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Most examples of speciation describe a population splitting (via various mechanisms) into two or more populations that eventually become separate species from one another. However, what if the population never splits? In other words, at some point in the evolution of a single species, an individual member would become (theoretically) unable to reproduce with its own ancestors--thereby becoming a distinct new species. How can we say that this has happened, given the impossibility of the example?
Allopatric speciation [… ], also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model, is a mode of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with gene flow.
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that they occur together at least in some places.
Yes, sympatric speciation appears to be a thing but estimates of how common that is avery hard to come by.
Now, what you describe is not so much sympatric speciation as you refer to a case of reproductive isolation between extant individuals and their ancestors. I am not sure that was really what you had in mind because otherwise you would have not put it in opposition to allopatric speciation but it is what you seem to phrase. Of course, such type of "temporal reproductive isolation" is obviously a thing.