Two recent contributions to my files on restrictive relativizer choice (other than the classic which/that business and the use of that for human referents, both of which are objects of unmerited prescriptivist scorn): a who for human-institution referents and a who for (non-human) animal referents. In both cases, which or that is the prescribed relativizer, and the use of who is an extension granting symbolic human-like status to non-human entities.
Who for human institutions, or: Corporations are people too. This morning on NPR’s Morning Edition, “Whole Foods Founder John Mackey On Fascism and ‘Conscious Capitalism’ ”:
A conscious business is one who understands: one, that it has a higher purpose besides just making money, and number two, it understands how these stakeholders are connected together in a large business system.
Note relativizer who, but anaphoric personal pronoun it; they, either as “singular they” or referring to a business as people in a collectivity, would certainly be possible, but genuinely singular personal pronouns like he or he or she are totally out of the question.
I have a considerable collection of these (which were noted by the compendious grammarians, such as Jespersen, long ago). When I brought up the subject on ADS-L in 2007, Chris Waigl collected some cites from Project Gutenberg, going back to the 18th century and up to very recent times. These are natural semantic extensions of the use of who for human referents.
Who for (non-human) animals, or: Animals are people too. A letter to the NYT on 1/7/13:
Americans consume roughly nine billion land animals a year, and almost all of them are treated in ways that would warrant felony cruelty charges if they were dogs, cats or other animals who have legal protection from abuse.
It’s undoubtedly significant that the letter came from Bruce G. Friedrich, senior director for strategy at Farm Sanctuary, a national farm animal protection group. There is certainly a solidarity-with-animals tone to the lexical choice; that would have been unremarkable (or which, if you’re not cowed by prescriptivist rants against it).
This one seems to be be rarer than the human-institution extension, and sometimes there’s evidence that writers contemplate the choices, as in this posting on NYT Online noted by Wilson Gray on ADS-L on 10/31/11:
And, our relationships to the animals with whom (or rather which, to be grammatically correct) we live is given very little status in our society. (link)
(Wilson was primarily interested in the number agreement in relationships … is, but the relativizer choice came along as a bonus. This is a case where the choice is starkly between whom and which; that is unavailable as the object of a preposition.)