2001). As well, both use vision in the detection of conspecifics and mates (Mollon, 1989). (Detection of predators is probably visual for butterflies but does not depend on color vision; it probably depends on motion vision instead.) Thus, various aspects of the color vision systems of butterflies and primates may have undergone convergent evolution (see below).

Phylogenetic analyses indicate that the opsin gene family duplicated several times before the radiation of metazoans, giving rise to as many as seven protein subfamilies (Terakita, 2005), including the ciliary and rhabdomeric opsins, each associated with a distinct photoreceptor cell type (Arendt, 2003). All photoreceptor cells expand their membranes to express opsins, but ciliary photoreceptor cells expand their ciliary side, the side closest to the cell body, and express ciliary opsins, whereas rhabdomeric photoreceptor cells expand their apical side and express rhabdomeric opsins (Arendt, 2003). In general, vertebrates have ciliary opsins that are expressed in the photoreceptor cells of the retina, whereas insects have rhabdomeric opsins that are expressed in the photoreceptor cells of the ommatidia of the compound eye.

Color vision adds to the complexity of the eye. With a single spectral class of photoreceptor, only achromatic (brightness-contrast) vision is possible. Both mammalian long-wavelength-sensitive (L) cones and butterfly L photoreceptors provide outputs for brightness processing (Kolb and Scherer, 1982; Jacobs, 1993). Color vision, on the other hand, is the ability to discriminate between different wavelengths of light, regardless of relative intensity and depends on the presence of at least two spectrally distinct classes of photoreceptors, as well as appropriate neuronal connections in the brain. Natural selection has recruited both the vertebrate ciliary opsins and the insect rhabdomeric opsins for use in achromatic and color vision. Moreover, mammals use all their cone photopigments for color vision, and although not yet fully investigated, butterflies likely use all their major spectral receptor types for color vision (Kelber and Henique, 1999; Kelber, 2001; Kelber et al., 2003).

There is variation in both the photopigment sensitivity and the range of color vision in mammals. Color vision among placental mammals is typically dichromatic based on outputs from short-wavelength-sensitive (S) and L cone pigments encoded by distinct opsin genes expressed in the cone photoreceptor cells of the vertebrate retina (Jacobs, 1993; Ahnelt and Kolb, 2000). Some primates have even evolved trichromatic color vision based on an additional middle-wavelength-sensitive (M) cone pigment, which allows them to discriminate colors in the green-to-red part of the visible light spectrum. The most parsimonious explanation for the variation in primate color vision systems is that the common ancestor of primates contained a single S opsin variant, but the ancestor was polymorphic at another locus for alleles encoding M and L cone pigments (Tan et