The evolution and design of the locomotor system in lizards of the southwest United States

Abstract

Understanding the adaptation and diversification of complex phenotypes is a central task of evolutionary biology. However, because many phenotypes are complex, constructed in a in a hierarchy of biological organization that often must perform multiple functions, studies frequently lack the detail required to fully understand the evolution of these systems. To gain a deeper understanding regarding the evolution of complex phenotypes, I integrate evolutionary modeling and functional morphology approaches to investigate locomotor evolution in lizards of the Southwest United States. I use evolutionary models based on habitat use, predator escape behavior, and foraging mode to examine what drives diversity in sprint speed, acceleration, and exertion in these lizards. I then examine how the locomotor system is designed for specific locomotor tasks. Because muscle provides the force required for locomotion, I focus on muscle cross-sectional area (CSA). I first examine the relationship between each performance and muscle CSA, and then explore how these relationships and CSA distribution vary with function (flexors vs. extensors) and anatomical location (hip, knee, and ankle). Finally, because the locomotor system is composed of numerous traits that can vary in their functional roles, I explore what drives the evolution of each trait individually. I model the evolution of each trait, and compare the best fitting modelsacross traits to determine how the locomotor system evolves as a whole. Overall, I demonstrate that multiple selective pressures act on both locomotor performance and morphology from the cellular to whole animal level. However, selection is not uniform within the locomotor system. Performance related selection acts across many locomotor traits, but some are more constrained or not subject to selection. Behavioral variation is an important driver of phenotypic diversity as behavioral shifts guide the evolution of performance, muscular design, and form-function relationships in these lizards. Finally, the complexity of the locomotor system may promote adaptation and diversification through the mitigation of potential functional trade-offs and allowing alternative locomotor designs to yield similar performance capabilities.