The Virtues of Junk DNA
A MAJOR DIFFERENCE between eukaryotes, especially the higher plants and animals, and their distant prokaryotic relatives concerns what may be called their DNA thriftiness. Prokaryotes practice the strictest possible economy in DNA content. 'Their genome contains hardly a single nucleotide that is not involved in coding or control. In the words of Harvard chemist and Nobelist Walter Gilbert, the bacterial genome is "streamlined," probably as a result of strong evolutionary pressure favoring fast proliferation.1
In striking contrast, the eukaryotic genome is made mostly of noncoding DNA without obvious function, sometimes called "junk" or "ballast" DNA. Less than 5 percent of the human DNA has a coding function. Salamanders do much better--or worse, depending on one's point of view.2 Some of these animals have twenty times more DNA than we have, with those in the west of the United States beating those in the east by severalfold. Fortunately for our self-esteem, DNA quantity is not by itself a measure of overall quality. Western salamanders are not obviously cleverer than their eastern congeners. Having more DNA does not automatically make salamanders superior to us.
The amount of apparently useless DNA in the genome of higher plants and animals requires an explanation. According to Britain's ethologist Richard Dawkins, the explanation lies in the "selfishness" of DNA.3 The unit of selection is DNA, not the body. The body is no more than a means of replicating DNA, just as a chicken has been said to be an egg's way of making another egg. To quote Dawkins: "The true 'purpose' of DNA is to survive, no more and no less. The simplest way to explain the surplus DNA is to suppose that it is a parasite, or at best a harmless but useless