Dr. Gabriel Rivera

Gabriel Rivera
NSF Postdoctoral Fellow
Ph.D., Clemson University
M.S., Old Dominion University
B.S., Old Dominion University

E-mail: grivera@iastate.edu

Link to CV

General Research Interests
My research is broadly grounded in the fields of evolutionary morphology and biomechanics. More specifically, my interests focus on understanding morphological variation, including the functional consequences of, and the evolutionary mechanisms responsible for it. My current projects focus on the role of aquatic flow velocity in generating intraspecific differences in turtle shell morphology.

Turtles as a Model System for Studies of Flow
Semi-aquatic freshwater turtles represent an excellent novel system in which to investigate the relationship between morphology and aquatic flow regime for several reasons. First, as tetrapods, turtles represent a phylogenetic lineage distinct from the osteichthyan fishes that have been studied to date. Second, unlike fishes which live exclusively in water and thus are maximally influenced by hydrodynamic characteristics of their habitats, freshwater turtles use both aquatic and terrestrial environments and thus have to balance pressures encountered in both media. Third, because the shell limits axial mobility, propulsion in turtles is limited to forces generated by movements of the limbs, which results in a decoupling between the morphology of propulsory structures and overall shape. In contrast, studies examining the association between flow velocity and the morphology of fishes have to interpret the complex interactions between modifications of the body and fins that reduce drag and those that increase propulsion. Lastly, studies examining patterns of intraspecific phenotypic variation across different flow regimes in freshwater turtles have found that shells display habitat-associated differences in morphology.

Current Projects
PROJECT 1: What are the relative effects of genetic and environmental factors (i.e., flow) on morphology?
A major step toward understanding phenotypic diversification is quantifying the importance of genetic divergence and phenotypic plasticity in producing divergent morphologies. However, determining the relative importance of these two sources of phenotypic variation on resultant phenotypes requires an experimental design that addresses both simultaneously. I am currently conducting a common-garden experiment in which I am raising hatchling river cooters (Pseudemys concinna) from different parental habitats (lentic and lotic) under different flow conditions for two years. I will use changes in shell shape to examine the effects of genetic and environmental factors on morphology. The results of this study will be used to test two predictions based on the available information from fishes, the current vertebrate model: (1) both genetic divergence and phenotypic plasticity play a role in producing phenotypic differences between flow regimes, and (2) genetic divergence plays a more important role than plasticity in producing phenotypic differences between flow regimes.

Project 2: To what extent does water flow drive repeatable and predictable phenotypic divergence?
Parallel evolution is considered strong evidence for the process of natural selection and is typically demonstrated by the production of similar phenotypes under similar environmental conditions across multiple independent lineages. In the southeastern United States, several physiographic features (e.g., Appalachian Mountains and Apalachicola River) and historical events (e.g., Pleistocene glaciation) have produced a major phylogeographic discontinuity between the Atlantic and Gulf drainages that is repeated in a wide range of taxa, including plants, mammals, amphibians, fishes, and freshwater turtles. This discontinuity divides the range of Pseudemys concinna into two phylogeographic lineages, those inhabiting Atlantic drainages and those inhabiting Gulf drainages. This system provides an excellent opportunity to test for patterns of repeated evolution across flow regimes for several reasons: (1) both drainages possess multiple lentic and lotic habitats, (2) I have identified patterns of phenotypic variation between the two flow regimes within Gulf coast drainages, and (3) preliminary evidence suggests that P. concinna within Atlantic drainages also show phenotypic divergence between flow regimes. I will be using shape data collected from turtles inhabiting multiple pairs of connected lentic and lotic riverine sites to determine whether the Atlantic and Gulf coast populations demonstrate the same morphological patterns.