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Variation in the limb skeleton was studied in a large sample (452 individuals) of adult newts (Taricha granulosa) from a single population in central California. A standard morphology is found in the hands and feet in more than 70% of individuals, but there is important variation in the remainder of the sample. Although twice as many individuals express variant tarsal (18.9%) than carpal (9.3%) patterns, the carpus reveals a greater diversity of variant conditions. Only a few of the many possible arrangements (including loss, addition, and amalgamation) of the seven carpal and nine tarsal elements are encountered. Most of the observed patterns are readily classified on the basis of phylogenetic relationship and models of development. Five patterns are bilaterally symmetrical. Two of these five arrangements are atavisms that represent plesiomorphic states. The other three symmetrical variants duplicate patterns found elsewhere among derived urodele taxa. These homoplastic patterns correspond to apomorphies of other genera within the same family or apomorphies of deeply nested and highly specialized members of other families (e.g., several plethodontid genera). This variation reveals the potential of the generative system and suggests the existence of limited domains of phenotypic expression. Both the origin of novelty and the restoration of ancestral states reflect the action of design constraints that not only place limits but also provide opportunities during the morphologic and phyletic diversification of urodeles.
Shubin et al. (Sun,) studied this question.