All right, I have finished reading "The Greatest Show on Earth" and "Climbing Mount Impossible".  Next on my reading list is "The Blind Watchmaker", after which I hope to finally tackle "The Selfish Gene" (and yes, I realize I am doing this backwards).  Hopefully, this will stave off the "why don't you just go read a book" responses the last time I asked a question about evolution...

My current understanding of "speciation" is that it occurs when two segments of a population diverge sufficiently that the members of the two segments either cannot or do not interbreed.  The "do not" bit confuses me a bit, since sometimes it seems to involve something as seemingly trivial as slightly different-colored plumage and it's hard to see why one group should be considered a different species if it is actually cross-fertile with another group (via in vitro fertilization, say) and the resulting offspring is not sterile.  That  may or may not be relevant to this discussion, however.

With regard to what causes speciation, my current understanding is that it usually happens when populations become separated and isolated from other populations of the same initial species, whereby natural selection then has a chance to work differently on the different groups over time.  I have also read that this process can actually occur over relatively short time spans (hundreds of years or even less), especially for organisms with short breeding cycles.

Finally, my current understanding is that the wide variation observed between the various "races" (and I use the term cautiously) of humanity are the result of different groups becoming separated and isolated for tens of thousands of years.  I don't really want to get into a discussion of what "race" actually means or how sharply the lines between different races are currently defined, but I think it's safe to say that natural selection worked differently on populations who lived in extreme northern climates compared to those who lived in the tropics, for example.  And yet, despite this fact, we are all obviously the same species since we can (and do) interbreed with no problems (my wife and I are different races entirely, and yet we not only wanted to interbreed we also have successfully done it...)

So here's my question.  Assuming everything I just said is accurate, why don't we have multiple species of humans?  Based on everything I've read, the separated populations that gave rise to the different races were certainly isolated long enough for speciation to occur.  And yet, it apparently didn't.  I've given this a lot of thought and have come up with a few possible answers:

  • Since humans have longer life spans and breeding cycles than, say, finches, thousands of years of isolation isn't really long enough for human speciation to have occurred after all. [Do we have evidence of other animals with long life spans that speciated over shorter time periods?]
     
  • Since humans have evolved intelligence, we are no longer driven solely by chemical and visual clues that prevent other animals from interbreeding with slightly different members of the parent species.  [Do we have evidence that other "intelligent" species such a dolphins or chimpanzees have also avoided speciation?]
     
  • Perhaps humans were already almost completely adapted to their environment at the point that populations started becoming isolated, so any further changes after that point would necessarily have to be minor. [Is it fair to say that adapting to life on the sub-Saharan plains is anywhere close to being "completely adapted" to life in the Arctic Circle?]

Any thoughts?  I realize there may not actually be a definitive answer, but I'd love to see what other people who know more about the subject think about it.