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Extreme Function: Why Our Brains Respond So Intensely to Exaggerated Characteristics

How quirks of perception drive the evolution of species

Marian Stamp Dawkins, an animal behavior expert at the University of Oxford, has championed the idea that aspects of sensory processing can influence the evolution of communication signals; for example, a nocturnal species whose predators are color-blind would not evolve colored warning splotches. Our idea complements hers but takes it further, by arguing that higher-order principles of perception may also play a role.

Another principle that may affect evolution is known as grouping. The visual system has an obsessive desire to make whole objects from fragmentary evidence—such as a lion largely obscured by leaves and shadows. Like-colored fragments are interpreted as bits of a single object that is partially hidden by another, closer object. As naturalists have long recognized, this tendency is cunningly exploited by reef fish, which evolved bold colored splotches that “break” their outlines and confuse predators seeking continuous contours.

Proofs of Concept
If perceptual laws influence the development of species, what would evolutionary biologists expect to see? For one, the progressive “caricaturization” of easily recognizable physical traits over time. And indeed, such trends are commonly seen in the evolution of mammoths, ankylosaurs, titanotheres and other animals.

Another prediction from the theory is that unseen parts—internal organs—would not be subject to perceptual selection pressures and hence should diverge considerably less. Overall, this appears to be true. A rhesus monkey’s liver doesn’t look much different from a human one.

Finally, because plants do not have sophisticated sensory systems, they should vary less in appearance than animals do, except when selection has been done for them by animals. This would explain why leaves and trunks look much alike, whereas flowers, which “compete” to be visited by insects and hummingbirds, are stun­ningly conspicuous and variable. There is even one species, the bee orchid, whose flower perfectly resembles a caricature of
a female bee—a superbee—to attract pseudocopulation and cross-pollination by male bees.

Ultimately, our hypothesis is not a mechanism outside Darwin’s theory but an unexpected interaction within it. His principle of natural selection leads to the emergence of brain mechanisms that enable an animal to quickly detect healthy sexual partners of the same species. But inevitably these cognitive processes have side effects. They evolved to increase a species’ fitness but may lead to perceptual quirks that do not promote fitness—and may even work against it. Thus, the study of visual illusions—and the laws they exploit—offers clues to certain otherwise mysterious trends in evolution.

This article was originally published with the title "Illusions: Carried to Extremes."

(Further Reading)

  • On the Origin of Species. Charles Darwin. John Murray, London, 1859.
  • The Descent of Man. Charles Darwin. John Murray, London, 1871.
  • Sensory Bias and the Adaptiveness of Female Choice. Marian Stamp Dawkins and Tim Guilford in American Naturalist, Vol. 148, pages 937–942; November 1996.
  • Phantoms in the Brain. V. S. Ramachandran and Sandra Blakeslee. HarperCollins, 1998.
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