Hi Barnes,
Cancer is a disease that causes cells to grow uncontrollably. Checking out the Wikipedia entry, the causes can be genetic, but there are many other causes.
A genetic change (mutation) can cause cancer, but usually not. Cellular reproduction almost always includes at least a few mutations, and the vast majority of cells are not cancerous. Every fish, reptile and mammal began as a single cell that split many, many times to become billions of cells. Almost every cell division resulted in a at least a few mutations, yet cancer is rare.
A cell's internal machinery does possess mechanisms protecting itself from genetic accidents, but mutations slip through nonetheless.
barnes writes:
Adaptation precludes no physical change but improvements relative to the environment.
Adaptational change below the species level most definitely includes physical changes. For example, the finches of the Galapagos that have been studied so extensively change the size and shape of their beaks in response to environmental factors.
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--Percy