[AlotOfReading]

There isn't a single answer to give re: the viability of interbreeding. It depends on the particular individuals and the particulars of their offspring and a whole lot of environmental luck besides. As individuals get farther apart, the chances of successful reproduction simply decrease. Beyond a certain point, fertilization simply ceases to take place and the issue is moot.

In general, the babies that are born have already passed through a nightmarishly difficult biological gauntlet to survive. If they fail at any stage, the pregnancy is simply terminated. The majority of fertilizations never lead to births as a result.

[lurquer]

It isn’t a continuum, though. There are distinct discontinuities. Chromosome count is one such example. Polar bears and grizzlies have 74 chromosomes. A panda has 42.

You can’t gradually go from one to the other.

Absent some mechanism we don’t know about, conception simply doesn’t work when the sperm and eggs have different numbers of chromosomes.

To be clear, the concept of chromosome isn’t as objective as one might think… nevertheless, it’s an example of the type of genetic difference that simply precludes reproduction.

[AlotOfReading]

Horses and Donkeys have differing numbers of chromosomes and yet (extremely rarely) produce fertile mules. In a more common case, women with down's syndrome have an extra chromosome and yet are sometimes fertile. I understand that this is extremely common with plants, but that's pretty far outside my wheelhouse.

I don't think we're disagreeing here, just simplifying like all conversations about biology.