Light-morph Jaguar (typical)

Dark-morph or melanistic Jaguar (about 6% of the South American population)

The adder Viperus berus: normal and melanistic colour patterns

Polymorphism in biology is when two or more clearly different phenotypes exist in the same population of a species. The words forms or morphs are sometimes used.

Polymorphism is common in nature. The most common example is sexual dimorphism, which occurs in many organisms. Another example is sickle-cell anaemia.

In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a population with random mating.