Today’s early issue of PNAS includes a paper by Tim Weaver, Charles Roseman and Chris Stringer who revisit the chance or natural selection issue in regards to Neandertal and modern human speciation. You may remember they published a paper in August of 2007, in which they basically concluded that natural selection really didn’t have anything to do with why Neandertals are so morphologically different. According to these older results, there really hasn’t been a real benefit to their really robust skulls with large noses, rather those traits just sprung about by chance. That goes against a lot of adaptionists hypotheses that proposed Neandertal robusticity has been selected in response to the cold and harsh environment they lived in.
The new paper is titled, “Close correspondence between quantitative- and molecular-genetic divergence times for Neandertals and modern humans,” and Weaver et al. apply their older data set of, “37 standard cranial measurements collected on 2,524 modern humans from 30 globally distributed populations and 20 Neandertal specimens” to some improved algorithms. The abstract indicates they built upon the older model, that used changes in microsatellites as a framework for understanding selection versus random genetic drift. With this ‘clock’ of sorts, they were actually able to present a divergence time of neutrally evolving morphological measurements.
I don’t have access to the PNAS paper just yet. For some odd reason my institution’s library access is always a day behind,
so if anyone would be so kind enough to email me the paper, nevermind I got it. Anyways, I’ll be more than happy to figure out how they were able to use a ‘reverse’ molecular clock. See usually molecular clocks are calibrated on the fossil record, but in this situation it seems like it is the opposite. It seems the authors were able to figure out a clock for random genetic drift and correlate that to morphological traits.
They were able to pluck out two dates, which differ by around 120,000 years because of difference between ‘within-population variation.’ The low end of the speciation time for Neandertals is at 311,000 years ago with a high end at 435,000. The confidence intervals for the 311,000 year old date are pretty gnarly, but in general this date is beginning to fall much more in line with the genetic data. Anyways it looks like a good paper, but I really don’t know the details,
if someone out there wants to send me the PDF, please do. Got it.
- WEAVER, T., ROSEMAN, C., STRINGER, C. (2007). Were neandertal and modern human cranial differences produced by natural selection or genetic drift?. Journal of Human Evolution, 53(2), 135-145. DOI: 10.1016/j.jhevol.2007.03.001