The facts of Darwinism allow two interpretations:
Darwinism A: The structure of an organism arose from factors like selection, drift, and descent. Calling these processes “successful” means only that the organism got lucky, and “lucky” means simply that it happened to survive till now for one reason or another. The primary lesson of selection, therefore, is that there is no ideal organism, just one possible way of interacting with the environment that happened to work.
Darwinism B: The structure of an organism is such that it makes the organism strive to attain goods, above all the good of its own intrinsic operation or self-expression. Reproduction arises from this striving, and as a material process it may occur with copying errors. These copying errors, however, can in turn be exploited by the organism (usually unconsciously) in its striving for its own goods. The primary lesson is that factors like descent, selection, etc. either arise from the organism striving for its proper good or find their only meaning in this context.
One interpretation of the difference is whether substances or operations are basic. Does form follow function (Darwinism B), or vice versa? Should the organism be taken as a sort of function, i.e. a striving for some sorts of actions, self-expressions, and goods (B); or should it be taken as a structure that might act well or poorly – who cares, so long as it finds a way to survive? (A)
More simply, is survival apart from any good or evil self-expression of the organism a sufficient explanation of structure (A) or is the organism’s striving for its own well-functioning state the sufficient explanation of how processes like descent or the exploitation of mutation (selection, etc.) can be beneficial or not?
Do we say “look, a think can survive regardless of whether it performs its proper function or not – even if it doesn’t have one (A)” or do we say “Survival is itself done by a thing striving for its own good”.
I forget who it was that said the best explanation of selection was a child’s game which was a bunch of shapes in a can, with the bottom of the can such that any shape could fit out if it happened to find its slot. You then shook the can and bet on which shape will fall out. This seems closest to an account of Darwinism A – the shapes don’t contribute anything to success at falling out – indeed as far as they are concerned it makes no difference whether they fall out or not. A better metaphor for Darwinism B would be betting on a football game.
There are deep conflicts in the vision of nature behind A and B that cannot be solved by mere accumulation of data. Is nature fundamentally a striving for goods or not? Does form follow function or vice versa? Is nature characterized fundamentally by action or inertia? Is consciousness a moment when nature finally becomes aware of the process that has been driving it all along (knowledge of good) or is consciousness merely a peculiar way of action, a mere phenotype among others, which sheds no particular light on processes that are non-conscious outside of human life?
For Aristotle, function was primary both in experience and causality, and so his biology saw the functional goods of organism as basic and the structure of the organism as consequent to them. For us, it’s the phenotype of the organism that is basic, and the function might happen or not, or even exist or not. On the first account, man (the substance) is a certain way of getting virtue to occur; on the second a collection of traits is seen as merely what survived, apart from whether virtue is sought or even if it does not exist at all. We collect a lot of data on organisms in an attempt to find what is really common to them and so necessary to characterize the population; Aristotle collected a lot of data in an attempt to find what was a paradigm in some nominal class. In a class of students, we are looking for which traits among the population really do characterize all of them; Aristotle is looking for the one student in the population who best most counts as a student.