Hi InGodITrust, and welcome to the fray.
Basically, natural selection can act faster than crossbreeding over a wide range can counter it.
Remember that the key to natural selection is that the organisms survive to breed: no survival, and no contribution to the next generation; no breeding, and no contribution to the next generation. You need both.
Another example of how speciation can occur is with timing of breeding: if one population breeds at one time of the year and another population breeds at another time of year, then they can diverge even though they live quite close.
In the early study of malaria, scientists were puzzled as some populations of a mosquito species seemed to be carriers, while other did not. Turns out one breeds in the morning and the other in the afternoon, and further investigation determined they were different species, no longer able to interbreed.
Enjoy.
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