Evolution by natural selection is key to understanding life and of considerable practical importance in public health, medicine, biotechnology, and agriculture. "The Next Generation Science Standards" (NGSS) include natural selection among several evolutionary concepts that all third-graders should know. This article explores a novel approach to developing and testing curricula for teaching natural selection and related concepts to children. College students developed lesson plans with specific evolutionary learning objectives based on the NGSS and taught them at elementary schools. Learning was assessed with a pre/post-test design, and a subset of students was retested after two years. After just two hours of instruction and active-learning activities, students of all three grade levels tested (grades 3-5) demonstrated substantial improvement in their understanding of evolutionary concepts. Students who were retested in grade 5 scored higher than fifth-graders who had not participated previously. The most challenging concepts for all grade levels were common ancestry and natural selection, but fifth-graders showed more improvement than third- and fourth-graders. If this finding is substantiated by further research, an adjustment to the NGSS schedule might be warranted. Spacing evolutionary biology concepts out might be a better strategy than concentrating them all in grade 3.