Whereas ‘traditional’ evolutionary biologists believed in the omnipotent power of evolution by natural selection (i.e. adaptationist programme, Gould & Lewontin 1979), emphasis in recent years has been on processes that slow down or prevent adaptation in order to understand the actual mechanism of evolution. Examples of processes that slow down or prevent adaptation to take place are genetic, developmental and phylogenetic constraints, and evolutionary trade-offs. My main interest lies with the latter. To be able to test for the existence of evolutionary trade-offs and detect the potential underlying mechanism, I use an explicit ecomorphological approach. This consists of splitting up the adaptation process in two components, the performance gradient and the fitness gradient, as suggested by Arnold (1983). Although Arnold’s conceptual framework was originally intended for studying adaptation at the intraspecific level, the same approach can be used in interspecific comparisons. Instead of measuring how variation in design and performance affect Darwinian fitness, interspecific studies address the question whether trait variation among species represents adaptation to different lifestyles (Emerson & Arnold 1989, Garland & Losos 1994, Irschick & Losos 1999). However, in interspecific comparisons a trait can not be regarded an adaptation without taking the species’ historical background into account. Therefore, I use the comparative method (Felsenstein 1985, 1988, Harvey & Pagel 1991, Losos & Miles 1994) to analyze all interspecific data statistically.
As a model system I use locomotion. There are several reasons for this. Firstly, the notion of evolutionary trade-offs seems particularly relevant to the evolution of locomotion because the same design features (e.g. limb dimension, muscle fiber type) affect several components of locomotor performance (e.g. sprint speed, endurance, manoeuvrability) simultaneously. Secondly, locomotion is ecologically relevant for many organisms, i.e. locomotor capacities are (expected to be) important for an individual’s fitness. Therefore selection acts on locomotor traits. Thirdly, with regard to locomotion, fairly simple biomechanical considerations (e.g. based on whole body modeling, simple lever systems) can improve interpretation of the mechanistic basis of the co-variation between morphological variables and locomotor performance.
As model organisms I use lizards because different locomotor tasks have been shown to be ecologically relevant in these animals, the variation in locomotor traits has been demonstrated to be (at least partly) genetically based in some lizard species, and different locomotor measures are inter-correlated. From a practical perspective, assessing locomotor performance traits and their morphological correlates is relatively easy in lizards, making it possible to gather data for large sample sizes.