Specialization and evolutionary longevity in the Arthropoda

The phenomenon of selective biotic extinction is commonly explained as the result of the extinction of specialized organisms and the survival of generalized organisms during a time of environmental stress. Using 48 orders of aquatic free-living arthropods, we test the hypothesis that generalized organisms tend to have longer geologic ranges than specialized organisms. The degree of specialization, as expressed through morphological complexity, is measured by the diversity of limb pairs on the arthropod body. The resulting “tagmosis value” reflects the functional differentiation of limb pairs to perform specific tasks and thus may estimate ecological specialization. No statistically significant correlation exists between an order's typical degree of morphological specialization and its geologic range. Degree of specialization is not an important determinant of evolutionary persistence at this taxonomic level. Evolutionary longevity may depend on the adaptive type (e.g., trophic role, relationship to substrate, etc.) represented by the taxon and on the assortment of these adaptive types.