Using parsimony (2 of 2)

What is parsimony?
The tree-building process explained above is based on the principle of parsimony. The parsimony principle is basic to all science and tells us to choose the simplest scientific explanation that fits the evidence. In terms of tree-building, that means that the best hypothesis is the one that requires the fewest evolutionary changes.

For example, we could compare these two hypotheses (right) about vertebrate relationships using the parsimony principle:

Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a bony skeleton evolving independently, twice. Although both fit the available data, the parsimony principle says that Hypothesis 1 is better — since it does not hypothesize unnecessarily complicated changes.

This principle was implicit in the tree-building process we went through earlier. However, in most cases, the data are more complex than those used in our example and may point to several different phylogenetic hypotheses. In those cases, the parsimony principle can help us choose between them.