Rubbish DNA: The functionless fraction of the human genome
Because genomes are products of natural processes rather than intelligent design, all genomes contain functional and nonfunctional parts. The fraction of the genome that has no biological function is called rubbish DNA. Rubbish DNA consists of junk DNA, i.e., the fraction of the genome on which selection does not operate, and garbage DNA, i.e., sequences that lower the fitness of the organism, but exist in the genome because purifying selection is neither omnipotent nor instantaneous. In this chapter, I (1) review the concepts of genomic function and functionlessness from an evolutionary perspective, (2) present a precise nomenclature of genomic function, (3) discuss the evidence for the existence of vast quantities of junk DNA within the human genome, (4) discuss the mutational mechanisms responsible for generating junk DNA, (5) spell out the necessary evolutionary conditions for maintaining junk DNA, (6) outline various methodologies for estimating the functional fraction within the genome, and (7) present a recent estimate for the functional fraction of our genome.