© 2000 by The Society for Integrative and Comparative Biology
Axial versus Appendicular: Constraint versus Selection1
1 Biomechanics Laboratory, Department of Orthopaedic Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
SYNOPSIS.Constraint theory has suffered from its many and varying definitions and from repeated confusion with the action of selection. Constraints are difficult to isolate, more difficult to quantify, and their consequences may be simulated by the action of agents such as stabilizing selection. This paper presents an alternative method of constraint analysis using a simple one-dimensional measurement. A total of 29 skeletal elements from 15 species representing 11 orders of mammals were measured for length and normalized for body mass, after which means, standard deviations, and variances were generated. A coefficient of variation analysis was performed to normalize for mean element length. The axial skeleton was found to be less variable than the appendicular. The appendicular demonstrated a trend where the more distal elements were the most variable, and variation decreased with more proximal positions. The three most variable of the 29 elements were finger V, toe V, and metacarpal V. In summary, the axial skeleton was found to be more conservative in the lengths of its elements, the more distal appendicular elements were less constrained than proximal ones and these constraints were probably the results of genetic, developmental, and mechanical factors. It is also proposed that stabilizing selection played some role in maintaining the curb on length variance in these structures, based on mechanical performance. The results of this study are intended to promote discussion of alternative methods of constraint analysis.