This is a good question, although coffee house is not the right place for it, it's clearly a legitimate 'Biological Evolution' topic.
The chances are that such a supernumerary finger could be the result of an environmental effect during development rather than the sort of heritable genetic mutation involved in evolution. I could be wrong, but you would need some familial history to find out.
Having said that there are plenty of genetic mutations which are known to produce polydactyly in humans.
In a situation such as you described with the snails then a genetic mutation leading to an additional small finger might well come to prevail, provided that there really was any sort of fitness advantage which I don't think your hypothetical example really shows.
Even were the extra finger to become predominant I see no reason why it would lead to speciation in terms of two co-existing reproductively isolated populations, unless the people decided for themselves to practice segregation on the basis of finger number. Whatever the human race is like in a million years, in the unlikely event we make it that far, it would very likely constitute a distinct species to modern man, simply because of the genetic change which is bound to accrue over such a long period. Such a hypothesis cannot be supported however due to the impossibility of testing interfertility between populations separated by such large spans of time.
Anyway, tell me, knowledgeable people here, are teratological features facts of evolution as intended by the theorists of the evolution theory?
In general no, in specific instances yes. But not 'facts of evolution' like an organism magically growing a new organ from nowhere but as in a fact regarding the role that genetic mutation has in producing phenotypic variation and a fact relevant to the role of the genes affected by the mutation on normal development and possible repercussions on our view of how that gene may have changed during evolution.
TTFN,
WK