Joined: Jan. 2009
|Quote (mammuthus @ June 11 2009,18:08)|
|Quote (deadman_932 @ June 11 2009,15:57)|
|Sanford's "genomic (mutational) meltdown" scenarios are a hoot. Even DaveScot was bright enough to see that Sanford's proposed mutation rates were out of line with reality: fast-reproducing sexual species that have existed a few million should have all been extinct by now, but they're not. Sanford inflates deleterious mutation rates and disregards compensatory mechanisms.|
His argument is a little more involved than that. †It seems to revolve around genome size; the smaller genome size of something like P.falciparum prevents genetic meltdown, but it would occur with larger genome sized mammals. †So genetic entropy is a problem for the latter (if not on Sanford YEC timescales). †You can't just take fast reproducing things like P.falciparum and apply the Genetic Entropy failure in this case widely. †At least that's how I read it.
|It occured to me recently that Sanfordís projected rate of genetic decay doesnít square with the observed performance of P.falciparum. P.falciparumís genome is about 23 million nucleotides. At Sanfordís 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 didnít 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. Thatís 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 donít 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.
Here is an abstract which might inform this particular question