Background:
Cross-grade studies are valuable for the development of sequential curriculum. However such studies are time and resource intensive and fail to provide a clear representation to integrate different levels of representational complexity. Author proposed a cladistics approach in conceptual evolution to construct a hypothetical Conceptual Evolution Tree (CET; also called Evolutionary Pathway of Student Mental Models) in electric circuits which overcome the limitations of earlier cross-grade studies. The aims of this study are to validate this hypothesis and to integrate with the CET graph to better understand Taiwanese students' conceptual evolution in electric circuits.

Material and methods:
This study uses a web-based mental model diagnosis system to collect 1,441 cross-grade students’ data.

Results:
The results show that the empirical cross-grade survey data closely meet the hypothesis. The CET graph in electric circuits well explains the relationship between student conceptual evolution from Grade 1 to 9 and the curriculum influences by comparing to the empirical data.

Conclusions:
The validated CET graph in electric circuits could be a useful tool for science educators in designing sequential curriculum content.