Students' explanations of biological phenomena are frequently characterized by disconnects between levels and confusion of levels. The purpose of this research is to investigate the effects of a hands-on lab activity that aims at fostering the ability to reason across levels. A total of 197 students (18 years of age) participated in a randomized, pre-post-test design study. Students in the experimental group engaged in a lab activity focused on artificial selection and designed to demonstrate how selection affects both phenotypes and genotypes. In contrast, the lab activity in the comparison group focused on phenotype alone. Data sources for the study included pre-tests of basic concepts in genetics and evolution and two post-test items requiring the students to reproduce and apply their knowledge about artificial selection. The findings indicated that the lab activity which allowed students to explore the interplay between different levels, provided vertical coherence and enhanced students' ability to explain evolutionary change in both reproduction and transfer items. In contrast, the lab activity in the comparison group failed to do so, and most students did not improve their ability to explain evolutionary change. Implications for instruction and recommendations for further research are discussed in light of these findings.