Cancer is often described loosely as cells "growing out of control," which undersells what's actually happening. A more precise, and more useful, way to understand it: cancer is the outcome of Darwinian natural selection playing out among a person's own cells, over years or decades, inside a single body.
A normal cell accumulates the wrong mutations, one at a time
Cells divide constantly throughout life, and each division carries a small chance of a copying error. Almost all such mutations are harmless, get repaired, or cause the cell to die. Very occasionally, a mutation happens to land in one of a small set of genes that control cell division itself — genes that normally act as accelerators, called oncogenes when mutated to be overactive, or brakes, called tumour suppressor genes when mutated to stop working. A cell that happens to accumulate several such mutations gains a real, heritable advantage: it and its descendants divide faster, or die less often, than the normal cells around them.
Selection, not just mutation, drives the disease forward
That single advantaged cell's descendants outcompete their neighbours for space and resources, exactly the way a fitter organism outcompetes others in a population — an internal evolutionary process biologists call somatic evolution, or clonal evolution. Further mutations that arise within that already-advantaged lineage, if they confer even more advantage — faster growth, evading the immune system, the ability to invade new tissue — get selected for again, compounding over years. What looks like one runaway process from the outside is actually many rounds of mutation and selection, compressed into a single patient's tissue.
What we're still unsure about
Understanding cancer as an evolutionary process has real, practical stakes that are still being worked out. Treatments that kill off the most vulnerable cancer cells can inadvertently act like a strong selective pressure, clearing space for any resistant cells already present in the tumour to take over — one leading explanation for why cancers so often return in a treatment-resistant form. How to design treatment strategies that account for this evolutionary dynamic, sometimes proposed as deliberately less aggressive dosing to avoid strongly selecting for resistance, is an active, unresolved area of cancer research and clinical trial design, not settled practice.
This sits inside Neoplasia & Cancer Biology, one of seven topics in Pathology, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.