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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?

charlie_sar
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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.

mgkrebbs
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Remi.b
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  • 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? – charlie_sar Nov 01 '18 at 00:15
  • @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. – iayork Nov 01 '18 at 15:21
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    @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. – Remi.b Nov 01 '18 at 17:14
  • 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."
    • – charlie_sar Nov 12 '18 at 22:51
  • 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? – Remi.b Nov 12 '18 at 23:35
  • Btw, It's OK to answer "I don't know." can be considered a bit rude. It suggests that I am being too pretentious (or dishonest for some other reason) to admit that I don't know what I don't know. Please believe, my goals are to help, to improve my knowledge in biology, to improve my communications skills and not to impress. – Remi.b Nov 12 '18 at 23:41
  • Allow me to try this again. I am specifically NOT describing a situation with or comparison between two (or more) extant species. I am referring to ONE extant species (say a hermit crab, E.gilli). At some point in the evolutionary past, E. gilli did not exist--because its ancestor had not yet evolved into E. gilli. Agreed? If so, how can we show when speciation has occurred, i.e. when the ancestor evolved into E. gilli ? For the purposes of this question, let's assume the definition of a species is that in which reproduction is possible and produces fertile offspring. – charlie_sar Nov 17 '18 at 21:24
  • @charlieK3 Yes, I understood that you are talking about question of species distinction among two populations where one is the ancestor of the other (and hence only one is extant). You have now suggested a definition of species to work with which is good. The definition is not sufficient as explain in the post How could humans have interbred with Neanderthals if we're a different species? but let's work with that anyway.... – Remi.b Nov 17 '18 at 23:28
  • Reproduction is not possible as they don't exist at the same time but of course, I expect it is not what you want to hear. Maybe you want to hear there is no absolute solution to test whether two individuals would be fertile without actually trying to mate them. Of course, some cases are obvious (such as reproduction between an oak tree and a snow leopard) but there is no general solution for knowing whether two individuals can reproduce without actually trying to make them mate. – Remi.b Nov 17 '18 at 23:28