p53 is an important tumor suppressor gene. Around 50% of cancers are associated with loss of function in p53.
Humans have only two copies of p53 in their genome (one on each homologous chromosome). Elephants on the other hand have 20 or so copies of p53, and they also rarely die of cancer.
From an evolutionary point of view, I find it very curious that humans (and most other animals, I guess) haven't evolved more copies of p53 on their genome. Duplications of genes happen all the time, so there is variation in copy number for natural selection to work upon.
Are there known to be evolutionary forces at play (e.g. positive or balancing selection) which determine the differences in number copies of this gene between species?