Homoplasy: The Result of Natural Selection, or Evidence of Design Limitations?

Similarity in morphological form may arise from common ancestry (failure to evolve), from parallel evolution, from convergence, or from reversal to an apparently ancestral condition. Homoplasy from convergence, parallelism, and reversal is common, and its ubiquity creates difficulties in phylogenetic analysis. Convergent evolution often is considered one of the most powerful lines of evidence for adaptive evolution. But an alternative explanation for convergence and other evolved similarities is that limited developmental and structural options exist. Identical forms can be obtained when particular developmental phenomena are triggered by very different kinds of stimuli or when constraints exist that shape external form or limit morphological expression to a few options. Examples from plethodontid salamanders are used to illustrate an approach combining internalist and externalist analytical methods. In order to explain how morphologies evolve in lineages, both functionalist and structuralist approaches are necessary, combined in a context in which phylogenetic hypotheses and their tests are continuously pursued. When homoplasy is rampant, as in salamanders, we can expect discordance with phylogenetic analyses based on nonmorphological data sets.