Yes--isolation and then rearranging the female lines through raids. It's also interesting to note that Africans have a great amount of genetic diversity.
I don't see any evidence here that the female lines move around. If the female lines were rearranged wouldn't the mtDNAs be spread all over Melanesia instead of isolated to islands and even sections of populations? The total genome of Melanesia may have a lot of diversity but apparently at any particular local it's not diverse.