Question: At what point does one species become another through evolution?

Anthony Redmond answered on 30 Jan 2019:

In animals we generally consider two populations from a species to have become separate species when they have become different to the point that they are no longer capable of physically or genetically breeding with each other to produce fertile offspring. For one species becoming another through evolution this would then be the point at which the descendants have become so different from their ancestors that they could no longer interbreed successfully. For a species with a lifespan similar to our own this would probably take hundreds of thousands or even a few million years to occur.

In reality it is a little more complicated than this and there are some grey areas! For example we classically consider neanderthals and modern humans as separate species based on anatomy, but genetics has shown that the ancestors of most humans alive today interbred with neanderthals. We know this because except for indigenous sub-saharan africans modern humans carry a small amount of neanderthal DNA.