Many "pure" cladists would say that you can never identify the taxa that correspond to the ancestral nodes of a cladogram (at best, you can identify taxa that would be very similar both in terms of morphology and stratigraphy to the taxa representing the ancestral node. I think that in some cases where the fossil record is very good, such as the Plio-Pleistocene of Ethiopia (my speciality), you can identify potential ancestral taxa almost with certainty.

Also, by definition, cladists recognize only cladogenesis and do not consider other modes of speciation (such as phyletic change, also called anagenesis) and therefore cladograms do not allow even considering other modes of speciation.

Therefore, if you run a cladistic analysis with taxa that include chronospecies (usually species that represent stratigraphic variants of a long-lived lineage, affected by phyletic/anagenetic change), you are likely to retrieve the oldest chronospecies as a basal taxon sister-group to a more-derived group including the younger chronospecies and maybe some other taxa that derived later on from the younger chronospecies (by cladogenesis or other modes of speciation). In that particular case, it is difficult to interpret the cladogram if you do not take into account the status of chronospecies and the stratigraphic positions of the different taxa analyzed.

FInally, I would disagree with the previous comment stating that a "common ancestor cannot persist following a speciation event". In some cases, the ancestral species persists after giving rise to one or more descendant species. Modes of speciation are very diverse and can include such things as birfucating cladogenesis, budding cladogenesis (with persistance of the ancestral taxon), or phyletic change.

Without going deep into the fossil record, I am pretty sure that among the numerous cases of speciation documented in recent times (historic or even modern), there are instances of ancestors persisting along their descendants (although usually not in the same place, or by adapting to very different ecologies).

Hope that it helps (sorry for the long read!).

Best!

The implication is that at the node, the common ancestor diverged into the descendant lineages, which are represented by the subsequent branches. By definition, a common ancestor cannot persist following a speciation event, and is replaced by the resulting species.

Fossils can correspond to these internal branches because they allow us to look back "into the tree", thereby providing a view of a given spot along one of the internal branches. The idea is that if speciation/extinction occurred along the "branch" of that lineage following the time at which that fossil was alive, it will no longer be an extant species.

I hope that helps.

I can think of two reasons. First, by definition an ancestor is a precursor of a line that carried on. The ancestor continues in that line, so it does not disappear as much as loses its species identity by natural selection into the new species. By this logic, dinosaurs never went extinct because they continued on as birds. We just identify them differently because we are imposing on a continuous germ line the construct of species. Second, the new species has an evolutionary advantage so it is able to occupy a slightly different niche or take over the niche of the ancestor. In a way, both explanations are saying the same thing.

Many "pure" cladists would say that you can never identify the taxa that correspond to the ancestral nodes of a cladogram (at best, you can identify taxa that would be very similar both in terms of morphology and stratigraphy to the taxa representing the ancestral node. I think that in some cases where the fossil record is very good, such as the Plio-Pleistocene of Ethiopia (my speciality), you can identify potential ancestral taxa almost with certainty.

Also, by definition, cladists recognize only cladogenesis and do not consider other modes of speciation (such as phyletic change, also called anagenesis) and therefore cladograms do not allow even considering other modes of speciation.

Therefore, if you run a cladistic analysis with taxa that include chronospecies (usually species that represent stratigraphic variants of a long-lived lineage, affected by phyletic/anagenetic change), you are likely to retrieve the oldest chronospecies as a basal taxon sister-group to a more-derived group including the younger chronospecies and maybe some other taxa that derived later on from the younger chronospecies (by cladogenesis or other modes of speciation). In that particular case, it is difficult to interpret the cladogram if you do not take into account the status of chronospecies and the stratigraphic positions of the different taxa analyzed.

FInally, I would disagree with the previous comment stating that a "common ancestor cannot persist following a speciation event". In some cases, the ancestral species persists after giving rise to one or more descendant species. Modes of speciation are very diverse and can include such things as birfucating cladogenesis, budding cladogenesis (with persistance of the ancestral taxon), or phyletic change.

Without going deep into the fossil record, I am pretty sure that among the numerous cases of speciation documented in recent times (historic or even modern), there are instances of ancestors persisting along their descendants (although usually not in the same place, or by adapting to very different ecologies).

Hope that it helps (sorry for the long read!).

Best!

@ Weston Testo, @ Tee Goidotti, @Nitish Ranjan and @ Antonie Souron, thanks for your replies. My another linked question is: Is it necessary that all the internal nodes designate a true common ancestor that have really lived on this earth? Here "true C.A." means that CA has ever actually lived on earth once upon a time.

When we deal with terminal branches, species alive now, the question seems to me to be a matter of definition only. An ancestral species may split into two or more lineages over time. We can separate the lineages, but since we do not have the relevant data for the actual ancestor, we do not know to what extent each has changed. It is likely that the various lineages have changed to different extents, as well as in different ways, relative to their common ancestor, especially if selection has influenced which changes persist. Whether you call any terminal lineage "ancestral" depends on your assessment of the amount of change that has happened, and on your sense of how much (or how little) change permits you to regard forms as being the same or different species. And you don't have real data for the actual ancestor!