One of the claims often made by opponents of the theory of evolution — there are some still left, mainly in Kansas — is that because natural selection is a phenomenon we can’t directly observe, the theory is untenable. And while creationists insist that species are immutable despite a staggering amount of evidence that they do adapt and evolve, it is true that we can rarely see the origin of species in the field, and account for it with reference to natural selection. But that is precisely what biologists have now done.
In a paper in today’s Nature, Chris Jiggins and colleagues at University College London and the Smithsonian Tropical Research Institute, Panama, show that two tropical sister-species of butterfly have arisen from a common ancestor. The species mimic two different species distasteful to predators, and the authors demonstrate that they have diverged due to natural selection. They also show that differences in coloration are crucial in choosing mates: Butterflies choose mates with the same color pattern.
Poisonous species of animals, like some butterflies, have evolved bright color patterns, and predators, like birds, learn to avoid feeding on them. The caterpillars of poisonous butterflies eat plants containing toxic substances that are retained in the tissues of the adult. Such bad-tasting, bright butterflies are described as having warning coloration.
Some perfectly edible animals will then be favored by natural selection if their coloration is similar to that of poisonous ones. The edible cheats are called “mimics,” and the poisonous species they copy are called “models.” This is a special type of mimicry called Batesian mimicry, after the Victorian biologist Henry Walter Bates who discovered it.
Mimicry is a textbook example of natural selection within a species because it is easy to grasp how animals will have an advantage if they look like a poisonous animal. But Bates thought that mimicry could explain how a species could diverge into two distinct species through natural selection. It is this idea, which Bates first aired in 1862, that has now found support.
Jiggins and his colleagues studied the Heliconius butterflies of Central and South America. This is a group of insects where there are several color morphs within a species (a morph is a color variation that is not different enough to make a new species). Each morph has a separate color pattern, and all morphs are poisonous.
Predators learn that each morph pattern is poisonous. But if two different morphs mate, they will produce offspring that are intermediate in form between the two poisonous patterns. Such offspring are not mimics — their color patterns don’t match a recognized poisonous form.
Jiggins and fellow researchers were particularly interested in two Heliconius species, H. melpomene and H. cydno. These sister species have recently diverged to mimic different model species. (H. melpomene mimics a black, red and yellow species, while H. cydno mimics a black and white species.)
The scientists carried out experiments in which each type of butterfly was given the choice to mate with different-colored butterflies. They found that “assortative mating” was strong among the test subjects — that is, butterflies strongly preferred to mate with other butterflies with the same color pattern.
This makes sense. Since the butterflies have started to mimic different species, if they mate with a butterfly with a pattern different than their own, their offspring will not have a color pattern recognized as poisonous. Predators, therefore, will pick them off.
“Mimicry therefore provides an example of a trait under strong ecological selection that is also used as a mating cue,” write the Nature authors. It looks like Bates was right, 140 years ago, when he argued that mimicry among Amazonian butterflies demonstrated the origin of species by natural selection.