Assortative pairing alone can lead to a structured biota in organisms with cultural transmission

Spatial separation is often included in models of ethnic divergence but it has also been realised that urban subcultures can, and frequently do, emerge in sympatry. Previous research tended to attribute this phenomenon to the human tendency to imitate self-similar individuals and actively differentiate oneself from individuals recognized as members of an outgroup. Application of such a model to non-human animals has been, however, viewed as problematic. We present a parsimonious model of subculture emergence where the algorithm of social learning does not require the assumption of an 'imitation threshold'. All it takes is a slight modification of Galton-Pearson's biometric model previously used to approximate cultural inheritance. The new model includes proportionality between the variance of inputs (cultural 'parents') and the variance of outputs (cultural 'offspring'). In this model, assortment alone can lead to the formation of distinct cohesive clusters of individuals (subcultures) with a low within-group and large between-group variability even in absence of a spatial separation or disruptive natural selection. Sympatric emergence of arbitrary behavioural varieties preceding ecological divergence may thus represent the norm, not the exception, in all cultural animals.