DNA is surrounded by a priesthood of enzymes anxious to correct its smallest errors. Without them, it would soon fail. If the repair enzymes are biased in their belief about what the correct message should be, then that version of the genetic creed is bound to take over. Other methods of genetic purification can homogenize a DNA sequence. In places with many copies of a particular string of letters, the segments tend to mispair, rather like the teeth of a zipper done up too quickly. One version may as a result have a built in tendency to increase at the expense of the other and to drive it out.
Such behavior hints that genes have an evolutionary agenda of their own. Perhaps, to parts of the DNA, species are no more than a place to live, great continents of animals linked by sex. Different species (such as southern and northern midlwife toads) may look much the same, but for the molecule’s point of view each is an island isolated form its neighbors by a sexual barrier. As a result, each evolves to its own internal rules.
Different species (such as southern and northern midlwife toads) may look much the same, but from the molecule’s point of view each is an island isolated from its neighbors by a sexual barrier. As a result, each evolves to its own internal rules.