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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?

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Allopatric speciation

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

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.

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  • $\begingroup$ Thanks for adding the sympatric clarification... yes, this is what I'm referring to. Obviously there is reproductive isolation...isolated by time, not geography. So at what point do we say that speciation has occurred; how do we know that a given individual has become the next iteration of speciation? $\endgroup$ Nov 1, 2018 at 0:15
  • $\begingroup$ @charlie-k3 now you are simply asking questions about definitions, not biological facts or theories. You're focusing on one definition of "species", of the many that exist, and you're choosing one that is explicitly not designed to address the question you're asking. Wikipedia introduces the question and there are many other discussions. Remi.b might want to update his answer with other pointers. $\endgroup$
    – iayork
    Nov 1, 2018 at 15:21
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    $\begingroup$ @charlieK3 Your follow-up question is an entirely other question and should be asked on a separate post. If the current question has been answered, please check it and consider opening a new post for your new question. That being said, before you open a new post, please have a look at How could humans have interbred with Neanderthals if we're a different species? that might already help you a lot. $\endgroup$
    – Remi.b
    Nov 1, 2018 at 17:14
  • $\begingroup$ 1) actually I'm not asking for the definition of a species, and I know there are many. 2) my first and second questions are variations on the same concept. 3) I'm specifically NOT asking the "Neandertals" question, as stated in my first post, because that is an example two separate species descended from a common ancestor. It's OK to answer "I don't know." $\endgroup$ Nov 12, 2018 at 22:51
  • $\begingroup$ I don't understand your follow-up question. To me your follow-up question seems to be How can you tell whether two populations are of the same or of different species in the special case where one population is ancestral to the other one? The answer is it depends upon the definition of species you are willing to considerate, whether or not the two populations of consideration exist in the same time or not. If I misunderstood your follow-up question, can you please clarify it? $\endgroup$
    – Remi.b
    Nov 12, 2018 at 23:35

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