The problem for colour vision can be summarised thus: "The focal length of a simple lens varies as a function of the wavelength of light. Short wavelengths (blue) are focused at shorter distances from the lens, whereas long wavelengths (red) are focused at longer distances from the lens. This longitudinal chromatic aberration (LCA) leads to chromatic blur that degrades image quality." Commercial lenses that are corrected for LCA make use of components made from materials with different refractive indices.
"The biological solution to LCA is the multifocal lens. Such a lens has several distinct zones that focus monochromatic light of a particular wavelength at different distances. Since the refractive power of the multifocal lens is also a function of the wavelength of light, each of the zones of different focal length for monochromatic light focuses a different band of wavelengths on the same plane in the retina. A sharp colour image is created on the background of defocused light. This solution seems at first sight to be inferior to the technical solution, but has the important advantage that the optical system is considerably smaller, which is of energetic benefit to the animal."
The biological solution is utilised in a wide range of animals. "The success of this optical design is reflected in the multifocal systems present in a variety of amphibians, reptiles and mammals (including primates). The evolutionary origins of this lens design, however, are unknown." The research team has set out to cast light on the antiquity of the multifocal lens. Their methodology is very interesting. The key to understanding it is "stasis". Once originated, body plans show little deviation. Similarly, complex structures having come into existence do not change significantly. Genes are conserved rather than modified. As Gould so forcibly argued: "stasis is data" and this principle appears to underpin this research. As we shall note, adopting this principle does not lead to findings that favour Darwinian gradualism.
The southern hemisphere lamprey, Geotria australis, showing its well-developed eye (Source: go here).
The researchers have chosen to work with lampreys, which are extant jawless fish. Recognisable lampreys are also found as fossils in Cambrian rocks, predating bony fish. The argument is: if multifocal optics are found in lampreys, this establishes the design as an early innovation, predating the cartilaginous fish (chondrichthyes) and the bony fish (osteichthyes). Conversely, if multifocal optics are not found, the innovation comes later. Genetic information relevant to this investigation concerns the presence of photopigments, and evidence was known to suggest there was value in an analysis of lamprey colour vision.
"The southern hemisphere lamprey Geotria australis possesses five photopigments [. . .]. At least three of the five visual pigments [. . .] are orthologous to the visual pigments in jawed vertebrates. Although rod opsin-like opsin type A (RhA) and type B (RhB ) of lampreys share similarities with the rod opsin (Rh1) and Rh1-like cone opsin (Rh2) of jawed vertebrates, respectively, the functional identity of the receptors housing these visual pigments remains elusive. It has, however, been suggested that all five photoreceptors in G. australis have close affinities to those of cones in gnathostomes."
The findings relate to all lamprey species studied, selected to be representative of the group:
"All four species of lamprey were found to have multifocal lenses, optically similar to those of bony fishes."
"This is the first study to show that the eyes of lampreys (Agnatha), extant representatives of the earliest vertebrates, possess multifocal lenses and therefore the optical apparatus for well-focused colour vision."
The implications are (1) that colour vision via multifocal optics is a Cambrian-Recent phenomenon; and (2) that multifocal optics is suggestive of a monophyletic origin.
"The presence of multifocal lenses in the eyes of lampreys confirms the early origins of colour vision in vertebrates, and suggests that other aquatic vertebrates and tetrapods have retained this optical feature, the latter despite the transition from an aquatic environment to a terrestrial habitat, where the cornea comes into play as an additional refractive element."
"The presence of multifocal lenses in representatives of all vertebrate classes studied thus far and their absence in cephalopods suggests a monophyletic origin for this lens design. Convergent evolution in so many vertebrate lineages is unlikely."
This should give Richard Dawkins (and others who think that eyes evolved easily) something to think about. Allowing an evolutionary perspective on the data, multifocal optics evolved once and then has been passed on to descendants with very little change. The same data permits a design perspective, as there is no hint here how such complexity evolved rapidly.
"The presence of at least four visual pigments [. . .] in the last common ancestor of jawed and jawless vertebrates suggests that the earliest vertebrates were able to sample a rich spectral light environment and enjoy the many advantages of colour vision. Our findings suggest that the eyes of these ancient animals were capable of forming well-focused colour images."
Early evolution of multifocal optics for well-focused colour vision in vertebrates
O. S. E. Gustafsson, S. P. Collin and R. H. H. Kroger
Journal of Experimental Biology 211, 1559-1564 (2008) | doi: 10.1242/jeb.016048
Jawless fishes (Agnatha; lampreys and hagfishes) most closely resemble the earliest stage in vertebrate evolution and lamprey-like animals already existed in the Lower Cambrian [about 540 million years ago (MYA)]. Agnathans are thought to have separated from the main vertebrate lineage at least 500 MYA. Hagfishes have primitive eyes, but the eyes of adult lampreys are well-developed. The southern hemisphere lamprey, Geotria australis, possesses five types of opsin genes, three of which are clearly orthologous to the opsin genes of jawed vertebrates. This suggests that the last common ancestor of all vertebrate lineages possessed a complex colour vision system. In the eyes of many bony fishes and tetrapods, well-focused colour images are created by multifocal crystalline lenses that compensate for longitudinal chromatic aberration. To trace the evolutionary origins of multifocal lenses, we studied the optical properties of the lenses in four species of lamprey (Geotria australis, Mordacia praecox, Lampetra fluviatilis and Petromyzon marinus), with representatives from all three of the extant lamprey families. Multifocal lenses are present in all lampreys studied. This suggests that the ability to create well-focused colour images with multifocal optical systems also evolved very early.
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