posted on Apr, 19 2007 @ 12:28 AM
It is true that cancer is a mutation, but usually in specific genes in the somatic cell lines, ie non-sperm and egg. These mutations usually result
in either a lose of function for a mechanism for controlling cell replication or a gain of function in a cell cycle mechanism. This results in the
uncontrolled growth of that cell, which will eventually lead to the growth of a tumor. These mutations almost never happen in the germ line, ie sperm
and egg, because the system there is geared toward extreme levels of replication already. So when a person gets cancer, in whatever cell type, the
mutation can't be transfered to the offspring because it isn't in the germ line. Furthermore most cancers present in the later years of life, after
the person has already had kids, so selection for the cancerous individuals wouldn't carry into the next generation.
You have to remember that natural selection is a passive process, where mutations and increased variability in the germ line will confer new traits to
offspring slowly over time. The traits that gives the organism the best ability to feed and reproduce get passed on in greater numbers, and over eons
this gives the species diversity we see today. The dinosaurs died out because in the environment created by the asteroid impact large plants
couldn't get enough sunlight and the temperatures were too cold to grow sufficiently. The large herbivores dependent on them started dieing off
because they couldn't get enough food, and the same with the large predators. Smaller animals were better able to find sufficient food amounts, and
so they lived on in greater numbers and keep passing on their genes.