Abstract. Efforts to understand the biology of cancer have taken two seemingly divergent directions, which can be reconciled by evolutionary theory. On one hand, molecular characterization of cancer cases has led to an ongoing subdivision of cancer into an ever-increasing number of molecular types and sub-types. On the other hand, it has emerged as an organizing principle of cancer biology that all cancer cells, of whatever molecular type, share the same set of phenotypic properties, described as the “hallmarks of cancer.”
These two patterns are reconciled by recognizing that all cancer cells share the same ecological niche within the host body, and therefore are all subjected to the same selective pressures during their somatic evolution within the host, shaping similar phenotypic traits from different stochastic mutations. This well-understood pattern also occurs in organismal evolution, where it is called ‘convergent evolution’. Recognizing this underlying process brings a coherent theoretical framework to cancer biology.