RESEARCH:: Celebrating Darwin

Purple Butterflies: One Species or Two?
By Dr. Timothy L. McCabe

Banded Purple butterflies, distributed throughout eastern North America, were once thought to represent two species: the southern Red-spotted Purple and the northern Banded Purple. Then, in the 1980s, published research demonstrated that these butterflies are a prime example of natural selection. The Banded Purple has the necessary genetic ability to produce mimics of the Pipe-vine swallowtail butterfly as a result of predator-driven selection.

The Pipe-vine swallowtail occurs in southeastern United States. As the name implies, its caterpillar feeds on the pipe-vine plant. The pipe-vine plant contains chemicals that are toxic to predators, most notably birds. The Pipe-vine swallowtail retains these toxic chemicals in the adult stage, affording them protection from birds. Among the purple butterflies, the best mimics—the ones that, by chance, look most like the protected species—survive in superior numbers because they are more frequently avoided by predators. The selective advantage is so great that eventually the entire population of Banded Purples mimic the Pipe-vine swallowtails. But this happens only where the model and the mimic occur together.

In northern New York there are no pipe-vine plants, hence no Pipe-vine swallowtails. In the absence of a chemically protected model, no predator-driven selection for Pipe-vine swallowtail mimicry takes place. Geographically, Albany lies in a north-south transition zone where the Red-spotted Purple form begins to lose its selective advantage and the species sports its primitive condition, the Banded Purple form. In this transition zone the two color forms freely interbreed, even in nature, and produce viable offspring. Total hybrid fertility is tantamount among the criteria we use when considering species limits.
The ability to interbreed and have fertile offspring is one piece of evidence that these butterflies represent one species, not two.

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Dr. Timothy L. McCabe is the curator of entomology at the New York State Museum.