My colleague Brendan Hicks
heads a research team that, among other things, has been looking at the evolution of some of New Zealand's freshwater fishes. A number of these fish species belong to the bully family - & it seems that they arrived here relatively recently (in terms of the geological timescale, that is!). Using mtDNA techniques (Stevens & Hicks 2009) Brendan & his team have found that the ancestor of our modern-day bully speices probably arrived in New Zealand about 20 million years ago - & that their closest living relatives are bullies found in southeastern Australia. (that 20mya date fits well with the fossil evidence: the oldest bully fossil in New Zealand is between 60 & 20 million years old.)
Now hang on a minute - these are freshwater fish. Yet New Zealand sits in the ocean - it separated from Australia about 70 million years ago, as Gondwanaland began to break up under the influence of plate tectonics. How can bullies (well, an ancestral bully) possibly have arrived here so recently?
Well, bullies have a marine life stage, & Brendan says that South-East Asia is the most likely origin for both Australian & New Zealand bullies. What seems to have happened is that the fish arrived in Australian waters as marine larvae, and then subsequently crossed the Tasman Sea in the same way before moving up into our waterways
This is borne out by genetic analyses by the resarch group. These trees indicate that our bullies evolved from a single ancestor that arrived from Australia & radiated into different species as it adapted to the range of new niches available to it (Stevens & Hicks, 2009). One factor underlying bully speciation appears to be loss of migratory behaviour, resulting in populations becoming isolated in individual catchments (Michel et al, 2008). And this process isn't finished yet. As Brendan says, "Bully evolution appears to be a work in progress. While some are of New Zealand's bully species are clearly genetically distinct, others are not. This suggests that this genus is still evolving in New Zealand, and [our] research... on the common bully in the Bay of Plenty confirms this."
C. Michel, B.J. Hicks, K.N. Stolting, A.C. Clarke, M.I. Stevens, R. Tana, A. Meyer & M.R.van den Heuvel (2008) Distinct migratory and non-migratory ecotypes of an endemic New Zealand eleotrid (Gobiomorphus cotidianus) - implications for incipient speciation in island freshwater fish species. BMC Evolutionary Biology 8: 49. doi.10.1186/1471-2148-8-49
M.Stevens & B.Hicks (2009) Mitochondrial DNA reveals monophyly of New Zealand's Gobiomorphus (Teleostei: Eleotridae) amongst a morphological complex. Evolutionary Ecology Research 11:109-123
My thanks to Brendan for providing me with the basis of this post :-)