13 Things That Don't Make Sense: The Most Baffling Scientific Mysteries of Our Time

by Michael Brooks

The power of natural selection is reduced as a creature gets older, Medawar pointed out, so a trait that gives an advantage before the creature has reached maturity (and entered into reproduction) will be selected for; a trait whose advantage only shows after the creature has ended its reproductive life will not. The converse is also true. A gene that disables you before you reach maturity will be (negatively) selected for; it will lower the chances of the organism passing on its genes. A gene that disables the organism much later in life will be, if not exactly selected for, at least able to survive into the next generation. And here, Medawar said, is the source of senescence. It's not about the inevitable ravages of time; it's the fact that late-blooming problematic mutations—genes for cellular machinery that breaks down late in life, for example—will be passed on, and will thus accumulate in a creature's genome. In humans, Huntington's and Alzheimer's disease provide examples of this process.