I would agree with Randy that it is important to consider that in a given population there is always variation, and not just single gene changes that lead to this variation. Two siblings differ at a number of genes, not just one. Between any two species there are many diferences, and speciation can result from a number of different mechanisms. Two parents having a new species offspring is generally not one, at least for large complex organisms. A great example of speciation occurring among salamanders in California illustrates the point. You see that the parent species is at one end of the state, whereas the newly forming species are on either side of the desert. Over time, there will be three species where there was originally one. Now, you can see how it is not correct to ask who the original two parents of the new species were. Here is the link to that video. http://www.youtube.com/watch?v=YCoEiLOV8jc Of course, many will say "that's microevolution, not macroevolution." Interestingly, most biologists don't find the term microevolution all that useful. This distinction really misses the point: These are the mechanisms that logically will inevitably lead to changes in species, and if the fossil record and genomic evidence is to be believed, it is reasonable to infer that these smaller changes do accumulate to bring about remarkable changes.
One can imagine that in some cases it might happen that a very few founder parents could give rise to a new species, in cases of founders to an oceanic island, for example. That would be a different mechanism, and again the changes giving rise to new species (think Galapagos finches) will not come suddenly from two individual parents having that new offspring, but rather due to the differential survival of the great grandchildren and great great great great great...etc grandchildren of those founder parents, with the selective pressures "molding" the direction of the changes over long periods of time.
If you want to learn more about natural selection, you can do worse than actually reading (or listening to Librivox audio version) of On the Origin of Species by You know Who. It's actually a very understandable and quite remarkable book (that is an understatement). http://librivox.org/the-origin-of-species-by-charles-darwin/