The puzzle of mimicry in nature

Perhaps no destination has attracted and inspired more great naturalists than Brazil. Charles Darwin, on his epic voyage on the HMS Beagle, first made landfall at Bahia in 1832; two fellow Englishmen, Alfred Russel Wallace and Henry Walter Bates, arrived at Para in 1848. Wallace roamed the Amazon for four years, and the indefatigable Bates for 11.

In 1852, a naturalist named Fritz Mueller arrived from Germany. Much less known today, Mueller, unlike his English contemporaries, moved to Brazil with his wife and young child and had no intention of ever returning to Prussia. A freethinker who refused to swear an oath to God required for his medical graduation, Mueller traded a medical career in Europe for a mud-floor hut at the edge of virgin forest in the Blumenau colony in Santa Catarina.

While Darwin and Wallace would conceive of the theory of evolution by natural selection, its acceptance was aided greatly by Bates and Mueller. And thanks to Bates and Mueller, perhaps no group of animals contributed more to the early growth of evolutionary science than butterflies. Their ideas continue to inspire naturalists even today.

Both men found Brazil ablaze with colourful butterflies. Bates noticed among his collections certain species whose bright wing patterns closely resembled those of other butterfly families in the area. In puzzling out why one species would mimic another, he realised that harmless butterflies were mimicking noxious species that were unpalatable to birds and lizards, and therefore not attacked by predators.

Only a few years after Darwin published “On the Origin of Species,” Bates suggested that this sort of mimicry – now called ‘Batesian’ – was timely proof of the principle of natural selection. While Bates was a full-time collector, Mueller was initially occupied with more basic concerns. For his first few years in Brazil, he eked out a living as a farmer, raising chickens and pigs and trapping game, while enduring floods and fending off hostile indigenous tribes, jaguars and tropical diseases.

As his family expanded, eventually to six daughters, Mueller moved to a coastal town to teach mathematics, natural history, and even some physics and chemistry. His position gave him a chance to explore more intellectual pursuits, and there he discovered Darwin’s new theories. ‘Origin’ so transformed Mueller’s understanding of nature that he was inspired to write his own book, “Fur Darwin,” that presented facts and arguments in favour of his theory, including Mueller’s own observations on Brazilian plants and animals. The two men struck up a lively and warm correspondence that would last 17 years, until Darwin’s death. Darwin referred to Mueller as the ‘prince of observers,’ and although they never met, Mueller considered Darwin a second father.

Mueller’s crucial observation was a new twist on mimicry. He noticed that unpalatable butterflies were also mimicking other species of unpalatable butterflies in the same area.
It dawned on him that unpalatable mimics would enjoy strength in numbers: Their unpalatability had to be learned by naive predators, and mimicking species would share the cost of those lessons, whereas a uniquely patterned unpalatable species would bear the full cost. He showed through simple algebra that two or more unpalatable species would each gain an advantage through a common pattern.

Natural selection thus explained why different species’ wing patterns would converge. But how were such similar but complex wing color patterns generated by different species? That was a much more difficult question, and its answer eluded scientists for nearly 150 years, until an international team of researchers recently revealed mimicry’s innermost secrets.

The most striking and famous examples of what is still called “Muellerian mimicry” involve Heliconius butterflies in South and Central America. In many instances, the wing patterns of different species in the same area are remarkably similar. And even more remarkable, each species may exhibit several different wing patterns, each specific to a given area. There are two fundamentally different ways Muellerian mimicry could evolve: Either each species independently evolved mutations that led to very similar wing patterns, or patterning genes were exchanged among species.

One of my favourite observations about scientific progress was offered by the Nobel physicist Jean Baptiste Perrin, who said that the key to any advance was to be able “to explain the complex visible by some simple invisible.” After being shrouded in mystery for more than a century, the revelation of the invisible genes that have generated such diversity is an exquisite example of the maxim.