...Here, we use high-quality data and novel methods to test two models of genetic and linguistic coevolution in Northern Island Melanesia, a region known for its complex history and remarkable biological and linguistic diversity. The first model predicts that congruent genetic and linguistic trees formed following serial population splits and isolation that occurred early in the settlement history of the region. The second model emphasizes the role of post-settlement exchange among neighboring groups in determining genetic and linguistic affinities. We rejected both models for the larger region, but found strong evidence for the post-settlement exchange model in the rugged interior of its largest island, where people have maintained close ties to their ancestral lands. The exchange (particularly genetic exchange) has obscured but not completely erased signals of early migrations into Island Melanesia, and such exchange has probably obscured early prehistory within other regions. In contrast, local exchange is less likely to have obscured evidence of population history at larger geographic scales.
Anthropology.net has already surveyed the paper, so if you're interested in this specific result just go there. Rather, I want highlight a general point: linguistic and genetic variation are correlated, but the large residual highlights differences between the two.
My native language is English. That of my parents is not English. I share the language of my social peers in totality; there is no hybridization between my parental (vertical) and cultural (horizontal) influences. I know couples where one parent is an immigrant and not a native English speaker, while the other is a native English speaker. Their children are native English speakers. Language facility is genetic, but specific language is culturally conditioned. In particular, peer group has an overwhelming influence on dialect details (autistic individuals are an exception to this, they will often speak with their parents' accents if their parents are immigrants). Not only change language change fast, but between group variation can easily dwarf within group variation. There is variation in dialect among the speakers of Spanish in the Mexico, and between speakers of English in the United Sates, but the two sets of dialects do not intersect with each other in terms of intelligibility. Language is an easy way for human societies to manifest strong ingroup-outgroup differences. Consider the origin of the word shibboleth. The ability to communicate is as functional as a shield, but the way in which one communicates is symbolic just as the crest or herald upon a shield is meant to indicate one's group affiliation (in the mÃªlÃ©e it is important to distinguish friend from foe).
Genes are different. I share genes with my parents (more precisely, 1/2 of my genes are identical by state with each parent, and by definition identical by descent), not with my peers. Transmission is purely vertical. Your genetic inheritance is by definition heritable (OK, actually, invert the relationship, but you see what I mean). Genetic evolution generally occurs more slowly than cultural evolution. Additionally, unlike cultural evolution it is straight-jacketed in terms of its transmission; you are a balanced compound of your parents (genomic imprinting the specific nature of the 23rd chromosome complicates this picture, but it works as a good first approximation). In contrast, in Judaism your identity is generally defined maternally, while in Islam it is defined paternally. In the United States people one can often choose their group identity, within broad limits. The rapidity and plasticity of cultural change means that it is an ideal driver of between group variation. On the other hand, genetically even small amounts of gene flow between groups can quickly erode differences. In fact, only 1 migrant per generation is needed between two populations to prevent random changes from generating between group divergence.
As an illustration, imagine two tribes, A and B. They both settle an island simultaneously from another location where they had been separated. Genetically and culturally they are very distinct in the first generation. But if there is migration between the two groups, for example exchanges of marriage partners, or they raid each other and "steal" women and children, between group genetic differences will quickly abate. Nevertheless, because of different nature of cultural evolution the two tribes may still speak totally different languages, worship totally different gods, and so forth, indefinitely. A concrete example are the Y lineages of Iran (e.g., M17). In western Iran they resemble those of the Middle East, while in eastern Iran they resemble those of Central Asia. But in both these regions the peoples are generally self-consciously Persian, speak Farsi, and adhere to Shia Islam, in contrast to their neighbors. But in the center of Iran there are large expanses of wasteland. Some scholars have surmised that this explains the genetic difference, for nearly 3,000 years the Persian peoples of western Iran were subject to gene flow from and to the populations of western Asia, and those of eastern Iran have had gene flow with the lands to their east and north. Even if they were originally genetically the same the cumulative effect of interaction with their neighbors has resulted in enough "turn over" of genetic variation that they are dissimilar now. At the same time men and women who moved into the Farsi speaking lands assimilated themselves and their offspring to a Persian cultural identity, so that the east and west of Iran retain their coherence as Persian domains.
The interesting point here is that biological evolution is constrained and relatively predictable. In contrast, cultural evolution has a much wider range of possible variance in the rate of its change. On the one hand, societies can rapidly transform their suite of culture-defining traits in one to two generations (e.g., the conversion of the Roman elite to Christianity in the late 4th century), but on the other hand they can maintain their coherence over long periods of time and not vary from a core template (e.g., the persistence of Jewish identity over 2,500 years).
Speaking for Iranians.... do you have more info about the genetics of Iranians?
Some of the papers I've read seem to vary significantly.
For example, when I was looking at Y-DNA Haplogroups, there's this paper from 2004...
And then there's paper from 2006...
One thing that stands out is the Y-DNA Haplogroup I in the first paper. But a lack of it in the second. I don't think the second one actually tested for Y-DNA Haplogroup I, but you'd expect it to then show up under Y-DNA Haplogroup F*, but F* shows nothing.
The word "autism" in your article caused it to pop up in my search this morning. I have been diagnosed with the autism spectrum disorder Asperger's Syndrome.
I do not know if I am unique in the AS group, but I have realized that I am unable to recognize voices by tone and timbre; I cannot even recognize my own children's voices. When someone calls me without identifying themself, assuming I will instantly recognize their voice, I do not recognize who I am talking to until they have spoken long enough to recognize their unique individual dialect.
Also of interest- I have a brother who moved from California to New York to England while his children were growing up. Each of his three children has a profoundly different accent.
My kids speak accent-free Japanese and English. 10 years in Japan and then 3 in the US. What is funny are the random English words my in-laws now use.
"In fact, only 1 migrant per generation is needed between two populations to prevent random changes from generating between group divergence."
OTOH, if you combine this with your other recent observation that much of our species' EEA is cultural, then we have a recipe for cultural differences driving genetic divergence via selection. Since the Neolithic (and further back) this has typically happened along geographic lines, but with lower search and exit costs distance matters less and preferential assortment more. The Ashkenazim might only have been ahead of their time -- assuming the robots don't kill us all, I sometimes wonder if the world of the future looks a bit like India, only moreso.
(An aside: how fluid were caste boundaries, historically? My cognitive anthropology instincts make me suspect that they were de facto somewhat more permeable than public pronouncements would lead one to believe, but I have no idea and am curious.)
Actually, I'd also relate this to your back-and-forth with Manzi over epistasis: if epistasis really was huge deal then it would be really hard for introgression (or just hybrid vigor) to happen since the probability of an allele being pasted onto a different genetic background being deleterious goes up *very* fast with the number of co-adapted gene-gene interactions in that background. Manifestly, this doesn't seem to be much of a problem between populations that haven't diverged too long ago, and in fact if anything the opposite is often true.
Just as manifestly this isn't so for culture: for something like language especially, you have to pretty much buy the whole package to get significant utility out of any part of it. (Loan words exceptions, and fairly small potatoes.) To the extent that culture supervenes on language, that raises barriers to diffusion for most of the rest of it too. In order to get any appreciable inter-group diffusion going the information has to be stripped of context for export -- the nonadditive made additive by passing through a bottleneck in the form of a shrewd translator who knows just as well what to leave out as what to put in. This is why (e.g.) Buddhism invaded China while Hinduism didn't.