This $300,000 National Science Foundation project grant to Texas State University will support the development of assessments and models to advance understanding of evolutionary tree-thinking among undergraduate students. Funded under the STEM Education program (CFDA 47.076), the two-year award beginning January 1, 2023 aims to create an evidence-based assessment measuring multiple conceptions of evolutionary relatedness. Researchers will model factors influencing tree-thinking abilities and explore how undergraduates scan tree diagrams. Data from over 200 participants at two institutions will be used to validate the new assessment's fit, validity, and reliability. Modeling will examine relationships between spatial reasoning, evolution acceptance, understanding and tree skills. Eye-tracking will observe students' scanpaths relative to abilities. Outcomes seek to better explain influences on tree-thinking and provide research-informed tools to evaluate student thinking relevant to evolutionary relatedness.