Dialects and speakers can exhibit robust competition in the morphological realization of forms. Variationist Sociolinguistic studies have revealed a structured hierarchy of constraints underlying these alternations. For example, verbal –s is highly favoured with noun phrase subjects, ‘the cattle all goes’, but is rare with pronouns, ‘they go’, unless there is an intervening adverb, ‘they often goes’. Constraints reorganize in contexts of diffusion making pronouns more likely to take –s and weaker constraints to fall away. The multiplex nature of variable constraints, their shifting weights and the resulting optimal forms exposes the inner mechanisms of the linguistic system and how its complexity plays out in contexts of change.