Dixon's theories of Australian Language diachrony have been based on a model of punctuated equilibrium (adapted from the eponymous model in evolutionary biology) wherein he believes Australian languages to be ancient and to have--for the most part--remained in unchanging equilibrium with the exception of sporadic branching or speciation events in the phylogenetic tree. Part of Dixon's objections to the Pama Nyungan family classification is the lack of obvious binary branching points which are implicitly or explicitly entailed by his model.
