“Can you point me toward racial classification definitions in the scientific literature?”
Well, there are varying definitions depending on what classification system you are talking about. Here is a good summary about the different kinds of models and their limitations:
http://anthro.palomar.edu/vary/vary_2.htm
...from the reference:
Since the number of "races" can be so easily changed by the way they are defined, it is clear that they do not really exist as distinct biological groupings of people. Instead, they are arbitrary creations that reflect our ethnocentric views of ourselves and other people. They are mainly cultural rather than biological groupings.
It's a social construct, much as Twain wittily conveyed as only Twain can, in "Pudd'nhead Wilson". "Pudd'nhead Wilson" was published 1894 and it does a better job blowing up the absurdity of "race" than any other publication I can think of.