Parsimony

Definition

An optimality criterion in phylogenetic inference that favors the evolutionary hypothesis requiring the fewest character-state changes; the principle that the simplest explanation consistent with the data is preferred. In practice, maximum parsimony selects the tree topology that minimizes the total number of evolutionary steps (e.g., nucleotide substitutions, morphological transformations) needed to explain observed variation among taxa.

Etymology

From Latin parsimonia, meaning thrift or frugality; adopted into phylogenetics to connote economy of evolutionary explanation.

Example

When reconstructing relationships among ant genera based on wing venation and mandible structure, a parsimony analysis would prefer a topology requiring 15 homoplasious character-state reversals over one requiring 25, even if the latter were biologically plausible.

Synonyms

• maximum parsimony
• Occam's razor (in broad philosophical usage)

Related Terms

• maximum likelihood
• Bayesian inference
• Homoplasy
• character state
• tree topology
• optimality criterion
• Cladistics

Usage Notes

Parsimony is one of several optimality criteria; it contrasts explicitly with model-based approaches (maximum likelihood, Bayesian methods) that incorporate explicit evolutionary models and may better accommodate homoplasy. Parsimony is particularly influential in morphological phylogenetics of arthropods, where complex character suites (e.g., genitalic structures, chaetotaxy) are coded as discrete states. The method is computationally efficient for small datasets but can be statistically inconsistent under certain conditions (Felsenstein zone). Specialists distinguish between 'parsimony' as a principle and 'maximum parsimony' as the specific algorithmic implementation.