Cancer is, unfortunately, governed by the same evolutionary rules that drive life itself. Cells in tumors are essentially competing to see which can divide the fastest. This competition drives them to pick up new mutations that can help them divide faster, survive immune attack, resist drugs, and expand to new areas of the body.

We can tell this by looking at the genetic changes that occur as tumors progress. Over time, we can trace the appearance of new mutations that confer abilities that are, from cancer’s perspective, useful for tumor cells.

Now, a new study suggests that an unfortunate side effect of these evolutionary changes is that human tumors are really difficult to study. Whether the tumor cells are put in a culture dish or grown in mice, they evolve changes that help them grow in this new environment. And some of these changes influence how the tumor cells respond to drugs.