Language vs. genes, similarities & differences

Genetic and Linguistic Coevolution in Northern Island Melanesia:

...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 mêlée 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).