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 today and have led to surprising new insights into how evolution works.
Two types of mimicry ...
Both men found Brazil ablaze with colorful 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 realized 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. 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. If they were already unpalatable, he wondered, what added advantage was there to mimicking other species?
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.
... and two types of evolution
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. The wing patterns are so similar that it is hard to tell species apart from even a short distance — and that is the point.
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.
Several genes controlling the production of the wing patterns have now been identified, enabling researchers to distinguish between these alternatives. The answer? Both mechanisms have been at work.
By analyzing the DNA sequences in two mimicking Heliconius species distributed across South America, researchers could determine that each species had independently evolved up to 20 different patterns that were nearly identical in each species. But in more closely related mimicking species, they found that color-controlling genes had been exchanged.
These discoveries are equally interesting. It is astonishing that so many patterns could be independently generated and replicated in different species. And it is surprising to have species swapping genes in the Amazon. After all, the inability to breed successfully with other groups has long been an operational definition of species.
But as we peer into genomes, we continue to detect evidence of past interbreeding — between Darwin’s finches, for example, and even between Neanderthals and our own species, Homo sapiens. Even if such interspecies matings are rare, a gene that confers a strong advantage, like mimicry, can spread quickly through a population.
An observation 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.
Building on the work of two 19th-century naturalists, researchers studying the similar appearance of various butterflies have found that different species share some of the same genes for color — which means they’ve been interbreeding.