[Matt R.]

Magic_Man,
Interesting scenario.

I think it certainly is possible, yet extremely difficult and unlikely. Basically, it requires a lot of assumptions.

First, you are assuming that this recessive mutation with "no other noticeable effects" can make the individual fertile (or more fertile) with other carriers of the gene and less fertile with non-carriers. It is really unlikely that a single mutation could have such a large impact on the overall reproductive process. It could make them infertile, sure. It could also make them more fertile with respect to some other members of the population. But for it to make them fertile ONLY with other carriers would take quite a mutation. At the very least I would think a phenomena like this would take dozens of interacting genes, probably a lot more. Also note that when you throw in those interactions it increases the complexity exponentially. One mutation can have thousands of potential phenotypic effects, depending on the genetic background.

And even if you have this one mutation, or series of mutation, which increases fertility only with "carriers" of this mutation, you have to consider what happens when the carriers inevitably mate with others of the "non-carrier" set. Now the gene is simply increasing in frequency throughout the population. If the gene is advantagous, it will tend to continually increase in frequency in the population... if it is neutral then random genetic drift is the deciding factor. Therefore this one gene that you're hypothesizing will cause speciation is no longer in just the little subset of carriers. In fact, the "subset" is somewhat ill-defined since they CAN still mate with non-carriers... if they couldn't then we'd already have another species! [img]/images/graemlins/tongue.gif[/img]

Basically, what tends to happen in your scenario is the gene tends to increase in frequency (if advantageous... if not then decrease in frequency) and it causes the ENTIRE population to change. For it to really cause speciation, it would pretty much have to cut that subset off (reproductively) from the rest of the population... which is the barrier to gene flow that I alluded to earlier. I think it would be extremely unlikely for a single mutation or a small set of mutations to cause something like this.