The process of reflective writing can play a central role in making meaning as learners process new information and connect it to prior knowledge. An examination of the written discourse can therefore be revealing of learners' cognitive understanding and affective (beliefs, feelings, motivation to learn) responses to concepts. Despite reflective writing being an important learning tool, the role of this genre in upper-division college biology courses has not been well studied. This paper examines how nineteen physiological ecology students wrote about their understanding of natural selection and adaptations in ten reflective essays and describes how a model of student meaning making was developed. Qualitative essay analysis (through a triangulation of data: class observations; essays; and transcribed interviews) revealed that students could be classified into four categories of writers: "subjective" (personal, affective connections); "objective" (conceptual, cognitive connections); "authentic" (both affective and cognitive connections); and "superficial" (no supportive connections or claims). In-depth case studies illustrating these four categories are presented. Implications for college science instruction are discussed.