ASPM – Replicated Typo

Return of the Language Evolution Tree

A while ago, some collegues and I noticed that two prominent books on Language Evolution -Christiansen & Kirby’s Language Evolution and Fitch’s Evolution of Language – both included a picture of an acacia tree in the sunset on their covers. On closer analysis, it turned out that they were the same tree:

Thus began the Acacia Tree Hypothesis of Language Evolution.

Following this up, I was thinking about Dediu & Ladd’s discovery that linguistic tone is has certain genetic correlates. Here’s the map of languages with linguistic tone:

However, I suspected the devious influence of acacia trees and so I found some information on their geographic distribution:

As I suspected, countries in which the acacia tree Acacia nilotica grows are significantly more likely to have tonal languages:

	Tone	No Tone
Acacia Trees	163	117
No Acacia Trees	104	237

(Chi-squared with Yates’ continuity correction = 47.1, df = 1, p < 0.0001, data from Crop Protection Consortium and the World Atlas of Language Structures).

The plot thickens …

Dediu, D., & Ladd, D. R. (2007). Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, Microcephalin and ASPM. Proceedings of the National Academy of Sciences, 104, 10944–10949.

Fitch, W. T. 2010 The evolution of language. Cambridge, UK: Cambridge University Press.

Christiansen, M. and Kirby, S. (2003). Language Evolution. Oxford University Press.

Update:

I’ve added the images David mentioned to the post:

Also, The Babel’s Dawn blog banner

Genetic Anchoring, Tone and Stable Characteristics of Language

In 2007, Dan Dediu and Bob Ladd published a paper claiming there was a non-spurious link between the non-derived alleles of ASPM and Microcephalin and tonal languages. The key idea emerging from this research is one where certain alleles may bias language acquisition or processing, subsequently shaping the development of a language within a population of learners. Therefore, investigating potential correlations between genetic markers and typological features may open up new avenues of thinking in linguistics, particularly in our understanding of the complex levels at which genetic and cognitive biases operate. Specifically, Dediu & Ladd refer to three necessary components underlying the proposed genetic influence on linguistic tone:

[…] from interindividual genetic differences to differences in brain structure and function, from these differences in brain structure and function to interindividual differences in language-related capacities, and, finally, to typological differences between languages.”

That the genetic makeup of a population can indirectly influence the trajectory of language change differs from previous hypotheses into genetics and linguistics. First, it is distinct from attempts to correlate genetic features of populations with language families (e.g. Cavalli-Sforza et al., 1994). And second, it differs from Pinker and Bloom’s (1990) assertions of genetic underpinnings leading to a language-specific cognitive module. Furthermore, the authors do not argue that languages act as a selective pressure on ASPM and Microcephalin, rather this bias is a selectively neutral byproduct. Since then, there have been numerous studies covering these alleles, with the initial claims (Evans et al., 2004) for positive selection being under dispute (Fuli Yu et al., 2007), as well as any claims for a direct relationship between dyslexia, specific language impairment, working memory, IQ, and head-size (Bates et al., 2008).

A new paper by Dediu (2010) delves further into this potential relationship between ASPM/MCPH1 and linguistic tone, by suggesting this typological feature is genetically anchored to the aforementioned alleles. Generally speaking, cultural and linguistic processes will proceed on shorter timescales when compared to genetic change; however, in tandem with other recent studies (see my post on Greenhill et al., 2010), some typological features might be more consistently stable than others. Reasons for this stability are broad and varied. For instance, word-use within a population is a good indicator of predicting rates of lexical evolution (Pagel et al., 2007). Genetic aspects, then, may also be a stabilising factor, with Dediu claiming linguistic tone is one such instance:

From a purely linguistic point of view, tone is just another aspect of language, and there is no a priori linguistic reason to expect that it would be very stable. However, if linguistic tone is indeed under genetic biasing, then it is expected that its dynamics would tend to correlate with that of the biasing genes. This, in turn, would result in tone being more resistant to ‘regular’ language change and more stable than other linguistic features.

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Genetic Components and Cultural Differences: The social sensitivity hypothesis

Cultural differences are often attributed to events far removed from genetics. The basis for this belief is often based on the assertion that if you take an individual, at birth, from one society and implant them in another, then they will generally grow up to become well-adjusted to their adopted culture. Whilst this is more than likely true, even if there may be certain cultural features that may disagree with someone of a different ethnic background (e.g. degrees of alcohol tolerance), the situation is not as clear cut as certain political factions may have you believe. Yet, largely due to studies on gene-culture coevolution, we are now starting to understand the complex dynamics through which genes and culture interact.

First, a particular culture may exert selection pressures on genes that provide an advantageous benefit to the adoption of a particular cultural trait. This is evident in the strong selection of the lactose-tolerance allele due to the spread of dairy farming. Second, pre-existing gene distributions provide pressures through which culture adapts. Off the top of my head, one proposed example of this is a paper by Dediu and Ladd (2007), which looked at how the distribution of the derived haplotypes of ASPM and Microcephalin may have subtly influenced the development of tonal languages. The paper in question, however, is looking more broadly at culture. Specifically, the authors, Baldwin May and Matthew Lieberman, examine recent genetic association studies and how within-variation of genes involved in central neurotransmitter systems are associated with differences in social sensitivity. In particular, they highlight a correlation between the relative frequencies of certain gene-variants and the relative degree of individualism or collectivism within certain populations.

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Continuity or Discontinuity: are our minds purely shaped by natural selection?

The debate concerning the origin of our minds stems back to the diverging opinions of Darwin (1871) and Wallace (1870). When Charles Darwin first discussed the evolution of our seemingly unique cognitive faculties, he proposed that there is “no fundamental difference between man and the higher mammals in their mental faculties.” (Darwin, 1871, pg. 66). Conversely, Wallace was suspicious of whether natural selection alone could have shaped the human mind, writing: “[…] that the same law which appears to have sufficed for the development of animals, has been alone the cause of man’s superior mental nature, […] will, I have no doubt, be overruled and explained away. But I venture to think they will nevertheless maintain their ground, and that they can only be met by the discovery of new facts or new laws, of a nature very different from any yet known to us.” In the intervening years, the debate surrounding the degree of continuity between animal and human minds still rages on in contemporary discussions (Bolhuis & Wynne, 2009; Penn, Holyoak & Povinelli, 2009).

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ASPM, Microcephalin and Tone

Disclaimer: I know this post is on a paper released over a year ago; however, I’m still going to write about it for three reasons: 1) I did a presentation about it earlier this week (20/01/08); 2) I think it relates to a recent buzz around gene-culture co-evolution; and, 3) It’s a bloody awesome paper.

So, what is the paper called? Okay, once you read this title, do not yawn, go to another website or… Linguistic tone is related to the population frequency of the adaptive haplogroups of the two brain size genes, ASPM and Mircocephalin. See, now we’ve got the hard part out of the way, I can begin to discuss exactly what the authors, Dan Dediu and Robert ‘Bob’ Ladd, found and why it’s important to our understanding of linguistics, genetics and evolution. It’s really interesting, honestly.
Continue reading “ASPM, Microcephalin and Tone”