A yet unresolved debate among language acquisition researchers is the functioning of innate language principles in SLA. UG-compatible analyses of interlanguage representations are not sufficient for this purpose. A satisfactory rationale for the involvement of UG can be established by the "poverty of the stimulus" arguments. The study endeavours to contribute to the body of knowledge on the POS argument from the perspective of distributional syntax with regard to the morphophonological and semantic constraints in the acquisition of dative alternation structures. To this end, a grammaticality judgement task was administered to three groups of Persian L2 learners along with a native control group. The results reveal that the L2 learners can acquire semantic and morphophonological constraints on dative alternation structures. It is the knowledge of abstract Case and Case assignment which is restricting the hypothesis space of the L2 acquirers. This in turn implies the operation of a domain-specific learning system in SLA and adds plausible support to the "poverty of the stimulus" argument the evidence of which corroborates UG access view and theory development in L2 acquisition research.