search of Google Scholar shows that the phrase was used to mean a proper taxonomic group at least as far back as 1818, before we had the concept of evolution or monophyly. And that’s just in English.
Which leaves me to wonder, did Linnaeus himself, or someone even earlier, use this phrase in Swedish or Latin, Ancient Greek, to mean what we now call a taxonomic group? It seems “clade” is a junior synonym of “natural group.” I wonder if the word “taxonomy” (first documented in 1819, according to the Oxford English Dictionary) is also?
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That’s my impression of how it’s been used, at least by taxonomists prior to widespread application of monophyly to taxonomy. Taxonomists have generally wanted to avoid “unnatural” taxa - groups containing “unrelated” organisms, or organisms less related to each other than to adjacent taxa - by whatever tools they had to assess that at the time. And post-Darwin, relatedness increasingly implied genetic and evolutionary relatedness, initially inferred by proxy using morphology and anatomy.
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I wouldn’t consider “natural group” and “clade” to be synonyms.
As you noted, “natural group” long predates the evolutionary synthesis; its antonym, I believe, is “artificial group”. The distinction, as I would understand it, is that the members of an artificial group are united on the basis of one or perhaps a few characters while being otherwise unlike in essence; in a natural group, the members are similar to one another in multiple ways that clearly distinguish them from non-members. Linnaeus’ sexual system for flowering plants, based strictly on the number and arrangement of reproductive organs, created artificial groups.
This maps very nicely onto cladistics, which replaces metaphysical grappling with essence and quiddities with shared descent. Now natural groups are understood to be united by a collection of synapomorphies, while artificial groups are homoplasious.
If I were deploying the term “natural group” today, I would use it to mean a clade where the members share observable and/or functionally important synapomorphies. (I believe some would also argue that certain paraphyletic groups deserve the label, if they are united by a significant group of symplesiomorphies which have vanished from the excluded nested clade, without extant intermediate forms.)
“Natural group”, in this sense, is of course a much more slippery and subjective notion than “clade”. Nonetheless, if I were considering whether a particular clade should have name and rank, and which rank, that’s the direction in which I would look.
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Clades include organisms of all systematic ranks and may refer to groups of organisms described in classification systems within different systematic categories (or parts of them) – therefore, in my opinion, organisms belonging to a given clade should always be called a “group”.
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Historically they may not have been synonyms. In modern taxonomy, I think they are used almost interchangeably. The one difference in usage I’ve noticed is that that “natural group” is sometimes used to refer to just the living member of a clade. But I’ve certainly heard “natural group” used (both by taxonomists and on this forum) to describe even those clades whose unifying features are obvious only to experts.
I think already Aristoteles was grouping organisms by common characters – without any reference to common descent, of course.
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