My name is Corey Hayes. I am in my final year of Anthropology at MacEwan University in Edmonton, Canada. My Minor's English, and I've been told I have a bent for creative writing, specifically, sci-fi. A few summers ago I wrote an article in response to Stephen Hawking's warning that humans might go extinct if we didn't migrate into space. I argued that even if we did migrate to other worlds, we would eventually still go extinct (by ceasing to be Homo sapiens, which you spoke of in National Geographic). I then began to speculate on the directions evolution might take us if, as you so brilliantly put it, "Some major new isolating mechanism" takes place. Suppose five arks head out in opposite directions (the Milky Way is about 100,000 light years in diameter), and those five populations (including Terran plants and animals) remain isolated from one another for hundreds of thousands, or millions of years, on a variety of planets (with different gravities, atmospheres, some with moons, some without, under reddish, bluish, or whitish suns) - would they, during a random encounter among the cosmos, and given the ample opportunities over such vast stretches of time for the loss of history to occur, even recognize one another? I reckon the DNA evidence would point to a common origin, and many phenotypical traits would be maintained. But I wonder whether linguistic commonalities could still exist along with cultural and ethical remnants. Or, if one population comes from a world illumined by a reddish sun, and another was illumined by a bluish, how might that complicate communication about, say, color? How might the absence of a moon, or the presence of two moons, affect menarche (though I understand that there is no scientific evidence the two are linked, the 28-day cycle is still a heck of a coincidence). How long would the populations have to be isolated for them to became separate species (or be unable to even breed hybrids)? I'm not necessarily asking you to answer these questions (but if you can, that would be great). But I would really appreciate it if you could point me in the direction of any relevant scholarly articles, essays, or even short stories you may have encountered that may help me answer these questions. And thanks for thinking ahead. I hope we make it there.
Thanks for writing.
One factor to consider is technology. Language change, for example, has really slowed since literacy became widespread; and the scope of nation-states with newspapers, magazines and radio and television have caused the disappearance or decline of many minority languages across the world. If the people who are part of some human diaspora live in habitats that are suffused with technology; computers designed and programmed on Earth, for example, they might continue their language and cultural evolution at a very slow pace – at least relative to how human societies have changed during prehistory.
I think the optimal population size for a diasporic group would be large enough to make drift negligible, even on the generation-ship timescale. Selection would kick in much more strongly when the groups reach their ultimate destinations. Then, anything goes.