Evolution is a topic that has been found to be notoriously difficult for people to fully comprehend. One possible explanation for this difficulty could be the clash between aspects of evolutionary processes and the way people intuitively think about plants and animals. This informal knowledge, known as folk biology, involves assumptions about species and interrelations among them. This study evaluates the relations between students’ folk biological and evolutionary knowledge and reasoning. Undergraduate General Biology 1 students (N=267) were given assessments of evolutionary understanding and folk biology (e.g. rigidity of categories, acceptance of variability, essentialism). Results showed systematic relations between the two: students who demonstrated higher evolutionary comprehension were more likely to believe that category membership was absolute, and more likely to believe that traits could be environmentally influenced. Likewise, students who showed more evolutionary misconceptions were less likely to endorse absolute category membership, and more likely to see category members as homogenous. These findings document ways in which students’ folk biological beliefs may both support and interfere with the acquisition of evolutionary knowledge. More generally, results support a growing body of evidence about the importance of accounting for pre-existing informal beliefs when teaching science, and suggest promising directions for future research on how such concepts may be incorporated into the biology curriculum.