The differences among individuals in a population.
are inheritable differences among the individuals of a single species. Often, with a quick glance, most individuals within a species look very similar, however, upon closer examination, many variations can be noted. Many can be easily observed, such as size, shape, color, texture, speed, etc. Others might be more difficult to observe but can be very important in terms of evolution. For example, there are often variations in the ability to produce offspring, hunting or hiding skills, and the ability to resist heat, cold, stress, drought, disease, etc. For variations to be acted upon by natural selection, they must be heritableHeredity:
The biological similarity of offspring and parents. (adj. heritable)
; they are often the results of genetic mutationMutation:
A change in the nucleotide sequence of genetic material whether by substitution, duplication, insertion, deletion, or inversion.

Polymorphism expressed as existing in several different colors. (adj. polychromic)
(many color forms in a single species) is shown by these Florida Coquina Clams (Donax variabilis). Predatory shore birds recognize a coquina as food by its appearance, forming a “search image.” If some Coquinas look different, they might not match the search image of the bird, and thus might be passed over as “not food,” providing a survival advantage. In such cases, polychromism is maintained in a population.

Variation in size, shape, and color of the Eastern oyster (Crassostrea virginica).

There are two main types of variation: continuous and discrete.

Continuous variation from light to dark color and discrete variation between male and female in the Eastern Oyster (Crassostrea virginica).