Why does phylogenetic analysis take so long?
When there are 4 species, there are 16 possible ways of arranging those species into a phylogenetic tree. For 7 species there are 945 possible trees and for 22 species there are 3×1023 possible trees. Because we have to evaluate different trees (using maximum likelihood or maximum parsimony for example), it can take an enormous amount of time to locate the best tree for many data sets.
