However; genetic isolation between human subspecies was never total. Their was always some adventurous young male roaming far from home in a quest for women. The amen in turn tended to be receptive to successful travelers.
There is also the issue of how alien viruses and prions will affect human DNA. Throw in extreme environments and speciation would be more rapid.
However; the biggest factor in the Honorverse is genetic engineering. The GEs might be designed to be a new species or the genetic engineering might be imperfect (Grayson) or the genetic modification will be further alter by natural selection.
JohnRoth wrote:TheGlyphstone wrote:To try and drag this back on-topic, what other variations of humanity might exist out there in the universe who adapted to their environments? Graysons were intentionally modified for pollutant filtration and mutated further from there to handle the toxic metal contents of their planet...the Mfecanes were subjected to genetic experiments on top of being residents of a heavy-gravity world. But how far might have some hypothetical undiscovered human colonies drifted to keep up with their homes in conditions other than simple gravity variances, and how far would they need to drift before they're not really 'human' anymore? (a more complicated question than 'can they breed together, since donkeys+horses can produce (sterile) offspring.
A long time ago I saw a comment that it takes about 5 million years for two groups of primates to achieve reproductive isolation - that is, strong form speciation.
We know that Neanderthals and modern Homo Sapiens could interbreed, and they parted ways 500 to 800 thousand years ago. Add in the mysterious Denisovians and the even more mysterious "archaic species" lurking somewhere in darkest Africa, and I wouldn't think 2000 years is anywhere near sufficient, regardless of how harsh natural selection happened. Extinction for that branch is more likely. Unnatural selection, now, that's possible, but speculating about it is also arm-waving.