To expand on the answer given by Dr. Jones, defining evolution means to define the process. Splits in a population producing a new species is a possible outcome of the process. Species produced in such a way would have a common ancestor. (Universal common ancestry, of course, should not be part of the definition - that is something that is determined by investigation into the historical course of evolution and is not a necessary consequence of evolution itself).
So, if we are producing a short and comprehensive definition we would probably not mention even speciation, as it is merely a possible outcome. And common ancestry is even further removed. A fuller treatment might well include speciation and could even include common ancestry in the general sense.
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