Robert, your formulation renders the term “race” redundant, since it becomes indistinguishable from the less vague term “ethnicity.” Most people who use these two terms as near-synonyms distinguish them on the basis of scale — a “race” being larger than an “ethnicity” — but then you concede that there could be a “race” at the level of a tribe. If you want a “usable” definition of race for this context, then simply use the term ethnicity instead, because it happens to be the proper term to use when referring to a “very specific tribe [that] is susceptible to a certain disease.”

The whole concept of race is that it is — quoting another of the Merriam-Webster definitions — “a category of humankind that shares certain distinctive physical traits.”

This is the more basic definition, and the truer one. To say it another way — I am running out of ways — one might try the biological metaphor: Race is phenotypical (a combination of a person’s physical appearance and how that appearance is perceived and altered by his social environment, these two things inexorably bound up together in the definition), while ethnicity is genotypical (i.e., a person’s real, not imagined or perceived, genetic ancestry).

It is not “inconsistent” to think that “race” is more alterable than “ethnicity.” This is because the two words actually have very different meanings, the one largely cultural and social and the other more “usable” and precise in the world of science.