Sturddlefish

Only when gametes are produced by the hybrids for their reproduction, during the meiosis the homologous chromosomes must pair and in such hybrids this fails.

On the other hand, when the first hybrid cell is formed by the fusion of gametes, and also during its later divisions, which will grow the hybrid body, the number and organization of the chromosomes matters very little.

What matters is just the amount of differences in individual genes and in their regulation, which should be small enough so that their functions remain compatible, which must be true in this case.

It is normal in animals for the number of chromosomes to fluctuate due to fusions or fissions and for the order of the genes to be modified, but such changes do not modify the complete set of genes or their functions and these changes may happen without any changes in the appearance of the animal.

It is a reasonably good predictor only for animals which are relatively closely related and only when in none of the two lineages there has been any significant modification of the way of life, like starting to eat a different kind of food or adapting to an environment where different locomotion or sensory skills are needed.

In this case the accumulated genetic distance consists mainly in random mutations with no or little adaptive value, for which the approximation of a constant "molecular clock" is valid.

The attempts to extrapolate this "molecular clock" hypothesis to other circumstances have produced a huge number of ridiculous research papers whose results have low usefulness.

The approximation of a constant "molecular clock" may be also be applicable in other cases then what I have mentioned in the beginning when the genetic distance does not refer to the complete genome, but only to some groups of genes that have not been involved in the specific adaptations which have been caused in one or both lineages by changes in the way of life, so they have still evolved with a constant mutation rate.

The approximation of a constant "molecular clock" fails in many cases because even if mutations really happen at a relatively constant rate, most of them lead to immediate extinction of the mutated cells and the mutations that we see in the genomes that are examined now are only a small fraction of all mutations, those that have been preserved in survivors.

How large is that fraction depends on where in the genome the mutation has happened, i.e. whether it was in a place with large or small influence on the chances of survival and also on whether the mutated animal lived at that time in an environment with great or little adaptive pressure.

As far as these being distantly related species, they are, but they're also as closely related as you can get without being in Acipenser. They're separate families, but those are the only two families within the suborder Acipenseroidei. Super neat fishes. I highly recommend looking up videos of baby paddlefish, as they're adorable. My favorite is when they're just old enough that they start switching to filter feeding. Handling sturgeon just makes you feel like you've got something ancient in your hands, which you kind of do.

What's interesting to me is that the paddlefish shares the Missouri River with both the shovelnose sturgeon and the pallid sturgeon. Also possibly the lake sturgeon if I'm remembering correctly that they sometimes come down the Missouri a bit especially during flood years. Makes me wonder if it's just luck that those sturgeon species aren't tetraploid or if tetraploid individuals are produced and selected against as they hybridize with paddlefish for potentially less well-adapted progeny.