Working out the times of important evolutionary events requires us to know “the” speed at which genetic changes accumulate in the genome. This tells us the speed of the genetic clock, allowing us to work out how long ago species split. Unfortunately we have been able to estimate the average number of mutations per generation.
Why is this unfortunate? Because once a clock rate is found, some people – most, apparently – immediately forget that the clock rate can change.
Years ago there was a dispute over whether the rate of evolution stayed about the same or varied. Stephen Jay Gould and others got some renown for supporting the Punctuated Evolution idea. I’ve said in the past that this wasn’t a particularly astonishing insight, but it seems there are many who still haven’t appreciated it. Or maybe they haven’t realised that if evolution speeds up, the rate at which genetic changes accumulate also speeds up.
A weird pattern appears in both mammalian and bird genomes, which appear to show the main groups of each split a long way back in the Cretaceous, alongside the dinosaurs, though the fossils don’t seem to reflect this. There is good reason to believe what actually happened was that the genetic clock accelerated massively after that great extinction, amongst those animals that survived and rapidly adapted to new niches.
No-one has done any simulations to research this, so no-one knows what genetic clocks will inevitably do in an “aftermath”.
However, John Hawks hasn’t heard of punctuated evolution apparently, or hasn’t realised it may be accompanied by punctuated genetic clock acceleration. For him, any estimation of clock speed is cast in stone. This is particularly annoying because in other ways he’s so good, but he not only spoils the ship for a ha’porth of tar, but his sound advice on other matters strengthens the impact of his mistakes.
In an earlier post I pointed out that if we assumed short-term clock speeds applied over longer periods, we would have to assume that not only did the main placental mammal groups diverge in the Cretaceous, but significant splits among the higher primates reached back almost that far – for example the ape-monkey split.
In his recent posting on the interesting new genome from Ust’-Ishim http://johnhawks.net/weblog/reviews/ancient-genomes/ust-ishim-fu-2014.html
. . .the authors find that the mutation rate would need to be fairly low, on the order of 1 times 10-8 mutations per site per generation, to account for the Ust’-Ishim data. That low rate is more or less the same as estimated from looking at parents and offspring living today, which is a good piece of evidence that the per-generation rate we measure in living people is not different from the per-generation rate averaged over longer periods of time. As the authors point out, this means that a lot of ancient events estimated from DNA only must have happened longer ago than was claimed prior to a couple of years ago.
You might find that if you remind him about the variability of genetic clocks, he will just dismiss you as a crank, as he did on Twitter when I pressed him about it (though he didn’t actually use my name). Or maybe he just thinks anyone who believed what he did in 2011, is, by 2012, a crank. This kind of block-headed arrogance damages science more than anything else.