Genetic Entropy and Malarial Parasite P. falciparum
|October 29, 2007||Posted by Dave S. under Intelligent Design|
Edge of Evolution I found to be amazing. It presented a case history of a eukaryote (P.falciparum) that has replicated billions of trillions of times within a span of a few decades. More importantly this is one of the most well studied organisms in biology due to its huge toll on human lives. In the last decade we’ve gone beyond phenotype analysis of the bug and have completely sequenced its genotype. This represents the largest test of evolution that we can hope to observe. The result of random mutation + natural selection being given billions of trillions of opportunities to generate significant novel biological complexity was essentially nil. Except for biochemically (but medically important) trivial changes in genotype the bug went exactly nowhere. It’s still the same old P.falciparum as its great grandparents billions of trillions of generations removed. It neither progressed nor regressed in an evolutionary sense.
All the negative reviews I’ve read of EoE nitpick at minutae while dodging the big picture. The big picture is that P.falciparum under intense scrutiny for billions of trillions of generations did exactly what ID theorists predicted – next to nothing. In contrast the ID deniers tell us over and over that the same evolutionary mechanism (RM+NS), in orders of magnitude fewer generations, turned a lizard into a lemur. Of course that’s a wholly imaginary story because the transformation of reptiles into mammals took hundreds of millions of years so can’t be confirmed by genotype observation. All we have is phenotype evidence based on fossils. Clearly *something* caused the transformation from reptile to mammal but I’ve yet to see any reasonable explanation for the observed failure of P.faciparum to evolve while somehow the same mechanism with fewer opportunities is imagined to have caused reptiles to evolve into mammals. Non sequitur!
Genetic Entropy I found less amazing because its basic conclusion was already obvious to me and unlike EoE it didn’t really present anything I didn’t already know. That’s not meant to detract from Genetic Entropy as many of its readers probably haven’t figured out for themselves that genetic entropy is, outside of environmental catastrophe, the force majeure in the evolution of species (or better put the eventual extinction of species).
One major disagreement I had with Genetic Entropy was the rate Sanford gives for random mutation. In virtually all the scientific literature I’ve read on the subject the given rate of copy errors in eukaryote DNA replication is one in one billion nucleotides. Sanford proposes, with references which I admittedly didn’t fisk, that the rate is really at least one and possibly two orders of magnitude greater. The lower rate Sanford gives is about one in ten million errors per nucleotide.
It occured to me recently that Sanford’s projected rate of genetic decay doesn’t square with the observed performance of P.falciparum. P.falciparum‘s genome is about 23 million nucleotides. At Sanford’s lowest given rate of nucleotide copy errors that means each individual P.falciparum should have, on average, about 3 nucleotide errors compared to its immediate parent. If those are nearly neutral but slightly deleterious mutations (as the vast majority of eukaryote mutations appear to be) then the number should be quite sufficient to cause a genetic meltdown from their accumulation over the course of billions of trillions of replications. Near neutral mutations are invisible to natural selection but the accumulation of same will eventually become selectable. If all individuals accumulate errors the result is decreasing fitness and natural selection will eventually kill every last individual (extinction). Yet P.falciparum clearly didn’t melt down but rather demonstrated an amazing ability to keep its genome perfectly intact. How?
After thinking about it for a while I believe I found the answer – the widely given rate of eukaryote replication errors is correct. If P.falciparum individuals get an average DNA copy error rate of one in one billion nucleotides then it follows that approximately 97% of all replications result in a perfect copy of the parent genome. That’s accurate enough to keep a genome that size intact. An enviromental catastrophe such as an ice age which lowers temperatures even at the equator below the minimum of ~60F in which P.falciparum can survive would cause it to become extinct while genetic meltdown will not. Mammals however, with an average genome size 100 times that of P.falciparum, would have an average of 3 replication errors in each individual. Thus mammalian genomes would indeed be subject to genetic decay over a large number of generations which handily explains why the average length of time between emergence to extinction for mammals and other multicelled organisms with similar genome sizes is about 10 million years if the fossil and geological evidence paints an accurate picture of the past. I DO believe the fossil and geological records present us with an incontrovertible picture of progressive phenotype evolution that occured over a period of billions of years. I don’t disbelieve common ancestry and phenotype evolution by descent with modification – I question the assertion that random mutation is the ultimate source of modification which drove phylogenetic diversification.