Aim The aim of this paper was to revise the historical biogeographical method for resolving complicated distribution patterns through a technique that has come to be called assumption 2. Assumption 2 was used to resolve multiple areas on a single terminal branch (masts) as well as paralogous and missing areas in two or more areagrams. Recent examples, however, have shown that assumption 2 may be using rather than resolving paralogy. The paper attempts to resolve this problem by formulating a separate procedure to avoid using paralogous (redundant) area data in area cladistic analyses.
Method The revision results in a new derivative method, the transparent method, to replace assumptions 1 and 2. It separates the procedures for resolving paralogy and for solving distribution patterns that occur in more than one area (masts).
Results Several hypothetical examples show how the transparent method reduces paralogy and masts. The results show that paralogy can be reduced if the paralogy subtree method is applied after uncovering all possible relationships as single components on the terminals of areagrams.
Conclusion The transparent method is a significant step forward in cladistic biogeography as it utilizes area relationships rather than generating general areagrams based on paralogous data.