July 14, 2012

Yet another Neolithic modeling study

A reader calls my attention to this paper or, more precisely, e-book:

Mathias Currat, Consequences of population expansions on European genetic diversity. De Gruyter, 2012. Open access.

Abstract
Population movements over space and time played a crucial role in generating the genetic patterns that are observed in the present day. Numerous factors, such as climate changes or cultural innovations, have the potential to induce large-scale movements, such as population expansions (i.e. increases both in density and range) or contractions to refugee areas. It is thus very important to take the spatial dynamic of populations into account when trying to reconstruct their history from genetic data. Computer simulation constitutes a very powerful tool for the study of the combined impacts of biological and demographic factors on the genetic structure of populations. The rapid increase of computer power opens many new possibilities for research in that specific area. A series of recent studies have focused on the consequences of population expansions on their genetic diversity. These studies extensively described one potentially important genetic process which may occur during a range expansion: the “mutation surfing” phenomenon. In this paper, we describe in detail this process and its potential implications for the establishment of the current genetic diversity in Europe. We also discuss the imitations and perspectives of such computer simulation studies in the field, and possible future improvements to them.

I'm generally distrustful of computer simulations for these matters, specially because they oversimplify everything so much that they can be totally wrong, regardless of results. For example they consider demic expansion as continuous and not, as it normally is in fact, intermittent, in irregular bouts followed by stagnation and even occasionally recession. There can be many many other reasons to impugnate or disbelieve them, unless they'd first make a major effort to approximate the real known facts on the ground, what they do not.

The foundations of all these simulations are too simplistic to be convincing:
what happened to all the "green" and "yellow" dots to the right of the wave front?

So take it please with a good dose of salt and assorted spices.

The results, anyhow, depending on what do you understand that the clines to be considered are, can either support or oppose demic Neolithic replacement. The greatest interest may be in results that, because of the assumption of the wavefront founder effect hypothesis, generate counterintuitive clines as result. Hence, according to Currat's simulations, in the context of West Eurasia, a SW-NE cline would be the product of a Neolithic SE->NW colonization, while a SE-NW cline would be rather the result of a Paleolithic expansion from the Franco-Cantabrian region.

I really appreciate the questioning of the apparently obvious, the food for thought, implicit on those findings, however the models, for what I can see, totally fails to incorporate a growing amount of "native" alleles at the wavefront as it advances, making any attempted conclusion meaningless.

4 comments:

  1. Manju,
    Thank you for your post and commentary. btw, regarding our earlier exchange..... To quote you:

    "I'm generally distrustful of computer simulations" -- except when they are used in climate science. Then I trust them completely.

    I like the skeptical Manju more.

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  2. Maju (not Manju) - Apologies. I need glasses.

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  3. One topic man, right? Why don't you take what you need and post it at your freaky-TeaParty-hockeysticknonsense forums?!

    Not interested. I do not debate modern climate change in this blog. I have a political blog for that (although in fact I'm quite more concerned right now by the use of the European Union against us European citizens and the consequences of Fukushima catastrophe - you will have to dig to find any reference to your single topic).

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  4. Regardless: global warming is a fact, man-made or accidental is debatable but warming is not. It's no "computer simulation" but factual observation.

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