Evolution: a Question

Evolution! The building block theory of biology, and demonstrable through multiple forms of evidence!

My question, is about evolution on a cellular level.

I’ve never met anyone who couldn’t agree that evolution, as inheritance of traits through generations, couldn’t be shown as a trivial matter given enough time. Dog breeds, cultivated plants, etc. all show inherited traits as a real thing. A reliably predictable, and modelled, and very useful thing.

And, the explanation goes, get enough of these randomized inherited traits, and enough time, in an isolated population, and you can get a new species.

Wolves, coyotes, jackals and domestic dogs are an example of this… they are all drifting away from each other, with different behaviours, and biologies. They are generally uninterested in interbreeding in the wild, so don’t view each other as potential mates. But, you can still get them to interbreed in stressful or compelled situations. Their pups are fully fertile. Chihuahua and Great Dane cross breeds are viable.

The formal split, between one species and another, taxonomically, is at the cellular level. DNA breaks into chromosomes, which pair off in sexually reproducing species: half from one parent, half from the other.

Horses have 64 chromosomes: 32 from one parent, 32 from the other.
Donkeys have 62 chromosomes: 31 from one parent, 31 from the other.

All sexually reproducing species generally require an even number of chromosomes to be able to reproduce.

If a mule is bred (from a horse and a donkey)–even though horses and donkeys look more similar than Great Danes and Chihuahuas–with 63 chromosomes the mule will likely be sterile, it may be a useful beast of burden, but not capable of reproducing.

That said, mules do come as male or female, and there are documented cases of mules having infants, but it’s rare. And is the result of breeding with a donkey or horse… so the foal is that parentage.

So… my question is… how do chromosome splits/fusions happen “in the wild” without massive bottle-necking of a population?

A little over 4 million years ago, donkeys and horses shared a common ancestor. Then they split.

Suppose the first 64-chromosome equid was “Silver” in ancient North America, in a herd of 62 chromosome Pleistocene Horses. Silver has a cellular mutation.

Silver might be a horny stallion, or mare, indeed! But… all of Silver’s progeny are going to be… 63 chromosome mules… and generally sterile.

Even if they aren’t sterile, the available mating population is 62-chromosomed Pleistocene Horses. So Silver’s grandfoals aren’t going to unusual… unless there’s some pretty intensive inbreeding. Silver and his offspring are going to have to be very interested in breeding each other… then you could have grandfoals with 62 chromosomes… who… again… better be very interested in reproducing with each other to get the new species off the ground.

So: Is it the case that the only way we can get successful chromosomal variations distinguishing species on a cellular level is through instances of what would generally be considered ‘unhealthy’ inbreeding with a very limited genetic pool… and that all new species have to go through this bottleneck to appear?

Doesn’t that suggest that most new species (on a chromosomal level) don’t make it, because of the recessive trait problems such intense inbreeding can create?

Sir Tainley