Brandon has a great post on some of the basic facts that clarify the question “Is Darwin’s theory teleological?” It clearly isn’t in the sense advanced by Paley, but Darwin himself insists that it is teleological in some way. This raises the question for a Thomist whether the theory is teleological in the sense that Aristotle and St. Thomas understood teleology. I don’t want to speak to that question, but raise one question that needs to be addressed prior to being able to answer whether an evolutionary theory is the sort of thing that acts for the sake of something.
(I’ve said before that the a diversity of species- where diversity is understood as a number over the whole of time, such that if five species give rise to one, and then the five go extinct, that this counts as six species- is a good, and so far as evolution is unintelligible without increasing diversity in this precise sense- a sense of diversity which, as far as I know, no biologist cares about- it is unintelligible apart from some good. Leave this sense of the end of evolution aside.)
One question that St. Thomas would have to clarify from the beginning is “what evolves?” In Contemporary terms: what is the unit of evolution? What does evolution act on/ exist within? Let’s get one answer out of the way from the beginning: assume someone said living things evolve. Very well. So does Lassie evolve? No. She can reproduce, die, interbreed, etc. but it’s not Lassie that evolves. So it’s not this or that individual living thing that evolves. So it must be something other than an individual. Here is the first difficulty: there are two things that differ from the individual: a multitude and a species as opposed to a multitude. I say “species as opposed to a multitude” because there is no impediment to understanding a species as a multitude of members.
Say we conclude that it is some multitude that evolves- or, to be pedantic, a species or population that is considered as a multitude as opposed to being considered as a unity- what kind of multitude is it? It seems that the extension of a multitude over time is meant. The population (multitude) changes over time due to selection (drift?). So is evolution something like a ticker-tape moving news sign, where the changes of various lights turning off and on give rise to a kind of intelligible movement, though none of the lights is moving? Again, it is like a movie reel, where the passing away of one picture after another gives rise to a sort of change of a whole? But this example might raise more questions than it clarifies.
The goal in figuring out what evolves is to be able to ask the question “is this the sort of thing that acts for an end?” Not all processes- even intelligible processes- are things acting for an end. Aristotle points out, an eclipse is not a process that occurs for the sake of something. We can predict eclipses thousands of years in advance, we know exactly what the relevent causes are, but there is no intention towards it, even in St. Thomas’s very broad sense of intention. One can speak of an intention in St. Thomas’s sense towards the equinox or solstices- for these are real terms of the motion of the earth- but an eclipse is not a real term of the earth’s orbit, even though it arises forseeably and necessarily from it.
So even after we figure out what evolves, and what sense of change we are dealing with, we still need to ask whether this sort of process is like the moon earth interaction that gives rise to an eclipse (which is not a real term of their interaction) or whether it is like the process that gives rise to the tides (which is a real term of their interaction). What does the initial question have to do with this? Because a multitude, precisely as multitude, does not have an end. Ends come from some sort of coordination or unity of the multitude.