[jamest (OP)]

Posting this one a little late as it was missing from last week.

From Girts: 'How similar do two organisms have to be to produce viable offspring. As far as I know, any female dog can mate with a male dog, but what about a dog and a fox, for example?'

[vhfpmr]

I'm not sure it answers your question, but ring species might be of interest:
https://en.wikipedia.org/wiki/Ring_species

[alancalverd]

It's a weird question, because we assign "similarity", i.e. species, principally on the ability to produce viable and fertile offspring. Some interspecies mating produces viable but infertile offspring (e.g. mules) but I think ligers and tigons (big cat  interspecies) can be fertile, and fertile dog-wolf hybrids are reported.

AFAIK the fox is more closely related to cats than to dogs, despite its canine appearance.

[paul cotter]

I would certainly expect the dog/wolf interbreeding to work since a dog is a domesticated wolf.

[alancalverd]

....but a wholly different species! There's the root of the problem; species is a label of convenience, not a definition of properties.

[paul cotter]

Compatible DNA would be the determinant. I am not familiar with equine DNA, the crosses like mules and ginnets(is that the right spelling?) are both sterile but they probably have the same number of chromosomes to allow conception.