Suppose that a single specie of sheep is gradually separating into two, and that the two parts learn to graze separately on opposite sides of a large mountain. Call the two extremes the Left (L) population, and the Right (R) population. There is never a 0% Fertility Ratio (FR) between L and R, at these extremes of contact, as they diverge for some length of time, yet have not become 100% relatively infertile. That is to say, if we arbitrarily bring together members from each of L and R, there wonít be COMPLETE INFERTILITY for a long time. Maybe there will be a 30% success in fertile offspring, or FR, at 10,000 years. As the separation time becomes longer, there may be only a 10% FR, then a 3%, then a 1%. Finely, the FR becomes identically zero. Do we have all of a sudden, a new specie? Would you call the population R, having a FR of 30% with respect to (WRT) the L, a new specie? What about at the 10% or 3% FRís? You see, there is no smooth transition between the FRís of 100% and 0%. Consequently, the TRANSITION TO A NEW SPECIE is not in discrete steps, but is a quasi-continuous process! It is at these, in-between, times that some want to call TRANSITIONAL FORMS (TF), but what if I take the in between point of these extremes, the Center (C). Those in C will evenually have an FR of 0% WRT both L and R. But at some time before that, it will interbreed with an FR that is greater than 0%, but less than 100%, with each end. Can you see where this is going? We can talk of the transitional steps, all-right, but the concept of SPECIE is far from a discreteness that must be forced into Natureís vocabulary!
Both the allopatric speciation and the more general discussion on the other thread are right on the money. This is precisely what is actually observed. The point that many creationists seem to miss is that it isn't a species that diverges magically into another. Rather, two isolated populations of a single species gradually accumulate differences. If the populations are in contact, there will be an intermediate area on the border of their ranges where genetic mixing can occur. Eventually, groups or populations far from each other will have developed either pre- or post-zygotic barriers that permit scientists to identify them as separate species. As you noted, this is merely a convenience. In reality, there is likely a gradation (at least if the populations are in contact) of allele frequency between the two groups (c.f., ring species, etc).
Creationists, who are career biologists, are in complete agreement! Of course speciation happens this way!
All we say is that the observed life on earth is due to this gradual process acting on populations of created kinds. Nothing more, nothing less.
The process you are describing did not necessarily lead to the origin of brains from no brain, immune systems from no immune system etc. You believe it did, we believe God created these things and then let them rip*.
* 'Let them rip' may be Australian slang (?) - it means 'let them go and do what they're supposed to do'.
[This message has been edited by Tranquility Base, 09-15-2002]