At what point does the capability of sexual reproduction diverge in evolution?

Question

I am asking this question as a layman in biology: What the title is supposed to mean is, as a species evolves, say humans and chimps from its common ancestor, at what point and why are humans or chimps not able to reproduce with it's common ancestor anymore? I'm asking this because two very different looking dogs can still reproduce, so how long would we need to keep breeding dogs until the resulting dogs can't reproduce anymore with the "older" dogs?

Answer 1

Zooming out a bit, biologists are usually interested in what practically stops gene flow, not whether gene flow is technically possible or not. A good example of this is chipmunks on the north and south ridges of the Grand Canyon, USA. A human could pick up a few chipmunks from the northern side, bring them to the southern side, and they'll breed perfectly fine with the local population. But in real life, chipmunks cannot naturally cross the canyon and so there is actually no gene flow.

Now to answer your question about when it's literally impossible for two different populations to mix and have children, it's always genetic. There is simply too much genetic difference between the two species to have a viable offspring. A great example is mules, the offspring of horse and donkeys. Horses have 64 chromosomes, and donkeys have 63, which is a different number. For the first generation of mules, it just so happens to not matter. But the dissimilarity prevents mules from breeding and having offspring. Between any pair of species, there will be genetic differences, and at some point, the genetic differences just become too great to allow reproduction. For example, you could never cross a horse with a plant because they are simply too incompatible. However, there is no one compatibility issue that is always responsible. Different species will have different issues. It is not always having a different chromosome number.