Population growth presents a unique opportunity to make the connection between mathematical and biological reasoning. The objective of this article is to introduce a method of teaching population growth that allows students to utilize mathematical reasoning to derive population growth models from authentic populations through active learning and firsthand experiences. To accomplish this, we designed a lab in which students grow and count populations of Drosophila over the course of 12 weeks, modifying abiotic and biotic limiting factors. Using the data, students derive exponential and logistic growth equations, through mathematical reasoning patterns that allow them to understand the purpose of these models, and hypothesize relationships between various factors and population growth. We gathered student attitudinal data and found that students perceived the lab as more effective, better at preparing them for lecture, and more engaging than the previous lab used. Through this active and inquiry-based method of teaching, students are more involved and engaged in both mathematical and biological reasoning processes.