Ed Yong in The Atlantic:
In the classical view of cancer, a cell picks up mutations until it shakes off the checks and balances that restrain its growth, allowing it to divide uncontrollably and turn into a tumor. This linear process is a macabre version of that famous image where a chimp walks to the right and gradually morphs into a human hunter. And both visuals are wrong. In reality, tumors quickly become seething masses of varied cells, all with their own mutations. One area might start growing faster; its neighbor might come to evade the immune system. Over time, the fittest lineages produce more descendants and rise to dominance—the essence of Darwinian natural selection. So forget the linear march. The better visual is that of a tree, with an initial trunk radiating into a web of branches. In 1837, Charles Darwin drew one such tree in one of his notebooks to represent how species evolve from a common ancestor. He could just as easily have been sketching the birth of a tumor.
This realization goes some way to explaining why the war against cancer has been so entrenched and unexpectedly difficult. Clinicians often diagnose these diseases by taking a biopsy from a tumor, but a single sample could miss important mutations with very different prognostic implications just centimeters away. And when we hit tumors with drugs or radiation, we create a potent source of artificial selection, effectively breeding for hardier tumors. That’s why relapses occur.