An online curriculum about biological evolution was designed according to the "Promoting Evidentiary Reasoning and Self-Regulation Online" (PERSON) theoretical framework to support evidentiary reasoning and self-regulation. An efficacy study was conducted with 83 suburban high school biology students using design-based research methods. Data sources and instruments included the Biological Evolution Assessment Measurement (BEAM), self-regulatory ability measures, discussion forum posts, and embedded evidence based reasoning assessments. Findings revealed that BEAM posttest scores were significantly greater than pretest scores for items designed to measure evidentiary reasoning. Performance on daily evidentiary reasoning tasks strongly predicted BEAM posttest scores. Self-regulatory ability did not significantly predict BEAM gain scores. Further, self-regulatory ability was not demonstrably improved as a result of this intervention. Implications for designing science instruction in asynchronous online learning environments to support evidentiary reasoning and self-regulation are discussed.