Can one use natural selection or genetic drift to explain human and Neandertal cranial differences?

That’s what Timothy Weaver, Charles Roseman and Chris Stringer, asked in a Journal of Human Evolution paper titled, “Were neandertal and modern human cranial differences produced by natural selection or genetic drift?

It is an interesting question to ask because if you’ve ever spent some time looking at a Neandertal skull and compared it to a modern human’s skull, you probably asked yourself, why the hell are most features in the Neandertal skull so robust? What made it that way? And, if you haven’t had the pleasure of holding and comparing these two heads, no worries… Please use the following image plucked from the article, to ask yourself those questions. On your left is a cast of a Neandertal from La Ferrassie, France and to your right is a modern human skull from Polynesia:

Neandertal and modern human cranial differences.

Now, Stringer, Weaver, and Roseman wanted to see if they can explain these differences due to some evolutionary method. They specifically wanted to see if the differences can be attributed due to genetic drift or due to natural selection. But you maybe asking yourself what is the difference between genetic drift and natural selection? I sometimes confuse the definitions, so forgive me if I’m being redundant by sharing with you some common evolutionary theory.

Genetic drift, in the most basic definition is just the probability an allele shows up in a population. The effect of the drift may cause an allele and the biological trait that it confers to become more common or more rare over successive generations. Ultimately, the drift may either remove the allele from the gene pool or remove all other alleles. So that being said, genetic drift is the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone.

It differs from natural selection, because natural selection in the most basic definition is when beneficial alleles become more common over time because they boost the survivability of the organisms and  reciprocally detrimental alleles become less common.

But genetic drift and natural selection aren’t mutually exclusive. In other words, both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstances such as population size. In a large population, where genetic drift occurs very slowly, a weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.

Before we get deeper into evolutionary theory, I want share with you the answer to the million dollar question asked in the title of the paper, ‘Were neandertal and modern human cranial differences produced by natural selection or genetic drift?’ No, Stringer and crew can’t explain human and Neandertal cranial differences with either evolutionary forces at this time… natural selection can not be supported nor can genetic drift by their measurements and calculations.  Instead, what they conclude is that the differences can be explained as manifestations of only two outcomes pooled from a vast space of random evolutionary possibilities. Which leads me into what Razib brings up. He mentions that,

“[he does] know that human bone structure and teeth have become less robust over the last 10,000 years, perhaps due to agriculture. This might simply be relaxing the selection for more robust physiques.”

Agriculture and other aspects of culture and technology can and does skew defining whether natural selection or genetic drift played a role on our bodies. For example, if one calculated and concluded that Mayan skull modifications were actually due to genetic drift and totally ignored or glanced over modifications being a cultural preference, then a incorrect conclusion would be made. Many cultures select for certain traits in skulls and bodies, which throws a big stick in the wheels of determining the evolutionary force in play. I feel that culture has been a large determining factor in deciding whether or not modern humans had more gracile skulls than robust ones. Take a second and ask yourself how our modern day popular culture is selecting for bodies?

So if I’m right, and culture has a big role, then I don’t know of any statistical method that could discern the effect of culture on traits in the skull. But, I’m thinking that one way where Stringer et al. or someone else can reproduce this work but also integrate measurements from other members of Homo, like H. heidelbergensis or archaic H. sapiens, like Homo sapiens idaltu to see whether or not gracility was selected for in the H. sapiens lineage or if it was a random? It’s a matter of simply expanding the sample size and type… maybe more can be derived or maybe more will be confused.

Homo sapiens idaltu

13 thoughts on “Can one use natural selection or genetic drift to explain human and Neandertal cranial differences?

  1. So that being said, genetic drift is the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone.

    hm. i am kind of confused by your def. here ;-) i know what you are saying because i know what drift is, but i’m a little up in the air whether i could extract it from this. basically for those who don’t know i just try to make it clear that drift within a population genetic model describes the process whereby a sample exhibits variance away from the expectation from generation to generation. and, the critical point is that the extent of this variation is inversely proportional to population size. the deviation from 50/50 for a heads or tails flip in relation to the number of trials works too….

  2. Razib, to answer your second question — yes I’m not just implying but I believe that social selection has had a big role on why we see more gracile traits in modern human than not. I do not have any imperical evidence to support this. I’m running on just my observations.

    Now about my definition on genetic drift. What I did mean to say is that genetic drift is situation where sample exhibits variance away from the expectation from generation to generation. I guess I overcomplicated the definition so thanks for clarifying it for me and others.

  3. The trouble with this way of looking at things is, it seems to imply that Neandertals and other “archaics” didn’t “really” have a culture! But Neandertals did have a culture! Their artifacts tell us so!
    Anne G

  4. Anne,

    I’m confused here, are you saying that Stringer et al. didn’t factor in the culture aspect? Or are you saying just as there may have been a cultural preference for more gracile H. sapiens, there may have been a cultural preference for more robust Neandertals?

    Kambiz

  5. The facts are in now that Neanderthal and modern humans are seperate species, therefore “drift” is a meaningless word as we are a different species, no kin to Neanderthal. The DNA testing available so far agrees with the findings. The question now is moot.

    The study released by Cambridge University on May 8th of this year (finally!) shows that all humans alive on this planet came from about 10,000 adult modern humans, some whom migrated out of Africa after the Mt. Toba ice age was ending around 55-50,000 years ago.

    Neanderthal may have been kin with Hablus or Erectus, but not modern humans. The Bradshawfoundation.com can provide much more facts and figures than I am able to include on this post.

    Who we are is back to square one.

    Ken

  6. Ken says,

    “we are a different species, no kin to Neanderthal. The DNA testing available so far agrees with the findings.”

    As far as I’m aware the only really definitive DNA testing is the mtDNA and Y-chromosome. We know that lines of these are replaced reasonably often in modern human populations let alone ancient ones. Besides which even the mtDNA shows moderns and Neanderthals separated a mere half million years ago. Many species that separated five times as long ago as that are freely able to form hybrids. The idea of calling the two populations separate species is still a bit premature. They may not have formed hybrids but the reason for that is unlikely to have been genetic incompatibility.

  7. I have a ques which may sound stupid but ill ask it anyway. How do skulls have teeth? Dont most people who die from old age lose most of there teeth. Dont your gums hold your teeth? After u die dont your gums disintegrate or something which would cause the teeth to fall out? So how do skulls have teeth?

  8. just a note on a comment in this article… modern culture is not selecting for body type. even though slim, trim and tall is in vogue – that does not mean these people are producing more offspring. there are plenty of non-hollywood types having children. if anything, they are having more.

  9. “just a note on a comment in this article… modern culture is not selecting for body type. even though slim, trim and tall is in vogue – that does not mean these people are producing more offspring. there are plenty of non-hollywood types having children. if anything, they are having more.”
    totally agree
    good stuff

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