"Social Darwinism actually predates Darwin himself (see my reference to the survival of the fittest man - Galton). it is my position that Darwin himself embracerd it (see his views on the Aboriginees), but I understand that this is debateable. What is clear, however, is that it is a natural and logical extension of the NS hypothesis IF this is taken in isolation."
Who cares if Darwin believed it? he clearly made great advances through his journeys, but we now know how traits are controlled (what chemical structure holds the genetic information) and how to measure allele frequency... it is clear however that humans are unique in at least one way.
Only in our ability to reason (which is computational in effect, and extended by our scientific tools) there are other social/co-operative creatures, ant colonies are a great example. However we can conceivable define our own evolution. Without even using genetic engineering, we have countless example of breeding programs, whether to get a pure breed of dog (and there are horrible results or in-breeding here, but they do manage to emphasis particular traits) or to develop different strains of bean. (i believe performed by some monks in middle age europe) and it has happened in humans (http://en.wikipedia.org/wiki/Slave_breeding_in_the_United_States) though they didn't speciate, breeding was common.
It is perfectly possible to develop a political philosophy which includes determining which traits are favourable. We are perfectly capable of defining what the 'fittest' traits are, which phenotypes are preferred by society. We could easily sterilise those who were determined to be undesirable.
We could force speciation, we could divide into classes which would develop into seperate species. We could but should we? Currently popular opinion says no. Popular political theory suggests that humans should be considered equals, slavery abolished and the Universal Declaration of Human rights respected.
It is conceivable within the current scheme of things to speciate and extend human rights to both 'classes' though i can't see why we would bother at the moment... as i suggested i think orbital colonies will be the first, but i think that offworld colonies will include more adaption of humans than they will involve terraforming. (though concept of Geoengineering will surely be tested - i think a non-rotating (zeroG) orbiting space colony will be easier to build, and you can't change the gravity on Mars...
How much gene transfer will occur is a question, and population sizes will start off pretty small but eventually we'll leave this rock.
It is likely that in the past large geographical distances and lack of safety/ability to travel seperate hominids, i postulate that the next time this will happen is when we can again expand our territory/habitat. (because at first these new colonies will be greatly seperated transport will again be dangerous and time consuming)
This seems more likely than an overturning of our current political institutions (though perhaps i'm not looking at the big picture and being very optimistic - it's entirely possible that civilisation will collapse and we will again find travelling around the world both dangerous and time-consuming...or that standard conventions of human rights will be thought of as a backward failed ideology...)