Carl Zimmer has an article in the upcoming edition of Scientific American that looks at cancer from the perspective of evolutionary biology. The most obvious parallel is that of cancer cells within an individual modeled as an evolving population:
Rare mutations, for instance, may cause a cell to lose restraint and begin to multiply uncontrollably. Other mutations can add to the problem: They may allow deranged cells to invade surrounding tissues and spread through the body. Or they may allow tumor cells to evade the immune system or attract blood vessels that can supply fresh oxygen.
Cancer, in other words, re-creates within our own bodies the evolutionary process that enables animals to adapt to their environment. At the level of organisms, natural selection operates when genetic mutations cause some organisms to have more reproductive success than others; the mutations get “selected” in the sense that they persist and become more common in future generations. In cancer, cells play the role of organisms. Cancer-causing changes to DNA cause some cells to reproduce more effectively than ordinary ones. And even within a single tumor, more adapted cells may outcompete less successful ones.
But multicellular organisms have also evolved mechanisms to suppress such runaway populations of cells. A group of researchers, however, noticed that there is a tradeoff between regulating cell division and aging. Their study of the tumor suppressor gene p16 in mice revealed that:
Natural selection favors anticancer proteins such as p16, but only in moderation. If these proteins become too aggressive, they can create their own threats to health by making bodies age too quickly.
The effects of natural selection on tumor suppression genes is greatest prior to and during prime reproductive age — once you can no longer reproduce, your ability to transmit your alleles is greatly limited (grandmother hypotheses aside). Therefore, our body is optimized to fight early onset cancers, but is not so great at dealing with those that strike at old age.