Scientist discovers evolutionary paradox of predation

June 29, 2006, volume 36, no. 5
By Carol Thorbes

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Predators not only size up their prey based on their looks, but also help drive species evolution. Simon Fraser University doctoral student Patrik Nosil's finding supports a suspicion that scientists have long held but had trouble proving because predation is hard to study.

The Burnaby resident is the lead author on Experimental Evidence that Predation drives Divergence in Adaptive Radiation, which appeared in the June 2006 issue of the science journal Proceedings of the National Academy of Sciences (PNAS). Bernard Crespi, an SFU biology professor and Nosil's doctoral supervisor, helped Nosil with his research and article.

Nosil used the walking stick insect, a reproductively prolific organism that doesn't fly and is easily contained, to find out whether predation could influence species radiation or evolution. Nosil and Crespi discovered that the bug's fate at the beak of its main predator, the scrub jay, depends on how well the bug blends into its own food.

The duo's colleague Cristina Sandoval, a researcher at the University of California, Santa Barbara, recently discovered that that there are two variations of the walking stick insect. Each variation feeds naturally on a leafy green plant that matches its body colouring, size and shape.

One variety has a dark green body with a strip down the middle and feeds on a plant with long dark skinny leaves that look like conifer needles. The other bug type, which has a solid bright green body, munches on a plant with bright green broad leaves.

The two varieties differ from each other in appearance in 11 features. But in experiments where the survival of each variation was measured on both of the food sources, Nosil and Crespi found that camouflage abilities most ensured their escape from scrub jays. Hence, their conclusion that the walking stick insect's main predator has forced it to evolve body types that are easily camouflaged by particular food.

 “We believe that the ecological variety naturally found on long skinny leaves has evolved to have a white strip down the middle of its body because that makes it look like two skinny leaves,” says Nosil. Similarly, the solid bright green bug found refuge from predators when it dined on bright green broad-leafed plants.

Species evolve when organisms adapt genetically and behaviourally to cope with challenging ecological conditions. They break off into new species when some organisms within one species become so different that they can no longer mate with the others.

Nosil says the discovery that predation drives species' evolution as much as competition for food is crucial to furthering our understanding of biodiversity and preserving it. Notes the Victoria native, “If scrub jays didn't exist, these insects wouldn't have evolved to be so different and diverse.”

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