Conflict in the Hypaxial Musculo-Skeletal System: Documenting an Evolutionary Constraint

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Abstract
The identification and demonstration of an evolutionary constraint is suggested to be a four step process: 1) recognition of a possible mechanism of constraint, 2) formation of an historical scenario of the consequences of the constraint, 3) elucidation of the causal mechanism in the modern analog or model, and 4) phylogenetic correlation of the constraint with theproposed effect in extant lineages. Steps 1 and 2 represent the formation of two interdependenthypotheses, and steps 3 and 4 are tests of those hypotheses. This approach is illustrated with an example from the musculo-skeletal system of tetrapod vertebrates. Consideration of the anatomy and mode of locomotion of lizards led to the hypothesisthat they may not be able to run and breathe at the same time. Analysis of the pattern of ventilatory airflow of lizards supports this hypothesis. Tidal and minute volume increase above resting levels during slow walking (i.e., speeds below 10% of maximum running speed), but decline rapidly at higher speeds. Furthermore, electromyographic monitoring of the hypaxial muscles indicates a clear conflict between locomotor and ventilatory functions. Key anatomical characters, suggested to be responsible for the conflict, can be traced back to the earliest tetrapods. The organization of the two extant lineages that do breathe while running (i.e., birds andmammals) suggests that the evolution of an ability to breathe during locomotion required modifications of the ancestral configuration that separate locomotor and ventilatory function.

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