There is a huge literature on characterizing terms like fitness, adaptation, and natural selection, and trying to figure out how they fit within "evolutionary theory". Here are just a few that might be of particular relevance to us:
Discussions of the propensity interpretation of fitness in light of effects of variance include:
In 1984, Elliott Sober's The Nature of Selection defended the view that natural selection is a force which causes evolutionary change (today sometimes called the "dynamical interpretation") which is still the focus of debate in philosophy. Ariew and Lewontin's piece is considered to be in the "statistical" camp which opposes the dynamical account.
One example of the tautology objection from a creationist is
An example of how population geneticists think about fitness is Allen's Orr dual pieces: