Classical r-vs.K-selection theory describes the trade-offs between high reproductive output and competitiveness and guides research in evolutionary ecology.While its impact has waned in the recent past,cancer evolution may rekindle it.Herein,we impose r-or K-selection on cancer cell lines to obtain strongly proliferative r cells and highly competitive K cells to test ideas on life-history strategy evolution.RNA-seq indicates that the trade-offs are associated with distinct expression of genes involved in the cell cycle,adhesion,apoptosis,and contact inhibition.Both empirical observations and simulations based on an ecological competition model show that the trade-off between cell proliferation and competitiveness can evolve adaptively.When the r and K cells are mixed,they exhibit strikingly different spatial and temporal distributions.Due to this niche separation,the fitness of the entire tumor increases.The contrasting selective pressure may operate in a realistic ecological setting of actual tumors.