I guess I'll respond to my own post, since no one else did. In the example phylogeny the character that groups the taxa A and C would require 3 steps if it were plesiomorphic, but only 2 steps if it were convergent. If we use a parsimony approach to reach a conclusion then the group A C would be considered polyphyletic. This example was actually easier to resolve than I first thought. I'll have to try to recall an example where the hypotheses of plesimorphy or convegence are ambiguous and this renders the distinction between para and polyphyly ambiguous too. I think have come across these at times when optimizing characters in MacClade, but it may take a little time to actually come up with one. Under a statistical model it is possible that the two alternatives are equally likely (using maximum likelihood or bayesian techniques) even in this case. I will work on this a little in my spare time and if I come up with an example that is ambiguous under parsimony, I'll post it. If anyone else can think on one please post it. Otherwise I might have been proven wrong.

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