Esther Pilla reports on a discovery in evolution
It is very rare for scientists to catch examples of parallel, convergent evolution, but earlier in March Professor Rosemary G. Gillespie and her colleagues from the University of California, Berkeley, published a study that highlighted parallel evolution in Hawaiian stick spiders. The group analysed a genus of spider, Ariamnes, whose ancestor probably first arrived on the oldest Hawaiian islands and later spread to the youngest ones. The ancestor used other species’ spider-webs to feed, either on insects or on the host. When Ariamnes started free roaming and hunting, they had to face the challenge of hiding from predators. Hence, they evolved into three differently camouflaged species: matt white (to hide in lichens), dark (to hide in moss) and gold (to hide on the underside of leaves). Both the dark and the gold coloured stick spiders live on four islands of the archipelago, whereas the white species live on only two.
To understand how they evolved, the team analysed their DNA and, surprisingly, they found that the spiders living on the same island were more closely related than similarly camouflaged spiders on the other islands. This could only mean one thing: the ancestors of Ariamnes on the four different islands all evolved into the dark, gold and white coloured spiders in parallel. This kind of deterministic evolution was already observed for the spiny-leg spiders of the genus Tetragnatha, which display different mimetic colours depending on their feeding and living habits. In the case of Ariamnes spiders, we could hypothesise that the strategy adopted to escape predators by camouflaging to match different backgrounds was so successful that evolution ‘had to’ repeat itself. These results may help in understanding what determines the evolution of different species in parallel and hopefully predict why evolution repeats itself in some cases but not others.