real life wrote:In your example, X ~~~~~~~~~~ Z, Z doesn't have to be able to breed with the original X to be considered part of the species.
Yes, it does. If it can't breed with it, then it's a different species, by definition.
real life wrote:He only needs to be able to breed with living contemporary members of the species, who in turn met the same requirement when they were born, who in turn met the same requirement etc. stepwise back to the original X.
That is incorrect.
You are applying finite rules to infinite sets. Your logic is invalid.
Interestingly, the concept of a 'species' is only functional for us because there are no intact transitional lines going all the way back to an ancestor which is no longer able to breed with a descendant.
If such a line did exist we would not be giving names to different species.
Instead we would be saying that X and Z have become sufficiently different along the transitional line to be considered different species. The question of species would only be used as a comparison between two selected points in the transitional line.
In reality, life has no boundaries between generations no matter how deep. We create artificial boundaries by assigning names to certain conditions (breeding capacity being one), but the flow is seamless and endless.
