The aesthetic theory of evolution that we are proposing also revolves around mating behavior, but it differs from sexual selection. For one thing, sexual selection only explains why secondary sexual characteristics in males (the peacock’s tail, the rooster’s wattle, the unwieldy antlers of the Irish elk) become exaggerated. The peak-shift effect, in contrast, helps to explain extreme traits and behaviors that pertain to all members of a species (both male and female giraffes must identify potential mates, which helps to explain why both genders have long necks).
Because humans (taxonomists included) are diurnal and hence visual creatures, we tend to place a strong emphasis on appearances. But the principle of peak shift can apply as easily to nonvisual signals. For nocturnal critters such as rodents that use smells to find mates and interpret their world, attractions to strong scents could drive evolutionary change. These changes would be hard to see but just as real. If dogs were taxonomists, the evolutionary trees in their textbooks would look very different from ours.
The Gull Chick Principle
Other rules of aesthetics besides peak shift can also be invoked to explain the astonishing diversity of species. One is what we call the “gull chick principle.”
Niko Tinbergen, a pioneering investigator of animal behavior, experimented with herring gulls 50 years ago, but the relevance of his work to evolutionary theory has not been widely appreciated. The adult herring gull has a long, yellow beak with a red spot near the tip. As soon as a chick hatches, it starts pecking at this spot, which triggers the parent to regurgitate food into the chick’s mouth. How does the chick recognize its mother? Tinbergen found that it doesn’t: chicks will peck as intently at a disembodied beak.
Why is a beak sufficient? The purpose of vision is to identify and interpret objects and events while expending the least amount of mental processing power. Through millions of years of evolution, the chick’s brain has acquired the wisdom that this long thing with a red spot always has a mother, not an inquisitive ethologist, attached to it, and it makes an interpretive shortcut.
Tinbergen next found that a beak is not even required. He held out a long, yellow stick with three red stripes on it, and the chicks pecked it—more, in fact, than they would have pecked at a real beak. Tinbergen had stumbled on a superbeak!
Why does this happen? Clearly, there are neural circuits in the visual pathways of the chick’s brain that are specialized to detect the red spot on a beak as soon as the chick hatches. Perhaps the neurons’ receptive field embodies a rule such as “the more red contours the better.” So even though the stick does not look like a beak—maybe not even to the chick—this strange object is more effective than a real beak at activating the bird’s beak-detection system. Hence, we predict that a species of gull will emerge that has two or even three red beak stripes instead of just a bigger red splotch. Another, even more striking example of the gull chick principle is the idiosyncratic preference (demonstrated in the lab) that guppies show for potential mates that have been painted blue—even though in nature guppies are not blue. Again, we anticipate the emergence of a new species: the blue guppy. It’s not often in evolutionary theory that one can make such specific predictions.
The gull chick principle may apply widely, because the visual system of every animal is wired to use specific characteristics to identify others of its species. If a potential mate diverges from the standard in a way that more optimally excites “species-identifying” brain circuits, the genes that promote such supertraits will flood the population. Unlike the peak-shift principle, no obvious parameter is being exaggerated (such as a long neck); the changes in appearance are selected because of idiosyncratic aspects of neural wiring. Even the florid, almost comical, exaggeration of dance rituals in some bird species may be influenced by this principle.