Let's say that there is an alien species of sentient beings, called Alpha. They are suped-advanced in bio-engineering (like DNA manipulation, medicine, etc...).

At one point of their history, their society breaks into two major groups:

  • one, called Beta, that decides to apply bio-engineering on themselves to create hybrids, super-specimens, monstruosities and such
  • the other one, called Delta, that believes in the purity of their species and won't allow the application of those technologies on themselves.

At which point one could say that Beta and Delta (Alfa) have become two different species? Which parameters could be used to determine that Beta has become a different species from Delta (Alfa)?

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    $\begingroup$ This seems highly relevant: evolution.berkeley.edu/evolibrary/article/evo_41 $\endgroup$ – Tim B Jun 14 '18 at 14:13
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    $\begingroup$ From the moment the Beta decided to bioengineer themselves into another species -in fact, more than one. When a species assumes the genetic traits of another species, and said integrations is full and irreversible, then you have a whole new species, $\endgroup$ – Valerio Pastore Jun 14 '18 at 14:14
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    $\begingroup$ Morphology (anatomy) and genetics are the most common way of determining species, if you can show they are morphologically distinct and have not shared genetic material for a significant amount of time, you can argue they are different species. $\endgroup$ – John Jun 14 '18 at 14:18
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    $\begingroup$ this strongly depends on the criteria they use to define a species. $\endgroup$ – L.Dutch - Reinstate Monica Jun 14 '18 at 14:31
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    $\begingroup$ This is just a question about the meaning of the word “species”, and so has no undeniably right answer. Define precisely what you mean by that word, and your question will then answer itself. But others may be using a different definition and so they won’t agree with you. $\endgroup$ – Mike Scott Jun 14 '18 at 14:38

TL;DR: Different species can't interbreed. Your Betas are probably multiple species themselves

The term "species"

If you mean the classical term of biology "species". The term species is referring to a group of animals similar enough to one another to interbreed. This is not as clear as you might think.

When a population splits they can have different selective pressures and develop slightly different traits. But neighboring populations tend to still be able to interbreed while more distant populations might not be able to.

The following image shows this. The lines between the different populations show neighboring populations. Due to the ongoing divergence over time as the populations travel around the barrier they might become so different that the final specimens of both groups can no longer interbreed, although you can form a path of populations to them only with closely related ones.

This concept is called ring species.

ring species

Back to your populations

Since you mention the beta changing their own genes they probably become different species themselves as many differently modified specimen might still be able to reproduce with some, but won't be able to interbreed with all of the other betas. Some might modified so much that they can only interbreed with other who were modified exactly like them and thus being a singular species.

And thus not only would the Beta be different from the Delta, but they would also be a multi-species community.

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  • $\begingroup$ the geographical separation is a very interesting concept. What biological traits could cause the two populations to not be able to interbreed anymore? $\endgroup$ – R. Satriani Jun 14 '18 at 14:52
  • $\begingroup$ @R.Satriani: It's not that much specific traits that inhibit interbreeding, but more generally being too different. Let's say you have a huge lake with one side being more like a mountain range and the other side being flatlands. If the populations are not moving that much locally they can become better adapted to their side of the lake and once both paths meet at the end of the lake they have become too different despite being still able to interbreed with the intermediate populations. $\endgroup$ – ArtificialSoul Jun 14 '18 at 15:09
  • $\begingroup$ Different species can interbreed (some of them), but their offsrpings are fertile : horse + donkey = mule. $\endgroup$ – Hawker65 Jun 14 '18 at 15:46
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    $\begingroup$ @Hawker65 i think you meant infertile $\endgroup$ – Jared K Jun 14 '18 at 15:59
  • $\begingroup$ @R.Satriani Google "ring species" and you'll find plenty of info on the concept $\endgroup$ – Tim B Jun 14 '18 at 16:01

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