How butterflies came to look like dead leaves

The secret of how butterflies came to look like leaves may have been
revealed – fittingly, by an insect that gave wings to Charles Darwin’s theory of evolution.

The question of how the closed wings of dead leaf (or oakleaf) butterflies from the Kallima genus came to perfectly resemble brown leaves – from their veins down to tiny fungus spots – has been hotly debated.

In the 1800s, naturalist Alfred Russel Wallace collected Kallima butterflies in Southeast Asia and used them to advance Darwin’s theory of natural selection, which would suggest the butterflies gradually evolved to look like leaves to escape hungry birds.

But other scientists would propose alternative explanations. For example, in the 1940s, U.S. geneticist Richard Goldschmidt argued that the butterflies’ leaf mimicry originated suddenly, without intermediate forms – sometimes termed the “hopeful monster” theory.

Now, though, Japanese scientists claim to have discovered that Kallima butterflies went through at least four distinct intermediate forms before evolving into species that disguise themselves as leaves.

The team mapped small, incremental changes to markings on the undersides of Kallima butterflies’ wings over time “to provide the first evidence for the gradual evolution of leaf mimicry,” says study co-author Takao Suzuki, of the National Institute of Agrobiological Sciences in Ibaraki.

Now, modern-day scientists who explain leaf mimicry with Darwin’s theory have a way to validate their theory, said Suzuki, whose study was published recently in the journal BMC Evolutionary Biology.

However, whether Suzuki and colleagues have proved Darwin right is hard to say, partly because of the novel methods of analysis they used, cautioned Mike Speed of the University of Liverpool’s Institute of Integrative Biology in the U.K.
“The methods are highly technical and very specialised, and I’d need a fair bit of time to work through them to be confident,” said Speed, who wasn’t involved in the study.

Usually it’s assumed that for mimicry to evolve, a major change or mutation needs to occur in the organism’s DNA “that takes the animal into the ballpark area where it resembles the thing it’s supposed to be looking like,” he explained.

“People have generally been sceptical about explanations where these traits evolve in tiny little bits across many generations,” Speed added. However, leaf mimicry – a type of so-called “masquerade” mimicry – may be an exception, he said.

If, as in the case of dead leaf butterflies, the ancestor species already has a degree of camouflage, “then I don’t think it’s as hard to evolve (to become leaflike) by small steps,” Speed said.

“Where you already look a bit like the background but don’t have the shape of a leaf, and then evolve a trait that’s a bit leaflike, and a predator then tends to overlook you a little bit more,” he said, then other leaflike traits could gradually accrue.

“One of the big values of this work is actually that not many people have looked at masquerade mimicry,” Speed said. “It’s had a lot less attention than it deserves, and much less than other kinds of mimicry.”

The wing-pattern mapping method used in the new study could reveal how other winged insects came to masquerade as other life-forms, study co-author Suzuki said. In fact, he added, he has already applied the method to moth and butterfly species that mimic lichen or bark. With Suzuki’s work, we just may unmask more of the insect kingdom’s masters of disguise.