Reversing evolution: conspicuous mimicry vs. camoflage
Two papers this week on the type of mimicry named for Henry Bates, whose book on exploring the Amazon was published shortly after The Origin of Species. Batesian mimic species resemble foul-tasting or dangerous species, thereby avoiding being eaten, even though they are not actually dangerous themselves. Bates worked on butterflies whose wing patterns resembled those of other species. Butterflies are still the best-known examples of mimicry, but there are also examples of mimicry (involving behavior) in snakes and octopi.
The two papers are:
Once a Batesian mimic, not always a Batesian mimic: mimic reverts back to ancestral phenotype when the model is absent
by Kathleen Prudic and Jeffrey Oliver, of the University of Arizona, and
Colour pattern specification in the Mocker swallowtail Papilio dardanus: the transcription factor invected is a candidate for the mimicry locus H
by Rebecca Clark and colleagues in the UK, Australia, Kenya, and Germany. Both were published in Proceedings of the Royal Society.
The first paper asks how mimicry evolves when the distasteful "model" species is absent. Model species are usually brightly and distinctively colored, so the mimics are, too. But what happens if the model becomes too rare to "train" predators to avoid that pattern, or if the mimic moves into an area where the model isn't found? The mimic could, perhaps, die out under these conditions.
Prudic and Oliver used molecular methods to develop a family tree for admiral butterfly species. Based on this tree, they concluded that the common ancestor of the admirals was inconspicuous, black with a white band that apparently makes it hard to see. Some admiral species have evolved to mimic different model species. But some of these evolved further, reversing this evolutionary path and becoming inconspicuous again. These species are found, as you might expect, in areas where the model is absent. So, in this case, reverse evolution was fast enough to avoid extinction. The reversal itself could have been rapid, if only a small genetic change was needed. Or the conditions that increased predation on the conspicuous mimics could have developed slowly.
Evolutionary reversals are not necessarily rare. Peter and Rosemary Grant found that the populations of Galapagos finches (made famous by Darwin) can evolve rapidly in response to dry years, but the effects are reversed in wet years, so there may be little long-term trend.
When natural selection favors a phenotype (such as wing pattern) that it previously eliminated, restoration of the old phenotype may not involve an exact return to the original genotype. The second paper identifies an actual gene responsible for the appearance of another butterfly mimic, the Mocker swallowtail. The gene identified was previously found to be involved in wing patterns ("eyespots") in another butterfly species.
Both papers make heavy use of molecular tools that would have astounded Bates or Darwin, but they would perhaps be pleased that questions they raised are still of interest.