I was watching a couple of squirrels playing in a tree outside my window and remembered this comment I read in a paper (the 2nd letter to the editor). The squirrels we usually see in the Pacific Northwest are not native to this area. They are an aggressive new-comer that has pushed out two other native species. The writer was saying this is bad and that we should not encourage the interlopers.
"Ahh, but isn't that evolution in action? Darwinian evolution is the natural course of events and has been going on for millions of years successfully diversifying the bio-system, right? Why should we ineffectually subvert what should be happening?
"In addition, it has always been the case that various species have traveled as baggage of other more-mobile species. For example, seeds travel in the guts of birds. Even if the new squirrels in this area arrived as baggage of traveling humans, isn't that is an established mechanism of evolution? Therefore we should let evolution have its course and let the new squirrels take over, correct?"
The truth is that we instinctively see this explanation as doubtful. If Darwinian evolution was actually the origin of life, then we would not being living in a diverse eco-system. The aggressive species would long ago have eliminated the weaker species. What we actually observe is an eco-system that was more diverse in the past (via fossils), is still quite diverse, but is becoming less so (by extinctions).
We know the naive_evolution line of reasoning was wrong. It is desirable to preserve existing species because we don't see Darwinian evolution producing new ones like the old ones. Evolution's explanation for the origin of the diversity of life seems incorrect.