Variation, propagation, selection and inheritance: These are the bases of biological evolution.
Cancer cells are clones. Much like bacterial cells form clonal colonies on agar, tumors are colonies formed from single cancer cells. As these cells replicate, additional variations accumulate, leading to further diversification of the population in the tumors. Over time, cells may separate and found new tumors elsewhere. Some of the changes acquired by subsequent generations of cancer cells support better growth, others, better propagation or bypassing of self-destruct signals. Vascularization, penetration though tissue boundaries and resistance to chemotherapy agents can also be acquired and honed by selection. The cells in say, later stage cancers, often have accumulated new traits compared to the cells found in earlier stages.
It is perfectly reasonable to approach cancer as a providing a lesson in evolution. That is why, for example, when we evaluate new therapeutics, we also try to identify both the cancer cell populations that are likely to respond and the probable pathways to resistance. In many cases, the targeted therapies work only for a limited duration. We can often knock down the more populous, susceptible cells and achieve a term of remission but resistant clones frequently take over and drive the next emergence of the disease.