Competition and cooperation shape the characteristics of populations at all levels, from species communities to cells in a multicellular organism. Interactions between cells are especially important in tumours: cancerous cells outcompete healthy ones by multiplying faster, but relations between the cancerous cells themselves also affect growth. Different populations of cells, or subclones, coexist within tumours, competing for dominance yet also cooperating by sharing useful molecules, known as growth factors. Making these is costly, so selfish cells, which don’t produce them, have a competitive advantage over those that do; however, they'll grow more slowly when no producing cells are around to help. These conflicting pressures can lead to a stable mix of cells, represented by the mosaic above – patches of non-cooperative cells (in grey), existing among the cooperators (in pink). Understanding how this diversity arises is important, as the composition of tumours will affect their growth and response to treatment.
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