Our lab investigates the evolution of metastases in zebrafish. Metastatic capacity is the defining feature of malignancy. It is responsible for the vast majority of deaths from solid tumors. Cancers acquire this capacity as a result of an evolutionary game that capitalizes on four essential features of Darwinian evolution: 1) heritable mutations, 2) environmental selection of the fittest cells, 3) geographical distribution of individual cell types, and 4) clonal expansion. In this light, successful metastasis must always be considered as an interaction between the tumor cell and its host environment.