A very fascinating post from Elizer Yudkowsky’s Overcoming Bias blog on the upper limits on DNA based evolution, with a startling conclusion:
Among mammals, the rate of DNA copying errors is roughly 10^-8 per base per generation. Copy a hundred million DNA bases, and on average, one will copy incorrectly. One mutation, one death; each non-junk base of DNA soaks up the same amount of selection pressure to counter the degenerative pressure of copying errors. It’s a truism among biologists that most selection pressure goes toward maintaining existing genetic information, rather than promoting new mutations.
Natural selection probably hit its complexity bound no more than a hundred million generations after multicellular organisms got started. Since then, over the last 600 million years, evolutions have substituted new complexity for lost complexity, rather than accumulating adaptations. Anyone who doubts this should read George Williams’s classic “Adaptation and Natural Selection”, which treats the point at much greater length.
In material terms, a Homo sapiens genome contains roughly 3 billion bases. We can see, however, that mammalian selection pressures aren’t going to support 3 billion bases of useful information. This was realized on purely mathematical grounds before “junk DNA” was discovered, before the Genome Project announced that humans probably had only 20-25,000 protein-coding genes. Yes, there’s genetic information that doesn’t code for proteins – all sorts of regulatory regions and such. But it is an excellent bet that nearly all the DNA which appears to be junk, really is junk. Because, roughly speaking, an evolution isn’t going to support more than 10^8 meaningful bases with 1 bit of selection pressure and a 10^-8 error rate.
Each base is 2 bits. A byte is 8 bits. So the meaningful DNA specifying a human must fit into at most 25 megabytes.