What’s it for?
About 50 million years ago, a family of insects — the Phylliinae — evolved a distinctive piece of anatomy: a body that looks like a leaf. They also evolved the capacity for catalepsy or statuesque stillness. Their leafy body is so exquisitely designed that even predators with superb search images are fooled as they walk or fly by. But from the fact that the leafy body provides these insects with an invisibility cloak, and the fact that this enables them to escape predation, we cannot conclude that the leafy body evolved for predator evasion. What something is used for today may be different from what it evolved for — the difference between current utility and original function. To show that the leafy body evolved for predator evasion, we need to know more, which we do. For one, the leafy body is paired up with a requisite behavioral adaptation: turning to stone. If leaf insects fluttered about as actively as any other insect, their motion alone would cry out to the predators. Optimal effectiveness requires acting like aleaf. But acting like a leaf without the leafy body has its own independent benefits, paying off in terms of predator evasion, as well as sneaking up on potential mates. It would therefore make good sense if stillness evolved first followed by a leafy body. This is precisely what evolution’s record reveals.
The adaptive advantage that comes from statuesque stillness and a leafy camouflage can only be measured against the backdrop of today’s predator line-up. If some future-predator evolves more sophisticated abilities to discriminate real leaves from faux leaves, the Phyllinae will be out of luck. This new pressure from predators will, in turn, push for new evasive tricks, thus initiating the classic cycle of predator-prey evolution. What is adaptive for the Phylliinae today, may not be adaptive tomorrow.
The comparative study of the Phylliinae raises a class of questions posed by all evolutionary biologists, independently of their taxonomic biases or interests in physiology, morphology, or behavior: How did it originally evolve? What adaptive problem did it solve? Did it evolve to solve one adaptive problem, but over time shift to solve another — a case of what the late evolutionary biologist Stephen J. Gould referred to as an exaptation? Does the exaptation generate profits or losses for survival and reproduction? Is the trait associated with byproducts, incidental consequences of the evolutionary process? What effects, if any, do these byproducts have on survival and reproduction? These questions apply with equal force to evil as they do to language, music, mathematics, and religion. The fact that evil, relative to a leafy body, is more difficult to define, harder to measure, and impossible to experiment upon — at least ethically — doesn’t mean we should take it off the table of scientific inquiry. What it means is that we must be clear about what we can understand, and how we can distinguish between the various
interpretations on offer. When we explore the evolution of evil, what are we measuring and what evidence
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