My research program studies the causes and consequences of phenotypic variation in nature. I use observational and experimental field studies, genetics and genomics, and digitization and quantification of museum specimens to answer questions at different biological and spatial scales and across vertebrate taxa. Much of my work concerns Sceloporus lizards, a radiation of ~100 species that occur across North and Central America, live in a variety of habitats, co-occur regularly, and reveal ecological and morphological diversity. They are an excellent group in which to ask questions about how phenotypic novelty arises and facilitates coexistence, and (more importantly) I really like them. My interests in phenotypic evolution and evolutionary ecology extend across vertebrate taxa, including but not limited to other squamate reptiles like colubrid snakes and Anolis lizards, as well as some mammals (especially bats and rodents). You can explore some of the major themes of my research below.
5 species of Sceloporus that can be found in the Chiricahua Mountains of Arizona. L to R: S. clarkii, S. virgatus, S. slevini, S. jarrovii, and S. cowlesi
How do closely-related species coexist?
Closely-related species should be more similar and thus more likely to compete for resources. So why do they co-occur so often? I quantify resource use and the traits involved (e.g., eco-morphological relationships) to assess how co-occurring congeners are able to partition resources and coexist via an experimental and observational field program. In my main study system, the Chiricahua Mountains of southeastern Arizona, I found that co-occurring Sceloporus lizards are adapted to partition microhabitat space (Westeen et al. 2023a), and doing so allows them to further partition thermal (Westeen et al. in prep) and dietary resources (Westeen et al. 2023b). I built upon my fieldwork using museum specimens and published ecological data to demonstrate a relationship between habitat ecology and morphology across Sceloporus and found that communities are ecomorphologically overdispersed, suggesting limiting similarity (Westeen 2024).
What drives the evolution of phenotypic novelty?
Why do some groups of organisms show striking diversity in size, shape, colors, and patterns between species, while others don't? How does the evolution of phenotypic novelty arise from processes like competition or predation? How does phenotypic evolution intersect with speciation? Much of my work involves quantifying phenotypes from field-caught animals and/or museum specimens, using techniques like microCT scanning and 3D morphometrics, and then leveraging comparative methods and natural history data to ask how and why different phenotypes have evolved. In Colubroid snakes, I found that fang phenotypes are labile and associated with prey capture method and diet (Westeen et al. 2020). In Anolis lizards, we showed that colorful female dewlap ornaments have evolved repeatedly and influenced diversification (Yuan et al. 2022) and that selection on females, potentially for signaling space, drives patterns of sexual dichromatism (Westeen et al. 2025).
microCT scans of snake skull and maxilla diversity from Westeen et al. 2020
How do phenotypes mediate organismal responses across environments?
For my NSF postdoc I will move towards a framework that mechanistically links phenotypes and environments via thermal and hydric physiology in Sceloporus. Leveraging ecomorphological replicates in this study system, I will experimentally determine whether shared phenotypes lead to predictable physiological responses across environments. I plan to use 3D morphometrics, high-resolution climate data, and physiological trials across species at multiple field sites to identify specific phenotypic and environmental factors that influence organismal responses. More soon!
How can natural history collections and databases be leveraged and expanded to understand patterns in ecology and evolution?
Much of my work uses large databases and especially museums to ask questions about species' traits, distributions, behaviors, diets, and more. Because of this, I am committed to documenting natural history phenomena that I observe in the field through publications, natural history notes, and public outlets (Martínez-Fonseca et al. 2024, von May et al. 2019 highlight work done in Nicaragua and Peru, respectively). I additionally contribute to museums and databases via the deposition of specimens, tissues, and associated natural history data. I have digitized hundreds of museum specimens through photography and microCT scanning, many of which are available via public repositories like VertNet and MorphoSource (e.g., Shi, Westeen, and Rabosky 2018). Further, I am committed to improving natural history science, and in particular fieldwork, by making it more equitable and ethical (Ramírez- Castañeda & Westeen et al. 2022). Simply put: my work couldn't exist without museums and natural history databases, and these resources rely on collaboration, documentation, and fieldwork!
Principles for equitable fieldwork as outlined in Ramírez-Castañeda & Westeen et al. 2022 and illustrated by Camila Pacheco Bejarano.