Here’s an interesting observation I made a few years ago that I think makes a good illustration concerning the evolution of eyespots on butterfly wings.
Below is an image of a peacock butterfly, with its very obvious eyespots.
Next, here’s an image of the top edge of the peacock’s upper wings, with beneath it an image of the top edge of another butterfly’s wings that look extremely similar. You can make out the same type of eyespot pattern on both sets of wings.
The second butterfly in the image above isn’t a peacock however, as you can see from the image of the whole butterfly below. It’s a small tortoiseshell, and it doesn’t have eyespots at all.
Assuming that the two species evolved from a common ancestor, how is it that the butterflies might have evolved in such a way that their wing patterns (at least along the top edge) are so similar yet so different?
Here’s a theory.
The common ancestor of the tortoiseshell and the peacock started without eyespots – with wings that were perhaps similar in patterning to the present day tortoiseshell’s (at least in the region of the wings that we’re dealing with). Small variations to the colouring of the patterning on the wings of some of the butterflies then turned the abstract patches at the top of the wings into what we interpret as eye shapes. These eye shapes turned out to be very useful for the survival of the butterflies that possessed them, and thus the eyespot pattern was passed on and reinforced through the generations. Thus the evolutionary line that developed into the peacock diverged from the line that evolved into the tortoiseshell. It’s important to emphasise that the patterning’s resemblance to eyes is purely fortuitous: it’s nothing more than a small random deviation from the abstract patterning that the tortoiseshell butterfly still has – a bit of light colour added here, a bit of dark there. It’s just that this particular small variation in patterning has such an obvious effect, because most creatures are attuned to homing in on eyes (or anything that looks like eyes). Other small random deviations to the patterning would go almost completely unnoticed.
This explanation for the evolution of the eyespot echoes very nicely the theory of the evolution of the eye itself, in which random variations in a species’ makeup eventually lead to the evolution of what otherwise seems to be a near miraculous organ (See The evolution of the eye. )
It’s also possible that evolution took a different route, and that the common ancestor of the tortoiseshell and the peacock both actually possessed eyespots, and that through evolution the tortoiseshell gradually lost its spots. This however doesn’t explain how the butterflies both acquired their eyespots in the first place. I expect that it would have been through a process very similar to the one that I’ve just described.
One last thing.
A while ago I was watching a peacock butterfly sunning itself on the ground. I was watching it through a pair of close-focus binoculars, so that I has a very close-up view of the insect. It was vibrating its wings, which insects often do when they need to warm up their flying muscles. Through my binoculars the extremely close-up view of the rapidly vibrating eyespots on the wings looked truly sinister and scary. If I’d been a predator (which by definition would mean that I’d be very close to the butterfly and would be very much aware of the vibrating of the eyespots) I’d have fled. So vibrating wings may not only be good for warming up wing muscles, but also for warding off predators (which would be particularly useful when the butterfly hadn’t warmed its wing muscles up enough to be able to fly away itself).