A Gradient Written in Genes
Along the scrublands and Pampas grasslands of South America lives Leopardus geoffroyi, or Geoffroy's cat, a small wild feline roughly the size of a domestic cat. Its coat is typically a gray or tawny-yellow fur covered in small, black spots, providing excellent camouflage in its native habitat. Yet, this cat displays a remarkable pattern of variation. In the northern parts of its range, such as Bolivia and northern Argentina, nearly all Geoffroy's cats have this spotted coat. But traveling south through the grasslands of central Argentina, a dramatic change occurs: a progressively higher percentage of the cats are entirely black.
This phenomenon is a clinal gradient of melanism. The frequency of black-furred, or melanistic, individuals increases steadily with latitude. In the humid, open grasslands of the southern Pampas, populations can be composed of more than 70% black individuals. This change is not random. Genetic investigation revealed that melanism in Geoffroy's cats is caused by a mutation in the Melanocortin 1 Receptor (MC1R) gene. This is a different gene than the one responsible for melanism in domestic cats, which is a mutation in the agouti signaling protein (ASIP) gene. The independent evolution of the same trait through different genetic pathways in separate species points to strong environmental pressures at work.