Links between species

[genovese]

Apologies for this basic question which intrigues me.

A species can be defined as a group of organisms capable of interbreeding and producing offspring.

Is it possible to identify an individual within a species that is in transitional form to becoming another species? How many differences in DNA base pairs are needed to show a transition potential or effect actually in progress?

If some members of species A are in transition to becoming species B, then presumably they will be breeding at one time with species A until the transition is complete and will then only be able to breed with species B. Is it possible for the organism in transit to be able to breed with both A and B and produce fertile offsprings in both?

[alextemplet]

I've read in a couple of book about species called "ring species" that basically display the characteristics you're talking about. Basically, assume a species is divided into several subspecies. Subspecies A is the original type, and subsecies C is on its way to becoming a new species; subspecies B is a transitional form. A and C cannot or do not interbreed, but B can and sometimes does interbreed with both A and C. The only reason A and C are still classified as the same species is because of the link through B; once B goes extinct, A and C can probably be considered reproductively isolated and thus separate species.

I hope that doesn't sound too confusing; I've read about that in a few books and Finding Darwin's God by Dr. Kenneth Miller probably has the best explanation of the concept that I've read so far. Miller cited a few examples of species that display those characteristics today, if you're interested.

[genovese]

Thanks for that Alex