VESTIGIALIZATION AND LOSS OF NONFUNCTIONAL CHARACTERS

Reduction and total loss of characters are common evolutionary phenomena. Vestigialization of any morphological, physiological, or behavioral feature can be expected upon relaxation of selection on the trait. Direct selection of vestigialization is rarely documented. Most explanations of evolutionary reductions invoke indirect selection through energy economy or antagonistic pleiotropy arguments, while some invoke the effects of accumulation of neutral mutations. A few documented cases of trade-offs between fitness and wing reduction or pesticide resistance in some insects, and between fitness and resistance to phages or antibiotics in bacteria suggest that indirect selection is a plausible mechanism for evolutionary reductions. Expression of presumably useless genes suggests that neutral mutation arguments require a longer time than is available for the observed reductions. Rapid decay of useless behaviors may require explanations in terms of trade-offs among neural pathways for information processing.