Parsimony means efficient simplicity. In science and philosophy, it often guides the quest for the shortest, cleanest path or explanation. This word comes from the Latin verb parcere – to spare. It is usually a safe bet that most series of events proceed under the rule of ‘common sense’ where waste of energy and/or time is minimized. Understanding and theorizing about such processes can benefit from the heuristic of parsimony.
One example of parsimony-guided analysis is found in an area of evolutionary biology called phylogenetics. Here, closeness of relationships is determined by counting the number of evolutionary changes between taxa (groups of organisms). For example, DNA base pair differences can be enumerated to hypothesize a most likely ancestral tree with speciation, hybridization, and extinction defining the branches. Even for just a few taxa, many different phylogenetic trees are possible. Applying parsimony, the most likely is the one that requires the fewest genetic changes. This scheme offers the hope that an entire systematic taxonomy (tree of life) can be compiled.
Parsimony is a valid heuristic in comparing phylogenetic trees because each tree results from the same theory (evolution) in general, and ancestry in particular. However, parsimony must not be applied when comparing competing theories. This can give misleading, and sometimes completely wrong, results. The only basis for comparing theories is each one’s ability to explain current observations and make correct predictions, not their elegance or parsimony. Parsimony cannot be used as a logical principle. For example, it is a fallacy to use parsimony to argue against what is in fact a fundamental requirement for life: complexity.