Darwin and Heliconius

It’s the 200th anniversary of Darwin’s birth, and the 150th of the publication of his “On the Origin of Species…”, and Chris Jiggins wondered what he had to say about Heliconius. I have done a brief search on www.darwin-online.org.uk. Sadly, the great man seems never to have mentioned Heliconius butterflies.

Plate I of Bates' "heliconid" paper. The handwritten notes are by Alfred Russel Wallace, as this was photographed from Wallace's own copy of the paper

However, Darwin (1863) did comment on Henry Walter Bates’ 1862 paper “Contribution to an Insect Fauna of the Amazon Valley,” although he seems to have confined his comments to what Bates called the “danaoid heliconids” (today’s Ithomiinae) as opposed to the “acraeoid heliconids” (today’s Heliconiina). Darwin was clearly a bit put out that Bates seems to have titled his now famous paper with such a lack-lustre title: “… this memoir, which from its unpretending and somewhat indefinite title we fear may be overlooked in the ever-flowing rush of scientific literature.”

Darwin, today known more for his for “turgid” writing than purple prose, pulls out some superlatives to advertise Bates’ paper: “In these facts, of which only a brief abstract is given, we have the most striking case ever recorded of what naturalists call analogical resemblance. … Why then, we are naturally eager to know, has one butterfly or moth so often assumed the dress of another quite distinct form; why to the perplexity of naturalists has Nature condescended to the tricks of the stage?”

“Mr. Bates has given to these facts the requisite touch of genius, and hs, we cannot doubt, hit on the final cause of all this mimicry. … The mocked and common forms must habitually escape to a large extent, destruction, otherwise they could not exist in such swarms; and Mr. Bates never saw them preyed on by birds … he suspects that this immunity is owing to a peculiar and offensive odour … The mocking forms, on the other hand, … are comparatively rare, [and] must suffer habitually from some danger. … Now if a member of one of these persecuted and rare groups were to assume a dress so like that of a well-protected species … it would often deceive predacious birds and insects, and thus escape entire annihilation. This we believe is the true explanation of all this mockery.”

However, Darwin is most excited by a discussion of Mechanitis in which Bates suggests assortative mating by colour pattern. “We will notice only briefly one other point which has an important bearing on the production of new races and species; namely the statement repeatedly made that in certain cases the individuals of the same variety evince a strong predilection to pair together.” However, Darwin adds: “… we are by our profession as critics bound to be sceptical, and we think that Mr. Bates ought to have given more copious evidence. He ought also to have given in every case his reasons in full for believing that the closely allied and co-existing forms, with which his varieties do not pair, are not distinct species. Naturalists should always bear in mind such cases as those of our own willow wrens, two of which [i.e. willow warbler and chiff-chaff] are so closely alike that experienced ornithologists can with difficulty [scarcely] distinguish them …; yet these are certainly as distinct species as any in the world.

Today’s work with Heliconius does indeed suggest that mimicry evolution entails a coevolved change in mate choice by males, leading to assortative mating in many cases of interracial and interspecies choice tests. Darwin hit on one of the most interesting points in Bates’ memoir, and delivered Bates with a challenge that he, having already returned from the Amazon for the last time, could not fulfil: to get more data on the relationship of the evolution of mimicry to the origin of species.