Abstract: Cell competition shapes tissue dynamics and homeostasis by mediating interactions between genetically distinct cells, yet its role in adult pluripotent stem cells systems remains unclear— especially in regenerating organisms. Planarian flatworms, which rely on stem cells for continuous tissue renewal, offer a valuable model. This study explores how genetic heterogeneity influences neoblast competition using chimeric Schmidtea polychroa worms created via reciprocal tissue transplantation between strains. Imaging and sequencing revealed a consistent hierarchy: strains with higher mitotic activity outcompeted others. Results support a neutral competition model where proliferation rate—not antagonism— determines success. Computational simulations confirmed mitotic fraction as the strongest outcome predictor. These findings suggest planarians preserve genetic diversity through stem cell competition, offering insight into how tissue integrity is maintained. This model advances our understanding of regenerative biology and may inform stem cell research in tissue repair and disease.