5.9 The tree branching of cultural traits.

The application of phylogenectic methods in order to understand cultural variation has led to new insights into human migration, demography and the history of culture. For the most part these insights have been produced by the application of phylogenetic (tree branching) methods originally developed in biology that has also proved useful to our understanding of cultural traits and data. In genetic inheritance all members of a species including humans share a common biological history. Cultural lineages however vary in populations that share the same genes since cultural evolution has produced different cultural trait constellations. Genetic and cultural inheritance must therefore be treated as following separate evolutionary paths although all humans carry parallel genetic histories.

In the case of cultural evolution societies may also reach steady states where the cultural syndrome of traits remains relatively stable. The evolution of major religions have produced many enduring belief systems that although not unchallenged have remained remarkably similar for centuries. Religions have produced a multiplicity of linked beliefs systems that reflect both horizontal and vertical evolutionary processes that have become stable and rigidly adhered to through sanctions or enforcement.

The differential extinction of cultural groups through competition has reduced the variation of cultural traits in the world. To demonstrate cultural evolution requires that significant cultural differences exist between groups that in turn affect the group’s relative competiveness. Groups that lose in intercultural competition are replaced by the more successful groups (Boyd & Richerson, 1990). Cultural groups that lose in competition with neighboring groups are either dispersed or assimilated into the more successful group. Members of cultural groups that assimilate are subject to the forces of social conformity or sanctions that serve to maintain cultural differences with other groups. Selection from intergroup competition is a slow process and that is supported by what we observe in the historical and archeological records. For example there is a 5000 year time lag between the beginnings of agriculture and the next major step in cultural evolution when the first city-states emerged in the record.

5.10 Limitations of genetic and cultural co-evolutionary theory: Horizontal and vertical cultural evolution.

Researchers have advocated a dual inheritance theory that includes both genetic and cultural evolution (Cavalli-Sforza & Feldman, 1981; Richerson & Boyd, 2005). In co-evolutionary theory genes and culture are viewed as separate but interdependent systems. Each system is based on autonomous mechanisms for transmitting information, with genes encoding for biological reproduction and learning and imitation transmitting cultural information. However, in the case of genetic evolution information can only be transmitted vertically from parents to children. Therefore the major difference between the two types of evolution is that cultural trait information can also be transmitted horizontally between neighboring groups and in the life time of individuals (Page, 2003).

Horizontal transmission of traits creates greater complexity in tracing cultural evolution as compared to genetic evolution. Cultural evolution also occurs more rapidly compared to biological evolution. These differences place limits on the usefulness of the comparative phylogenetic approach. Horizontal evolution occurs when two cultural groups with interdependent relationships mutually infuse cultural ideas or when the dominant group transmits cultural traits to the weaker partner. Corresponding mutations within same generation are rarely observed in genetic evolution except epigenetic changes that we noted in regard to the poisoning by Agent Orange in Vietnam or other traumatic events like the holocaust. The divergence of cultural traditions that result from population splits when cultural groups migrate ensures separate development isolated from the parent culture that can however be traced by phylogenetic trees. Cultural divergence patterns initially evolved when populations expanded in search of new fertile and productive land. Nevertheless Neolithic expansions in various regions still left strong signatures associated with the growth and expansion of language families (Gray & Atkinson, 2003).

Cultural transmission of traits is a primary determinant of human behavior. At some point the adoptions of these traits had adaptive functions that affected the selective fitness of cultural groups. Adaptive fitness also determined what traits were transmitted from one generation to the next. The transmissions of cultural traits that affect adaptation to ecology demonstrate the mutual interdependence between cultural and genetic evolution (Bettinger & Eerkens, 1999). The record of artifacts reveals large and complex cultural variations that could only be brought about through evolutionary change over time (Shennan, 2008). Phylogenetic comparative methods can evaluate cultural evolution vertically by using tree branching structures to demonstrate the descent of cultural traits over time. The structure of these trees can provide insights to when cultural traits diverged, and can also be used to make comparative inferences about trait evolution (Currie, Greenhill, & Mace, 2010).

As we noted cultural traits that are very adaptive may spread rapidly horizontally between cultures. The use of horses and the invention of the wheel come to mind. Fashions also move rapidly between countries in the developed parts of the world, although such horizontal transfer is produced by little effort or costs and involve primarily social conformity processes. For example women may be exposed to two or more opposing forces of conformity in fashion decisions and the resolution is a compromise. One study (Hardy & Larsen, 1971) at a religious university showed that women responded to conflicting norms regarding dress (skirt length) by making a compromise between fashion and religious standards. Conformity to norms may also prevent adaptive change when it threatens fundamental beliefs or the entrenched power structures. However, dysfunctional beliefs create stress in a world dominated by the information revolution and will eventually be reformed or obliterated as we saw in Eastern Europe and currently in Arabia (and also in other events in world history). However, since there are significant entrenched interests in social structures and new forms of social organization are not easily diffused from one culture to another (Murdock, 1949).

Cultural world views and geographic distance serve to strengthen cultural stability and coherence allowing for less horizontal transmissions of traits. These factors ensure that cultural information proceeds vertically and supports the idea that phylogenetic methods are at least to some extent suitable models (Mace and Holden, 2005). Cultures can be considered assemblages where cultural information is transmitted vertically within the cultural cluster, but communicated to other cultures horizontally producing a cultural evolution more complex than can be measured by tree-branching models. However, whether human history is phylogenetic and tree like or requires more complex evaluation is an issue still evaluated in the literature (Gray, Bryant, & Greenhill, 2010).