A comma before "either-or" construction

Can you please tell me whether it is correct or not to insert a comma before either in this sentence?

"Finally, the populations of Russia and Turkey did not show any alterations (,) either positive or negative."

I am neither a native speaker nor am I savvy in grammar. Hereby, I want to apologize beforehand, if the question will seem a little dumb. Thanks in advance!

Solution 1:

I think it is correct to put a comma there. The comma separates Finally, the populations of Russia and Turkey did not show any alterations, which is the main clause, from either positive or negative, which is a non-defining relative clause (which adds extra information but can be removed without losing the meaning of the sentence).

Quoting from https://www.ef.co.uk/english-resources/english-grammar/non-defining-relative-clauses/,

Commas or parentheses are always used to separate non-defining relative clauses from the rest of the sentence.

Solution 2:

The comma is correct. It signals a parenthetical (nonessential or nonrestrictive) element.

That means you can leave off either positive or negative without affecting the meaning of the sentence; while the phrase drives home the point, it doesn't need to be there:

Finally, the populations of Russia and Turkey did not show any alterations.

These alterations, had they happened, would have been positive or negative, and we can presumably infer this from context without the aid of the phrase.

If it helps your understanding, you can consider the phrase to be (in traditional grammar terms) a reduced relative clause. The full relative clause would be reduced from something like:

Finally, the populations of Russia and Turkey did not show any alterations (which would have been either positive or negative).*

It is reduced by omitting the relative pronoun (which) and the to be verb (would have been), leaving the adjectives behind:

Finally, the populations of Russia and Turkey did not show any alterations (either positive or negative).

At What is a noun modifying clause? English and Language Usage's "resident" authority, linguist John Lawler, applies the linguistics term whiz-deletion to describe the concept:

The rule called Whiz-Deletion by linguists (from the fact that it deletes a Wh-word plus a form of be, quite often is; a monosyllabic variant of "Wh-is deletion"), when applied to a relative clause, creates a bare verb phrase without a tensed verb, but with whatever is left after the deletion. Any phrase of more than one word simply goes after the noun it modifies. . . . It has long been suspected that all attributive adjectives . . . are the result of reduction of relative clauses.

* _{Note that this whole exercise would be easier with an example in the positive (e.g. The populations showed alterations, either positive or negative reduced from The populations showed alterations, which were either positive or negative).}