I know you may read this & think I'm a bit odd… But anyway – just after the L3 exam I was talking with a student & she said, why did there have to be cockroaches in a question? My answer: why not? They're just another animal (even if many people don't like them much) & the examiner had obviously chosen this particular example to tease out understanding of evolutionary processes. (Could have been worse – they might have chosen tapeworms!)
Anyway, I did a bit of searching & found the original paper (Chinn & Gemmell, 2004). The abstract begins with the statement that [the] South Island of New Zealand offers unique opportunities to study insect evolution due to long-term physical isolation, recent alpine habitats and high levels of biotic endemism. In other words, here's an excellent opportunity to look at the potential for speciation events in a group of organisms that are isolated (no gene flow) and in a rapidly changing environment (new selection pressures). Chinn & Gemmell chose cockroaches as their study taxon & used DNA sequence data to test their hypothesis that cockroaches as a group showed rapid adaptive radiation in response to the habitat changes brought about by the formation of the Southern Alps.
Many NZ plants & animals have what are described as 'disjunct' distributions, where species of a particular genus are quite widely separated geographically. One explanation for this distribution suggests dramatic climatic and geological changes that divided habitats and generated new ones. These changes were probably due to the mountain-building event that produced the Southern Alps, an event which had a marked impact on wind and rainfall patterns (cooler & wetter on the west, warmer & drier to the east). Their effect was to kill off initially widespread plant & animal species over much of their former range, leaving the surviving populations isolated in refugia and subject to different selection pressures and the effects of genetic drift.
Chinn & Gemmell found that between 6 and 4 million years ago the ancestral cockroach (Celatoblatta) lineage split into 6, and that this correlated with the rise of the Southern Alps. Glaciation of the Alps during the Pleistocene (which had effects on both climate and landscape, and which would have further restricted the range of many species) was associated with a second, rapid round of genetic divergence. They discuss the example of C. hesperia populations: they are close genetically & seem to have moved into wet subalpine habitats fairly recently. They are also found on both sides of the Alps, which suggests dispersal through mountain passes in periods when the glaciers had receded: a suggestion supported by the authors' finding that genetic radiation of the populations occurred 1-0.5mya.
The genetic data also indicate a link between the cockroach species on Banks Peninsula and those found on the Chatham Islands. What's your suggestion for how the Chatham species got there?
W.G. Chinn & N.J. Gemmell (2004) Adaptive radiation within New Zealand endemic species of the cockroach genus Celatoblatta Johns (Blattidae): a response to Plio-Pleistocene mountain building and climate change. Molecular Ecology 13: 1507-1518
PS Off to Paris today & Madrid on Wednesday. I've been staying near Chambéry, which is really interesting geologically – & biologically: I saw Milanese eagles up in the mountains on Thursday, & apparently somone found a cave full of cave bear skeletons in a pass up behind Chambéry.