There is scientific data, gleaned from the geological strata as well as from currently living organisms, supporting the scientific theory of evolution. It is a theory expressed from scientific position. Argue against the data if you wish, but it is scientifically based, not philosophically.
Unviable genetic combinations happen all the time, its part of the process. That's the thing though, every conception is a chance at a random genetic mutation, which may be beneficial or detrimental to the organism, or may be completely unnoticeable. Thats why it takes so long. Once a beneficial mutation happens, the organism will be more likely to survive and reproduce. You throw telos in there like it means something. Purpose does not define a species. Physical attributes do. You summed it up nicely at the end here though, the telos of any organism is simply survive and procreate. Those that don't simply cease to be. You could argue the telos of say, decomposition related bacteria is to break down dead things. But in reality, thats just what they do in order to survive and reproduce.
The assumption that individual species don't have a specific purpose isn't really required for most people, although it is easily deduced if one is for evolution. You do see it come up though, mostly when someone tries to argue that god made everything with a purpose. It usually follows with a challenge for the evolutionary to prove a negative.