In the simplest case there is a default state (e.g. eyes have color of whatever pigment just happen to be produced there as a sideeffect of important genes). And then there are genes that override this default. They may disable the normal pigment genes, they may produce additional pigment, they may break down pigment. These genes can then exist in variants (alleles). Following the example, they may produce pigment molecules with different colors.
In the absence of selection pressure the override genes will mutate randomly. The end state of that process are broken genes that don't do much of anything (loss of function), and so the color goes back to the default.
For human eyes, the default is blue eyes and there are then some genes that overrides this by producing melanin, which turns the eyes brown.
In the absence of selection pressure the override genes will mutate randomly. The end state of that process are broken genes that don't do much of anything (loss of function), and so the color goes back to the default.
For human eyes, the default is blue eyes and there are then some genes that overrides this by producing melanin, which turns the eyes brown.