This is a guest post by Dr. J.
Dr. J. is a fellow warrior dedicated to debunking the various creationist bull shit found on Twitter. My thanks go out to him for allowing me to repost his rant. (I wish all rants were this informative)
This is written in response to @TamiHoshiyama who (as a creationist with an apparently deeply-rooted anti-evolutionary stance) claims genetic entropy is
A) a thing that genuinely exists
B) responsible for a steady degradation of the human genome, with presumably concomitant effects on human health
As far as I can tell, this stance is selected more or less to support the usual run of “EVOLUTION CAN’T HAPPEN” claims. Aside from the fact that evolution can happen (as we’ve observed multiple instances of random mutations followed by natural selection leading to novel behaviour, substrate utilisation, environmental tolerances and outright generation of novel species), this does pose rather a puzzle from a purely theistic perspective:
1) Humans are too complex and amazing to have evolved, they must be designed
2) Humans are mutating themselves to death because they’re poorly designed
If the argument is that both 1) and 2) are correct, it doesn’t really paint this hypothetical designer in a very good light. Rather than being omnipotent and making perfect creations (a common argument), god instead makes a whole bunch of species that barely hold together for a few thousand years before imploding under cumulative mutational load due to not apparently bothering to design sufficient repair enzymes.
A further note is that a huge number of mutations are due to nucleotide choice: thymidine, for instance, is highly prone to UV-induced crosslinking. Anywhere in the genome where two thymidines are adjacent is vulnerable to thymidine dimer formation. UV hits on thymidine, it crosslinks to the other. These then have to be chopped out and replaced.
With four bases to choose from, getting TT is a 1/16 chance, so in a genome of human size (3×10^9) that’s just over 187 million places to go wrong every second of every day. Repair systems are busy.
Note, incidentally, that uracil provides exactly the same base-pairing properties as thymidine but doesn’t crosslink (A-T pairing and A-U pairing both work): why doesn’t life use uracil in DNA? The answer is because of the OTHER major source of DNA mutations: cytosine deamination.
Cytosine contains an amine group (NH2) which has a tendency to fall off, because hey: thermodynamics. What do you get when cytosine spontaneously deaminates? You get uracil.
If DNA used uracil rather than thymidine, it would be extremely difficult for the repair mechanisms to distinguish between a legitimate uracil and one resulting from cytosine deamination. In a U-G mismatch is the U wrong, or the G?
As it is, the repair mechanism (uracil DNA glycosidase, or UDG) simply scans for any uracils and chops them out to be replaced with cytosine, because that’s a far easier mechanism to develop. It does this millions of times a day, PER CELL.
Repair systems are BUSY.
So on one hand we have a divine creator who deliberately selected a range of nucleotides specifically to be really prone to mutating and necessitating a massive array of repair systems just to stay reasonably intact but apparently not enough to prevent massive eventual mutational collapse of his/her magnificent creation….
….or we have naturally formed early life that, over a billion of so years, evolved a system using the least detrimental combination of nucleotides that can form spontaneously, because that’s all that was available. And since this system cannot prevent mutation, genomes will change. Successive rounds of mutation and selection will produce huge varieties of life, always selecting for “good enough”, because that’s all you really need.
Anyway, on to genetic entropy. (more…)