Long-term competition experiments reveal limited adaptive evolution in human cell lines

Cancer progression is driven by somatic evolution, where mutations and selection generate more adapted clones. However, it remains unclear whether human cells continue to evolve under stable laboratory conditions or instead reach a near-optimal state. We addressed this using two human cell lines, HeLa and 293, with isogenic subpopulations differing only by a fitness-neutral marker (inducible GFP expression). Mixed GFP⁺ and GFP⁻ populations were co-cultured for 12 months under constant conditions, with competition assays performed with or without periodic GFP induction and sorting. In a parallel experiment inspired by Velicer's microbial studies, GFP⁺ cells were exposed to independently cultured, non-evolving GFP⁻ populations to test whether competitively advantageous variants could emerge when competitors could not respond. Initially, we observed patterns consistent with genetic drift, followed by a gradual expansion of GFP⁻ cells. After 4-6 months, a likely beneficial mutation in the GFP⁺ 293 population reversed this trend, whereas GFP⁻ HeLa cells retained their advantage. Fitness differences were small, and observed dynamics were consistent with weak selection. These results suggest that long-term cultured human cells evolve slowly without rapid clonal expansion.