Parallelism and Integrated Design in the Foot Structure of Gekkonine and Diplodactyline Geckos

Abstract

The Gekkonidae is divided into four subfamilies (Eublepharinae, Diplodactylinae, Gekkoninae and Sphaerodactylinae) and various interrelationships of these taxa have been proposed. On a cladistic basis the Diplodactylinae may be regarded either as the sister group of the Gekkoninae or the sister group of the Gekkoninae and Sphaerodactylinae. With respect to diversity within the family the diplodactylines and gekkonines have occupied the greatest number of habitat types and their locomotor systems have developed independently but in similar ways. These two subfamilies have radiated largely in isolation from each other and in many instances parallelisms have occurred. With respect to locomotor mechanisms these parallelisms may be linked with the possession of a key innovation-a spinose Oberhautchen. Much of the apparent similarity seen is due to the way of life of the organisms concerned and within the family near duplicates have been produced in isolation. This is particularly evident in foot structure and reflects the method of contact of the organism with the substrate and exemplifies the interdependence of substrate type, locomotor pattern, morphology and behavior. The present paper attempts to delineate some functional correlates associated with different foot types. Subdigital, pilose pads have probably evolved on several occasions in the Gekkoninae but appear to have arisen only once in the Diplodactylinae. In both subfamilies secondary pad loss appears to be correlated with secondary terrestrialism but there has also been an adaptive radiation of primitively padless forms in both groups. Comparison of the structure of subdigital pads in both subfamilies shows how the various parts of the system are interrelated and interdependent and illustrates which parts of the system are fundamental to the successful operation of this type of adhesive mechanism and which parts are reflective of independent solutions of the same basic problems. The mechanical units of the foot are closely functionally integrated and with respect to the process of adhesion exhibit a continuity of pattern, through morphotypic series, reflecting continued functional efficiency in the transition between locomotor modes. The various attempts to solve similar mechanical problems have resulted in slightly different configurations of mechanical units. Similar selective pressures in different regions have presumably resulted in similar adaptive radiations in which many apomorphic character states occur in only slightly different configurations.