Stephanie E. Pierce
Thomas D. Cabot Associate Professor of Organismic and Evolutionary Biology
Curator of Vertebrate Paleontology in the Museum of Comparative Zoology
Functional adaptive landscapes illuminate major transitions in vertebrate evolution
Abstract: The concept of the adaptive landscape has been an invaluable tool to the field of modern evolutionary biology by providing a representation of how fitness and selection vary within populations of organisms. Trait variation can be visualized as a multidimensional “landscape” (analogous to a mountain range) with height on the landscape corresponding to fitness; selection acting on populations is expected to drive populations “up-hill” to cluster around “adaptive peaks” where fitness is maximized. Although originally implemented in the study of population genetics and microevolution, former Harvard paleontologist G.G. Simpson expanded the idea of the adaptive landscape to explain phenotypic change over macroevolutionary timescales. Specifically, he linked large-scale biological and geological processes, such as extinction and climate change, with shifts in adaptive peaks representing selection acting upon phenotypic traits that were of “functional benefit”. Despite decades passed, the practical application of Simpson’s adaptive landscape in macroevolution has been limited by computational challenges in quantifying the functional performance outcomes of morphological traits and how they vary across a vast phenotypic landscape. Here I will introduce a new method called “functional adaptive landscape analysis” which determines the functional trade-offs associated with morphological adaptation to differing selection regimes. I will then use this method to explore two major transitions in vertebrate evolution. First, I will tackle the fish-to-tetrapod transition and the origin of land locomotion by modelling the evolution of the humerus bone. I will show that the earliest tetrapods occupied a performance valley between water and land adaptive peaks, but that they may have had some capacity for terrestrial locomotion. Second, I will discuss the ‘reptile’-to-mammal transition and the evolution of the mammalian backbone. Using vertebral morphometric data and experimental biomechanics, I will test the lateral-sagittal functional paradigm and demonstrate how this long-held idea is too simplistic to explain mammalian backbone evolution. Together, I will establish the utility of functional adaptive landscapes in quantitatively testing longstanding questions in the vertebrate fossil record and its potential application to unravelling the relationship between form, function, and adaptation across deep time.