If a cancerous mutation occurs in a stem cell in the bone marrow, what process would cause the differentiated cells to also be cancerous?

Stem cell differentiation involves the changing of a cell to a more specialized cell type, involving a switch from proliferation to specialization. This involves a succession of alterations in cell morphology, membrane potential, metabolic activity and responsiveness to certain signals.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.