  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.
