What do you think about the AGP III system for flowering plants? http://onlinelibrary.wiley.com/doi/10.1111/… The approach, mainly molecular-based, has reorganized a few plant famillies, earlier classified based on more classical parameters such as those discussed here. It seems that defining what a species is difficult because the living world will always come with a specific example conflicting with the definition - therefore the amount of definitions existing for a species depending on the approach. I still could not find THE definition of a species, but if someone knows it, I'll be glad to get closer to God ;-)