One evolving species vs speciation.

Given the requirement for some type of isolation to arise within a population before speciation can occur (at least in the vast majority of multi-cell organisms), I'd say that your two scenarios were about the only way it could occur in humans. Emigration off-world or some type of catastrophe that geographically separates huminity into isolated fragments would be about the only way speciation could happen. With the trend today being quite the opposite (homogenization rather than isolation), I'm not sure how else it could happen. Edited by Quetzal, : No reason given.

My guess would be that increased radiation would not be required. The normal mutation rate would be sufficient if given long enough time spans. Of course, an increase in the level of ionizing radiation would likely increase the mutation rate - but probably at the same time decreasing the probability that anyone would survive.As to what type of catastrophe, my guess is it would have to be pretty global in scope and decimate the current population. I wouldn't want to hazard a guess as to how much, 'tho. It doesn't have to be an extraterrestrial "dinosaur killer" of course, or a global nuclear war. A nice airborne hypervirus could theoretically do the trick.

Depends, I suppose, on how far down the population gets reduced. The only example we have of human speciation is whatever caused the split between Homo sapiens and H. neanderthalensis, assuming they had a recent common ancestor. My guess is that it would require reduction to the point where you have isolated populations on each continent that have lost so much of the cushioning effect of "civilization" (for lack of a better term) that thousands of generations pass before reconnection occurs (if any). Especially given the fact that any surviving population of modern humans is likely going to have a pretty wide cross-section of the available human alleles due to the current effects of globalization.However, there are a couple of other factors to also consider in any such scenario. The most obvious is at what level would our species become non-viable. I don't know if anyone has done any modeling on a Minimum Viable Population for H. sapiens. If there is any correlation between us and, say, bonobos, then an MVP₁₀₀₀ would be 5-10,000 individuals in a freely inter-breeding population. Or more. Some models suggest long-term viability (i.e., greater than MVP₁₀₀₀) would require 50,000 or more individuals in the starting population due to potential loss of alleles, etc.Another factor that might be important when we consider H. sapiens is that our species seems to have something of a wanderlust. IOW, given that it appears there have been at least one and possibly two "out of Africa" migrations in our lineage, it strikes me as entirely possible that there wouldn't be sufficient time to allow isolation and natural selection to "do their thing" before gene flow between catastrophically isolated populations is re-established.The science here seems to be pretty soft, if you ask me. I'm not sure how realistic any of the above might be when you look at our species. It just might be that the only realistic scenario for human speciation would be the one where utterly different selection pressures operate on a totally or mostly isolated population for a really really long time (i.e., the "other planet" scenario).

Once again, I'm not convinced that an increase in ionizing radiation is required (i.e., ozone hole). The key, as WK pointed out, is the time frame of the isolating event. I can see the possibility of both phylogenetic speciation (in the event that the species is reduced to a single, viable population for instance), and allopatric speciation as long as the isolating mechanism remains applicable over geological time frames. It just seems easier for me to visualize something like this happening if we moved a portion of our species off to another world.As you and WK both pointed out, it's a question of time. I agree with WK's speculation that re-acquiring technology from a "stone-age" start point might not take long enough for speciation to occur. Our species seems to have an amazing dispersal ability.

Now THERE'S an interesting speculation. I assume you mean through genetic engineering of some kind? Homo superior has long been a favorite speculation of sci-fi authors. I see no reason why a sort of sympatric speciation based on large scale genetic manipulation wouldn't be possible, as long as the two groups never interbreed - and the "enhanced" group bred true. Of course, you'd need a LOT of manipulation, then some kind of genetic or behavioral isolation for it to work. Kind of an "instant speciation" thing.