Young children can struggle to learn difficult disciplinary content and important skills for practicing science. Problem-based learning (PBL) may be useful for addressing such difficulties, yet evidence to support its usefulness in elementary school-aged children is limited. We considered the role of a PBL unit in improving students' genetics content understanding and their skills specific to creating arguments with coordinated claims, evidence, and reasoning. First- through fifth-grade students participated in a 6-week PBL unit about evolution and genetics. Students worked in mixed-age groups and were charged with illustrating a fictitious alien species, called markles, based on a series of facts they collected about factors expected to impact markle adaptation. This work was particularly unique in its assessment of student groups' illustrated design solutions as arguments. Although students demonstrated weaknesses in coordinating claims and evidence overall, they were able to demonstrate success in gaining difficult genetics content knowledge and in preparing arguments with, at minimum, two components of well-constructed arguments, in most cases, providing a claim supported by reasoning. This work is informative for understanding student abilities, the potential of PBL, and considerations for its use.