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| Mimicry in Butterflies Is Seen here on These Classic “Plates” Showing Four Forms of H. numata, Two Forms of H. melpomene, and the Two Corresponding Mimicking Forms of H. erato. This highlights the diversity of patterns as well as the mimicry associations, which are found to be largely controlled by a shared genetic locus [15]. (Photo credit: Wikipedia) |
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by Sean B. Carroll
Perhaps no destination has inspired more great naturalists than Brazil. Charles Darwin first made landfall at Bahia in 1832; Alfred Russel Wallace and Henry Walter Bates arrive at Para in 1848. Wallace roamed the Amazon for four years, and the indefatigable Bates for 11.
In 1852, a naturalist named Fritz Muller arrive from Germany. While Darwin and Wallace would conceive of the theory of evolution by natural selection, its acceptance was aided by Bates and Muller. And thanks to Bates and Muller, no creatures contributed more to the early growth of evolutionary science than butterflies.
Bates noticed species whose wing patterns resembled those of other butterfly families in the area. In puzzling this out, he realized that harmless butterflies were mimicking noxious species that were unpalatable to birds and lizards, and therefore, not attacked by predators.
A few years after Darwin published "On the Origin of Species," Bates suggested that this sort of mimicry - now called "Batesian" - was proof of the principle of natural selection.
Muller crucially observed that unpalatable butterflies were also mimicking other species of unpalatable butterflies. 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 predators, and species would share the cost of those lessons, whereas a uniquely patterned unpalatable species would bear the full cost.
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? The answer eluded scientists for nearly 150 years, until an international team of researchers recently revealed mimicry's innermost secrets.
There are two ways in which what is still called "Mullerian 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 answers? Both mechanisms have been at work.
By analyzing the DNA sequences in two mimicking Heliconius species in 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.
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. After all, the inability to breed successfully with other groups has long been an operational definition of species.
Even if such interspecies matings are rare, a gene that confers a strong advantage, like mimicry, can spread quickly through a population.
Darwin referred to Muller as the "prince of observers," and although they never met, Muller considered Darwin a second father.
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Lifted from TODAY Saturday Edition, April 6, 2013; The New York Times International Weekly
