Process | Examples from eye evolution |
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Direct adaptive evolution | Gradual evolution of lens crystallin concentrations resulting in evolution of graded refractive index lenses in aquatic animals |
Exaptation | |
One structure has one function and takes on or switches to a new function in a new environment | The cornea, which has no refractive capacity in water, became the primary focusing structure after tetrapods moved onto land |
The lens became far less important in image formation in terrestrial vertebrates and became specialized for accommodation instead | |
One structure has one function but becomes modified enough to allow a shift in function | Circadian organ in early chordates became modified sufficiently that it became capable of visual functions |
An early protective, transparent layer of cells became sufficiently thickened and invaginated that it could begin serving as an early lens | |
Two structures perform the same function but become differently specialized | Though both cell types were probably found in the distant bilaterian ancestor, ciliary photoreceptors became the dominant type in vertebrates whereas rhabdomeric photoreceptors came to predominate in most other animals (see also duplication and divergence) |
A vestigial structure takes on new function | In vertebrates, rhabdomeric photoreceptors lost their microvilli and became retinal ganglion cells that function in circadian entrainment rather than in vision |
Duplication and maintenance of repetition | The compound eyes of arthropods are composed hundreds or thousands of repeated lens eyes called ommatidia |
Duplication and divergence | Opsin genes duplicated and diverged to become r-opsins and c-opsins, along with specialization of rhabdomeric cells with r-opsins and ciliary cells with c-opsins |
In certain taxa, duplications and diversification of opsins to respond to different wavelengths of light allowed the evolution of color vision | |
The rod cells of vertebrates are derived from cone cells, both of which are derived from a single ancestral ciliary photoreceptor | |
Gene sharing | Some lens crystallin proteins function both in the eye in light refraction and elsewhere in the body for other functions (e.g., cellular stress response) |
Collage | The first photopigment was formed by the combination of a preexisting light sensitive molecule derived from vitamin A (which became retinal) with a preexisting G protein-coupled receptor protein (which became the ancestral opsin) |
The first “eye” arose by the combination of a photoreceptor cell with a pigment cell | |
During the evolution of complex camera-type eyes, various types of tissue that already existed (e.g., blood vessels, nerves, muscles) were incorporated | |
Scaffolding | May apply to the evolution of phototransduction pathways or other relevant biochemical systems, but more data are required |
Constraints, trade-offs, and historical contingency | Trade-off between resolution versus brightness in pinhole camera eyes |
Trade-off between visual acuity versus size of compound eyes | |
Inverted retina in vertebrates | |
A narrow range of available wavelengths of sunlight is perceived in most animals, probably because eyes first evolved in water which filters most wavelengths | |
Convergence | Lenses, irises, and various other components of camera-type eyes emerged independently in vertebrates and cephalopods |
Parallel evolution | The same developmental or other genes may have been independently co-opted in different lineages (though homology is also a possibility) |