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.