A couple of days ago, when I wrote about my friend's death, I was also mulling over the idea of how the progression of cancer can be a good example of evolution in action. One of the hallmarks of cancerous cells is that they are capable of continued cell division. Plenty of opportunity, with all that mitosis, for mutations to accumulate and generate a range of genetically diverse cell lines.
Now take that genetically diverse population of cells & apply a strong agent of natural selection - a chemotherapy drug. You may strike it lucky & kill all the cancerous cells. Or you may get most of them - but some may possess a mutation that makes them less likely to succumb to the powerful chemotherapy medicines. So they'll be selected for & over time, & with continued exposure to the drugs, you'll end up with a population where most of the cells are resistant to whatever the oncologist can throw at them. That population of cancer cells has evolved. (Orac says all this much better than I can, but then, he's an expert in this field.)
But knowing how evolution operates can also offer the prospect of improved treatment. For example, if researchers can understand the selection pressures that may be involved in the transition from precancerous to cancerous cells, they can start to look at the mechanisms involved and how best to target them. Or using a combination of drugs that affect several aspects of the cell's functioning may delay the point at which some cells acquire the multiple mutations that confer resistance.
(It may also make you quite unwell. Many anti-cancer treatments target rapidly dividing cells. Unfortunately cancers aren't the only cells affected. Your gut lining can also take a pounding, as can hair follicles. But offered the choice of death, or life with bearable side effects - & the chance to spend more time with those who are important to me, I know which I'd choose.)