Dienekes has touched upon it in detail, so I don't have much to add. Except for two points: 1) The ancestry cline here is not due to isolation-by-distance, but the expansion of the Austronesian population rather precipitously ~4,000 years ago. As Dienekes observed this was rather clear by non-genetic means; this is just icing on the cake. 2) There is evidence of an Austro-Asiatic substrate across maritime Southeast Asia. For whatever reason it seems that Austro-Asiatic speaking agriculturalists ceased their push east at the Wallace Line.