Cancer: the selfish cell?
Most of the time, the body's cells work together harmoniously, each contributing to the greater good. An exception to this rule are cancer cells, which divide and colonise the body. They prosper at the expense of other cells, but also seal their own fate by eventually killing the host.
A process of selection is seen as cancers develop. Normally, when cells begin to go wrong – for example, if their genes are damaged – they receive signals that tell them to self-destruct. But a cell may acquire mutations (or it might have inherited them) that cause it to resist these signals. Other changes encourage it to start dividing – it is becoming a cancer cell.
A mass of cancer cells – a tumour – may build up. But then it will stop growing because oxygen cannot get to the cells in the centre of the tumour. But if another mutation causes cells to release chemicals that attract new blood vessels, it can start multiplying again. Often, then, many tiny tumours form in the body, but only one or a few go on to cause problems.
Even with a blood supply, a population of cancer cells is stuck where it originated. But some cells may acquire a new ability – motility. They may detach from the original tumour mass and travel around the body. They may acquire the ability to digest tissue and invade other organs, setting up metastases (secondary cancers).
So, by accumulating genetic changes, cancer cells acquire properties that enable them to overcome the body's defences. They gain a selective advantage, increase in number and spread. Selection within tumours also leads to resistance to anticancer drugs, such as tamoxifen, much as selection favours antibiotic resistance in bacteria.
But there is a big difference between this process and evolution in the outside world. A cancer cell growing out of control kills its host, so is soon 'extinct'. Natural selection is not so crude. 'Survival of the fittest' does not mean short-term survival of the strongest and the elimination of all other forms of life.
Dog days
Interestingly, though, there are cancers that have extended their existence beyond their original host. Some cancers are infectious – they can be spread from one animal to another and set up a new tumour. (Cancers caused by viruses are, in a sense, infectious; but it is the virus rather than the cancer itself that is transmitted.)
An infectious tumour is currently ravaging the wild Tasmanian devil population. And an infectious dog tumour exists. Genetic studies suggest it originated in either a wolf or an 'old' Asian dog breed such as the husky or shih-tzu, probably around 1000 years ago.