To study clonal evolution in myeloproliferative disorders, we used stochastic models of hematopoiesis for mouse and cat, species for which the in vivo kinetics of hematopoietic stem cells (HSCs) have been experimentally defined. We determined the consequence if 1 HSC became able to survive without the support of a microenvironmental niche while the rest of its behavior did not change. Neoplastic cells persisted and dominated hematopoiesis in 14% of mice and 17% of cats, requiring mean times of 2.5 ± 0.5 and 7.0 ± 1.2 years, respectively (n = 1000 simulations/species). In both species, when the number of neoplastic HSCs exceeded 0.5% of all HSCs, clonal dominance was inevitable. Our results can explain the absence of clonal myeloproliferative disorders in mice (lifetime, 2 years), are consistent with clinical observations in cats, and provide insight into the progression of chronic myelogenous leukemia (CML) in humans. They also demonstrate that competition for microenvironmental support can lead to the suppression of normal hematopoiesis as neoplasia evolves. Toxic or immunologic suppression of normal HSCs is not required.