What is Parsimony?

Among all the trees that could relate a set of taxa, the most parsimonious is the one that requires the fewest evolutionary changes of character state — the smallest tree length, the sum over characters of the minimum number of changes each needs on that tree. A character that fits the tree with one change is consistent with it; one that needs two or more is a homoplasy on that tree. Parsimony is not a claim that evolution is economical; it is the rule that a hypothesis should not multiply convergences beyond what the data force.