What does it mean when it says on a grad school application to choose recommender "familiar with the candidate's scholarly abilities"?
It doesn't necessarily mean your research advisor. It could be a faculty member you've taken classes with, someone you've worked on a project with, etc.

What the admissions committee is looking for is evidence that you will succeed in graduate school - and that means evidence that you can think independently, tackle difficult problems, and so on. Choose faculty members who can speak to your abilities as a student and a (future) researcher.

When I asked for LoRs, I asked (1) a faculty member I had taken several classes with and with whom I was working as an undergraduate research, (2) another faculty member I had taken several classes with and had a good working relationship with, and (3) a faculty member who I was currently taking a class with (and who admittedly was an alumnus of my top school) :-)