The cumulative case establishing the fact of evolution by natural selection is overwhelmingly persuasive and has been for decades. Full disclosure: I don’t just think it is true but also want it to be true. What other theory can claim to explain more with a simpler and more intelligible account?
But to take evolution by selection as a fact does not mean we see the basis of this fact (if there is one). One of the basic facts of Natural selection is that there is a connection between the desire for survival and reproduction, but a clarification of this fact gives interesting results. Notice that the individual animal or living thing doesn’t enhance its chances for survival in any way by reproducing. It’s not as if I expected to add any years to my life by having kids. The desire for survival and reproduction therefore cannot be linked together in the individual living thing. Now the response to this has been known for millennia: it’s not the individual that survives by reproduction, but the species. But here is where the question gets interesting, since by introducing evolution into descent, the species is just as much destroyed by reproduction as it is preserved (which is just as much true as any part of the individual or species, say, a gene). If reproduction were an activity that promised the survival of your species or its genetic code, then evolution would be impossible.
And so if we want to establish a per se connection between survival and reproduction as an explanatory principle of evolution, it seems we have to introduce life itself as the point of connection between the them. But this is the same as to introduce existence as such as the point of contact (since, for the living thing, to exist is to live). Notice, however, that is not existence pure and simple that we must introduce here but existence so far as it is being sought after (for reproduction perpetuates existence by seeking after it), which means we must introduce existence as convertible with the good. Seen from this perspective, evolution by natural selection relativizes the species that Aristotle took as absolute and replaces them, even within the biological order, with Plato’s understanding of the good itself. Eros (i.e. the desire to reproduce) is not explained by reference to a species (as Aristotle claims in De Anima 2.4), but by an interaction between the individual and the good itself (as Plato claims in Symposium). Reproduction is a result of the participation of the individual with the good itself, and this participation has both a positive connection that makes the good itself the per se motive of reproduction but also an insurmountable division that makes reproduction a substitute for attaining the good itself as opposed to being a means to attaining it.