The Weka, a flightless rail on New Zealand, shows clear genetic patterns across a narrow seaway between the two main islands.
New Zealand is comprised of two main islands (conveniently named North Island and South Island), surrounded by about 600 smaller islands. The two islands are separated by Cook Strait, which is 22 kilometers wide at its narrowest point. That doesn’t sound like a big distance if you can fly, but what if you are a flightless rail?
In a recent study, published in Molecular Ecology, Steve Trewick and his colleagues set out to answer this question for the Weka (Gallirallus australis), a flightless land bird that is endemic to New Zealand. The current distribution of this bird on both main islands can be the outcome of several scenarios. Perhaps some birds walked from one island to the other when sea levels where low. Or maybe the populations have always resided on one of the islands, never meeting each other.
A Lousy Choice
In addition to several genetic markers, the researchers also studied two lice species of the Weka populations. These lice are passed on from parent to offspring and provide an independent approach to study evolutionary history. A classic example of this method is the striking concordance between evolutionary trees of seabirds and their parasites. Finally, they compared the Weka results with other (flying) bird species in New Zealand. Do they show similar patterns?
The findings are clear: all genetic markers and the lice point to two primary lineages corresponding to North and South Island. Moreover, this division existed before the last glacial maximum when it was possible to walk across Cook Strait. So, Wekas from different islands did have the opportunity to meet, but did not interbreed (or very little).
The same pattern holds for some birds species, such as the Toutouwai Robin (Petrcoica australis) and the Whio duck (Hymenolaimus malacorhynchos), but not for others, such as Kereru Pigeon (Hemiphaga novaeseelandiae) and Karearea Falcon (Falco novaeseelandiae). Clearly, the dynamics across the Cook Strait are more complicated and probably species-specific. The authors conclude that “this narrow seaway is unlikely to have been the direct cause of lineage splits. Rather it likely represents an environmental step where spatial and ecological constraints intersect.”