The theory of evolution does not claim or demonstrate anything, because that is not what theories do. Rather, it provides a coherent conceptual framework with which to interpret and understand what is a very voluminous body of knowledge.
We can (and do) in fact observe the evolutionary mechanisms that lead to speciation. Without any prior knowledge of the organisms involved, I can construct a molecular phylogenetic tree of almost any gene that gives the genetic information about the common ancestor--even though I did not initially have that genetic information and did not know the common ancestor. And I can compare that tree to the phylogenetic trees derived from paleontological methods and find that they are almost identical. With this technique, I not only demonstrate speciation, but I can determine the relative lengths of time elapsed since the split of any two species in that tree. (I need other data to determine absolute lengths of time.)
As for the wide divergence of species between two very similar habitats, it is completely inexplicable within the context of the creation stories. Why, having created the perfect set of plants and animals for the Gobi, would God have created a completely different set for the deserts of the US? Why reinvent the wheel? And so on. There are many implications to what one would expect to observe if all plants and animals had been created at a single time and place, as described in Genesis. The bottom line is that we (the scientific community) have never seen any evidence of a single creation event on earth.
The fact that the book of Genesis refers to people and places that preexisted outside of the Garden of Eden is all the evidence I need to understand that the intended purpose of the biblical creation stories was to illustrate moral principles, never to serve as scientific documentation.
I am not defensive, but frustrated. I see a sizable movement of people who call themselves Christians, but then place the restriction that if you accept the scientific observations about the physical nature of the world, you cannot be a Christian. And in that argument, the faith community loses. Consequently, a young person who is raised to believe that one cannot accept science and be religious and as a result rejects religion, is he not likely to reject religious morality, as well? Don't we see ample evidence that millenials are, in fact, rejecting religious teachings, even to the point of openly mocking religion and religious people? Personally, I do not see these as good developments, and I do blame the rigid either/or choice that creationists (not Christians) promote. I consider Creationism as a separate sect--it is neither biblical nor scientific, but claims to be both.
I never said anything about who can be a Christian.
Your thinking seems to be tangled within a chaotic array of “isms,” and as a result you apparently bring frustrations from previous discussions to this one. Please try to clear your mind. Focus specifically on what I’m stating without dragging in other, unrelated ideas such as what you’ve experienced with “creationism,” or what you imagine to be the purpose of unrelated discussions.
Now back to our discussion. Instead of saying “We can (and do) in fact observe the evolutionary mechanisms that lead to speciation,” you should say “We can (and do) in fact observe the evolutionary mechanisms that might lead to speciation, if speciation is possible.”
To demonstrate speciation is to make it happen. All you’ve described is an extrapolation which constructs an imagined process of speciation by extrapolating from separate and independent observable patterns. But I’ll say again that speciation has never been made to happen.