When different eye colors, such as brown, blue, or green, appear among members of the same species, this variation is the result of specific biological mechanisms. The existence of these distinct, alternative forms of a physical trait within a single population is a fundamental aspect of genetic diversity. This variation is governed by heritable differences encoded in the species’ genetic material, which manifest as observable physical characteristics.
Polymorphism: The Scientific Term for Species Variation
The scientific term for the occurrence of multiple distinct forms of a trait, like eye color, within a single interbreeding population is polymorphism. Derived from Greek, meaning “many forms,” it applies to any discontinuous variation where individuals fall into two or more clearly separate groups. For a trait to be classified as polymorphic, the genetic variant responsible for the least common form must be present at a frequency greater than one percent within the population.
This frequency requirement ensures the variation is stable and not merely explained by new, spontaneous mutations. Human eye color is a classic example, as the genes controlling the trait have several different alleles, or variants, that lead to brown, blue, green, and other colors. These multiple alleles exist simultaneously within the human gene pool, providing a visible expression of the species’ genetic diversity.
The Biological Basis of Eye Color
The physical appearance of eye color is determined primarily by the concentration of a single pigment called melanin, not by an array of different colored pigments. Melanin is produced by specialized cells called melanocytes and is located within the iris. The amount of melanin present in the stroma, the connective tissue layer in the front of the iris, dictates the resulting color.
Brown eyes, which are the most common globally, result from a high concentration of melanin in the stroma, which absorbs most of the light entering the eye. Conversely, blue eyes contain very little melanin in the stroma, causing a different physical effect. The blue appearance is not due to a blue pigment, but rather to the scattering of light by the collagen fibers within the stroma.
This phenomenon, known as Rayleigh scattering, is the same process that makes the sky appear blue. When light enters an eye with low melanin, the shorter blue wavelengths are scattered back out, making the iris appear blue. Green and hazel eyes represent an intermediate condition, where a low to moderate amount of melanin combines with this light-scattering effect, creating the appearance of green or hazel.
Eye color is a polygenic trait, meaning it is influenced by multiple genes, with at least 16 genes identified as contributors. The OCA2 and HERC2 genes are the most significant, working together to regulate melanin production and distribution. A specific variant of the HERC2 gene can suppress the expression of OCA2, which limits melanin production and is strongly associated with blue eyes.
Heterochromia: A Specialized Form of Variation
While polymorphism describes differences in eye color across a population, heterochromia is the term for a variation in color within a single individual. Heterochromia occurs when the irises of a person’s two eyes are different colors, or when a single iris contains multiple colors. This condition is distinct from the species-wide variation because it involves an asymmetry of color expression in one person.
There are three main classifications of heterochromia: complete, sectoral, and central. Complete heterochromia is characterized by two entirely different colored eyes, such as one blue and one brown. Sectoral heterochromia involves a wedge-shaped section of one iris that is a different color from the rest. Central heterochromia presents as an inner ring of color around the pupil that contrasts with the outer part of the iris.
The causes are varied, with most cases being congenital, meaning present from birth, and often resulting from a harmless, isolated genetic mutation affecting melanin distribution. Acquired heterochromia, developing later in life, can be a sign of an underlying medical condition or can result from an injury to the eye. Acquired causes include eye trauma, certain eye diseases like Fuchs’ heterochromic cyclitis, or the use of some glaucoma medications.