So, the panel on human speciation went well, I thought! Whew. This was a little bit of an intimidating panel to moderate because all the other panelists were older than me (although frankly I am getting old enough that doesn’t feel like it matters so much) and one of the other panelists was Elizabeth Moon, whose books I’ve been reading since I was in high school. (Also G David Nordley, who among other things is an astronautical engineer; and Frederick Turner, who has written SF epic poetry (!).
It was an interesting panel because I think we all basically agreed with one another about a lot of things, including (a) it’s hard to define what a species is because lots of organisms are obviously distinct species regardless of whether they can interbreed with each other or not (my example for this was grizzly bears and polar bears); and (b) humans don’t speciate very readily; ie, tens of thousands of years of reproductive isolation has in the past not produced true species, because not only can we still interbreed when we get back together, we do so pretty readily even when morphological differences are fairly significant.
So many unknown factors would make a big difference for whether humans speciate or not after spreading out into space:
Do we find a way to manage FTL? Without that, population separation matters. With it, not so much.
Do we use extensive genetic engineering? Without that, speciation would be slow. With it, not so much.
And in my case, I declared that humans have two distinct competing instincts and it really matters which wins out in a particular situation:
Humans are very tribal …… humans will have sex with practically anybody
Behavioral reproductive isolation matters so much in the natural world. Do black bears breed with brown bears? No. Or not enough to blur the species lines in general. Do red wolves interbreed with coyotes? Yes, very freely, and thus conservation efforts for red wolves (which do appear to have started off as a distinct species) are doomed.
So far, behavioral isolation has not mattered much for humans. Not enough to promote speciation, for sure.
Elizabeth Moon made the point that the physical demands of a low-gravity environment and the metabolic demands of any radically different environment would tend probably to select for sharp divergence in important biochemistry, thus possibly producing postzygotic reproductive isolation. In other worlds, people might be willing to interbreed, but hybrid offspring might be betwixt and between, at a substantial fitness disadvantage. This would by definition cause speciation.
G David Nordley commented about epigenetics and how peculiar that is and how it could affect humans under weird environmental conditions, for some unknown number of generations. Really epigenetics is not at all well understood yet. Very cool branch of genetics, though.
I wound up the panel by mentioning a small number of SFF titles that arguably humans that have already speciated, some more obvious than others. Here they are, in no particular order — add others in the comments if any occur to you.
This Alien Shore by CS Friedman
Ammonite by Nicola Griffith
The Left Hand of Darkness by LeGuin
The Integral Trees and Ringworld by Nivan
Ethan of Athos (telepathy!) and Cetaganda by Bujold
Seveneves by Stephenson
And some of the far, far future SF, such as The Golden Age trilogy by John C Wright, often present truly extreme levels of human speciation. I mean, wow, do they ever.