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Figure 14-4. The type of red-green color blindness is determined by the arrangement and mutation of the X-linked opsin genes. This table shows phenotypes associated with mutant, deleted, or fused genes that have been observed. Spectral sensitivities were measured in two ways. Reprinted from Neitz et al. (Neitz et al.) with permission; see the original for details.

Figure 14-4. The type of red-green color blindness is determined by the arrangement and mutation of the X-linked opsin genes. This table shows phenotypes associated with mutant, deleted, or fused genes that have been observed. Spectral sensitivities were measured in two ways. Reprinted from Neitz et al. (Neitz et al.) with permission; see the original for details.

1995).The amount of variation probably should temper what we consider "normal" and is also relevant to the age-old philosophical question of whether all people see the same colors. Not only is there variation in the genes themselves, but there is a stochastic element in the opsins expressed by any given cone, meaning that not even the left and right retinas of the same person are exactly the same. Of course, even if they were identical, this cannot answer the question of whether people who detect the same thing perceive things similarly, which probably remains as philosophical as ever.

The patterns of red-green X-linked opsins found among primates has been used to infer whether their common ancestor had di- or trichromatic vision. Old World monkeys and apes have trichromatic color vision as we do, with both red- and green-sensitive opsins, suggesting that this was probably the condition or our shared ancestor. However, most New World monkeys have only one X-linked opsin, and there's a twist. The single X-linked opsin is polymorphic in many species that have been studied, and the observed alleles are variously sensitive to red- or green-range light. Depending on the alleles' frequencies, a female's genotype may be RR, RG, or GG, whereas a male is either R or G sensitive. The relative frequencies of these genotypes will be determined by the allele frequencies (e.g., pR2, 2pR(1 - pR), (1 - pR)2 in Hardy Weinberg Equilibrium, using pR for the frequency of the R gene).

What accounts for this widespread variation in the peak sensitivities and polymorphism among monkeys with but a single X-linked opsin? The phenomenon provides an opportunity to examine the nature of darwinian explanations (e.g., Mollon 1989; Nathans 1999; Yokoyama 2000). The typical suggested scenario is that there has been selection for ability to detect fruit or other plant foods (for example, leaves whose color indicates taste or toxicity) in the dappled background of tropical forests. Animals who could see colors appropriate to their dietary staples had higher fitness, over time spectrally "tuning" their opsins' peak sensitivity frequencies. The polymorphic nature of the single X-linked opsin cannot be quite so easily explained. Why was it not "tuned" to an optimal frequency? Instead, as it is now, members of the same population are variably sensitive to color in a strange way: males are differently sensitive and to only one of the available colors, in proportion to the R and G allele frequencies. Females vary in proportion to the respective genotype frequencies (as above), the RR's and GG's being dichromatic and only the RG's being trichromatic. How would selection produce this?

Assuming that trichromacy must be evolutionarily better and hence favored by selection (from a human-centered perspective), it has been suggested that the (at most) few trichromatic females in any small local monkey troop may have led their peers to food sources. Otherwise, only a single best allele would have been favored by selection. If the selective scenario is correct, one might expect a kind of balancing selection to have optimized the frequency of female heterozygotes, which will occur with allele frequencies close to 0.5 (which maximizes 2pR(1 - pR)); this does not seem generally to be the case (Cropp et al. 2002; Heesy and Ross 2001), although available samples are inadequate for a definitive answer.

Perhaps there is a simpler and more natural explanation that is also more parsimonious (though, as noted early in this book, there is no reason that evolution needs to have followed the simplest path). It is useful first to note that the placement of cones in the retina may be related to spatial as well as, or even rather than, color perception. Also, even fully trichromatic individuals use other features of light such as shading and lightness in image discrimination perception. Figure 14-5 shows the

Single R- or G-opsin Single polymorphic R-opsin Separate R- and G-opsins equivocal

Microchiroptera

Megachiroptera

Dermoptera

Tupaiinae

Ptilocercina

Adapinae

Cercamoniinae

Notharctinae

Loridae

Galagonidae

Daubentonia

Microcebus

Mirza

Allocebus

Cheirogaleus medius

Cheirogaleus major

Phaner

Lepilemur

Propithecus

Propithecus v. coquereli

Avahi

Varecia variegata

Varecia vriegata rubra

Hapalemur

Lemur catta

Eulemur fulvus

Eulemur mongoz

Microchoerinae

Omomyinae

Anaptomorphinae

Tarsius bancanus

Tarsius syrichta

Oligopithecidae

Parapithecidae

Callimico

Saguinus

Leontopithecus

Cebuella

Callithrix

Cebus

Saimiri

Aotus

Cacajao

Chiropotes

Pithecia

Callicebus

Alouatta

Lagothrix

Brachyteles

Ateles

Propliopithecidae Catarrhini color sensitivities of the red/green opsin genes in primates (Heesy and Ross 2001), but the distribution of peak sensitivities among these alleles (Figure 14-6) does not suggest tight selection for red-centered and green-centered alleles but rather suggests that the alleles are spread across the entire red-green part of the spectrum. Selection might only have ensured that an animal would have opsins sensitive to some part of the red-green range rather than any particular one.

Because of codon redundancy, the polymorphisms in the key amino acids that have mainly been responsible for peak sensitivities in this range (Nathans 1999; Yokoyama 2000) require only a single nucleotide change; therefore, recurrent mutation to similar alleles in different species is not implausible on an evolutionary time scale. However, the appearance of similar mutations in multiple lineages might just reflect genetic drift on an ancient polymorphism in the ancestral lineage. Consistent with this, in both squirrel monkeys and humans, there is less variation in the single blue opsin than in the X-linked genes (Shimmin et al. 1998), suggesting that the blue-sensitive gene has been under stronger selective constraint than the polymorphic X-linked gene in squirrel monkeys (Cropp, Boinski et al. 2002).

The presumed need for precise spectral tuning may be less than is typically thought if one considers the many ways in which visual signals are interpreted, that opsins have sensitivity at wavelengths around their peak sensitivity, and that colors are not perceived strictly on the basis of peak sensitivities. For example, vertebrate retinas compare the relative strength of signal detected by different parts of the system (e.g., from two types of nearby cone cells) in various ways, even before signal is sent to the brain (e.g., Nathans 1999). In addition, what an animal "sees" is affected by its past experience.

In a species with two X-linked opsins, it is plausible that selection could keep their sensitivities separated as part of their normal repertoire (gene duplication of X-linked opsins occurred at least twice in primate lineages; New World howler monkeys have trichromacy similar to that in Old World monkeys and apes). But these systems, too, have considerable normal variation, suggesting that selection has, at most, not been too stringent.

The evolution of color vision can also illustrate the potential importance of organismal as opposed to classical natural selection. Some fish live deep in the sea where there is little light and what gets through is of short wavelength. Coelocanths use a combination of two rhodopsin-related opsins that are sensitive only to such light (Yokoyama et al. 1999). The fish are adapted to life in the depths. A fish will

Figure 14-5. Phylogeny of primate X-linked color vision capabilities. Phylogenetic tree of primates showing species that have alleles conferring only red (R) or green (G) sensitivity, a single gene with observed polymorphisms conferring alleles with R and G sensitivity, two X-linked genes (R- and G-sensitive), or some less-clear sensitivity. One can infer that the original primates may have been "dichromats" but with a single gene that was polymorphic for peak sensitivities in the red-green range. Gene duplication led to two X-linked genes, one specialized for red and the other for the green ends of this part of the spectrum, enabling "true" (human-like) trichromacy in the ancestor of Old World monkeys and apes. Gene phy-logenies suggest that this occurred independently in the lineage of howler monkeys (Alouatta) in the New World. Taxa with no symbol indicate no data available. Redrawn from (Heesy and Ross 2001) with permission.

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