> In this sense, the development of word-senses after word-origination and with respect to developments in other parts of the language would be a fine argument.

That is true, but your arguments above don't speak to this point. Those are just random flights of fancy dedicated to defending a false idea. For example, snake is older than the phenomenon in question, always started with sn-, and hasn't changed relevantly in meaning.

By contrast, snack is generally believed to be an example of the sn-/nose correspondence. It's not because the concept was ever, or is now, related to noses, smells, or flavor. Rather, the connection is supposed to be that the mouth was viewed as part of the nose. (This makes a bit more sense as applied to animals than to humans.)

A basic prerequisite to arguing that snob developed the sense it did because of the supporting parallel existence of the expressions "turn up your nose" or "look down your nose" would be to show that those expressions predated the development in snob. That part of the argument works fine; "turning up your nose" is a very old concept. You'd also hope that the developments in snob didn't make sense on their own terms, because in that case there would be nothing left to explain by reference to noses, and in the case of snob that hope would be disappointed.

> I'm probably showing my ignorance here, but if etymology and ancestry are paired concepts, what is the name of the word that means "why this word took on this sense", or "why this word stayed in the lexicon and evolved while this other word died out in usage"?

My sense is that there is not a general word for your first question, but it is viewed as an important phenomenon and there are words for particular types of sense development, whereas there is not a general word for your second question, there is also no related specific terminology, and the usual feeling is that the answer to questions of that form is essentially always "chance", or in evolutionary terms "drift".

In fact, more 'study' has gone into your question than I think would have been wise. You might have heard about the theory of the "bear taboo", how the Indo-Europeans replaced the word for bear with euphemistic references to the features of bears.

You can find people arguing for this theory with a straight face today, but in my view there are two major problems:

(1) The word for bear was replaced in Germanic, Slavic, and Sanskrit. It was not replaced in Latin, Greek, or Persian; speakers of Spanish, French, and Farsi today are still going around referring to bears by their ancient proto-Indo-European root.

(2) In medieval France, the native word for fox was replaced by the Germanic word renard. In this case, we know there wasn't a taboo, and we know the source of the new word; we still have the body of literature - the Reynard the Fox stories - that supplied the modern French noun.

Do we know why common usage shifted? No; that's just a thing that sometimes happens. In English, the native word hound was replaced by dog, and we have no idea where the new word came from. But most native words for common objects did not change.

But this means that the only evidence we have for the bear taboo - the shift in usage - doesn't actually support the theory.

> Tracing, for example, the genetics of various finches in the Galapagos, one might find a tree of ancestry and say that "this finch species came from this other, which in turn are derived from this common ancestor" (etymology for birds) -- and that this is why their beak is shaped in such and such a way (because of their ancestral tree).

> To do so, however, would be to lose sight of the larger picture -- there are a variety of ecological niches in the galapagos suitable to different beak shapes, and these shapes (like crab body form) have evolved again and again.

One of Darwin's more important but less famous results from studying in the Americas was that tropical American plants more closely resembled temperate American plants than they did tropical old-world plants. This speaks directly to your point here, and labels it minor in comparison to the effect of common descent.

That is not to say that the effect isn't there (we know it is), or that the relative importance is equal between biology and language development (we don't know, but we can be pretty confident that they're not literally the same).

But languages, like biology, are characterized by quite a lot of stability, and this tends to limit the potential significance of any systematic force for change.