Macroevolutionary patterns of adaptation are a product of natural selection acting on genetic and developmental variation within populations, the basis of microevolution. In microevolution, an adaptive landscape is used to visualize the relationship between phenotype and fitness, through a series of peaks and valleys. The adaptive landscape, as a concept suggests that there is some phenotypic optimum, or a combination of phenotypes, that result in a maximum fitness. This peak is not stable but is a reflection of interactions between the environment and the flora and fauna within. To expand the adaptive landscape to macroevolutionary scales is to assume that there is some optimum that a species or population is adapted to, and that numerous species can be compared to one another on the same landscape. The world of phylogenetic comparative methods uses the theory of the adaptive landscape in investigating the trajectory of trait change but is often limited to extant organisms. The fossil record often represents a major gap in the use of adaptive landscape theory, due in part to the incomplete nature of specimens or difficulties in untangling evolutionary relationships. Within this gap, the Triassic Period (252.2 – 201.5 MA) is sparsely represented, due to the often highly incomplete nature of Triassic fossils and our constantly evolving understanding of their phylogenetic relationships. However, the Triassic Period is bookended by mass extinctions, and is thus a useful case study to explore the utility of adaptive landscape theory for organisms in a time of rapid environmental change. My dissertation explores convergence and growth through an adaptive landscape framework, to reconstruct how species were evolving, or populations adapting, to a changing environment.
The first chapter of my dissertation explores the evolution of a long snout in reptiles, with exploration of convergent evolution for both extant and extinct reptiles across the tree of life. The second chapter of my dissertation explores a statistical method to incorporate variation due to fossilization in estimating and quantifying growth curves. This second chapter was necessary to explore the third chapter of my dissertation, the ontogeny of a large-bodied mammal relative, Exaeretodon argentinus. With my third chapter, I quantify growth curves, compare them across other proto mammals closely related to Exaeretodon, and explore how diet may have changed over the lifetime of a single individual. These chapters focus on the adaptive landscape over different scales (population vs clade), and serve as a basis for future work in estimating dietary evolution.
Doctor of Philosophy
Evolutionary studies are often divided into macroevolution and microevolution. Microevolution can be thought of as the primary process of evolution, such as evolution by natural selection. Macroevolution, however, relates to patterns in evolution, such as the repeated evolution of flight in insects, birds, bats, and many extinct reptiles known only from fossils. Surprisingly, we lack any convincing models that can accurately relate evolution by natural selection with macroevolutionary patterns. One idea to bridge this gap is the adaptive landscape, an 80-year-old concept that relates a trait to the fitness of a population or a species. The adaptive landscape provides a visual representation of evolution through a series of peaks and valleys. Peaks embody high fitness, reflecting a population that is well adapted to their environment. Valleys on the other hand represent extinction traps, where populations are maladapted to their environment, and are in decline and at a higher risk of extinction. The adaptive landscape essentially quantifies how different traits of a population relate to their environment.
My dissertation seeks to explore evolution through the lens of the adaptive landscape, to better reconstruct hypotheses about how animals interact with their environments, particularly in extinct animals known from fossils. I view the adaptive landscape with a macroevolutionary lens by redescribing a fossil reptile and investigating the repeated evolution of a long snout in reptiles. I then attempt to look at the adaptive landscape at a microevolutionary scale, by investigating patterns of growth in a 230-million-year-old mammal relative. My dissertation views evolution across scales and sets the stage for future work to better connect those scales.