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Short Feature GEETANJALI SINGH H UMANS and monkeys have many similarities due to their close evolutionary ancestry. They fall under a common order called Primates. Primates possess five important senses: the sense of smell, taste, vision, touch and hearing. All these senses are unique and have evolved to make Primates adaptable to their habitat. An interesting sense in Primates is the sense of vision. Not all organisms in this world have colour vision. Some animals see only in black and white and some can see only limited colours. A topic of great interest to scientists is: Why have some monkeys acquired colour vision? A trait is a genetic feature shaped by both genetic makeup and environmental influences. The trait of colour vision has these phenotypes: monochromacy (can see no colour, or colour blind), dichromacy (can see two wavelengths of light and hence can see some colours) and trichromacy (can see many colours). Thus, a trait is often a measurable and observable characteristic of an organism and is the result of genetic information contained in the DNA. A trait is described as ‘adaptive’ when it helps in the survival of the organism in terms of producing successive generations. Adaptive traits are quite varied and may include body colour (required for mating, camouflage, etc.), behaviour (e.g. feeding behaviour), function (e.g. sweating) and structures (e.g. fins, wings). Likewise, the sense of smell, ability to hear and colour vision are also adaptive traits. Organisms that fail to acquire adaptive traits do not have an advantage over their competitors in a habitat, which leads to lower chances of survival or a need to leave a particular habitat. However, not all adaptive traits remain useful over time because the organisms live in an ever-changing environment from one generation to the next. When a particular trait continuously enhances adaptability of Marmosets and tamarins only see blues and greens an organism to its environment and ensures successful reproduction, then the trait gets fixed in a population. This means that every organism in the population possesses that trait. However, this does not lead to complete abolition of phenotypic variation due to the phenomenon of mutation and recombination. This is the reason why all humans may have trichromatic vision but still there are some colour-blind individuals. An adaptive trait may cease to be useful and eventually be lost when the environment in response to which it was formed changes. Loss of eyes of cave crustaceans is one such example. These organisms once had eyes and vision, but when they started living in dark caves for many generations, they no longer required visual function. Study of adaptive traits in primates gives important insights in understanding human evolution. Humans and related species belong to the order Primate, which is divided into two suborders, Prosimii (lorises, lemurs, and tarsiers) and Anthropoidea (Platyrrhinithe New World monkeys, Catarrhini-the Old In Platyrrhines, only a small population is trichromatic like Howler monkey. SCIENCE REPORTER, JANUARY 2013 38 Short Feature Since there is a common ancestor involved in this trait, therefore, the colour vision trait is a homologous trait. World monkeys and Hominoidea). These two parvorders (Platyrrhini and Catarrhini) superfamily (Hominoidea) include monkeys, apes, and humans. The colour vision trait in primates exists in monochromatic, dichromatic and trichromatic states. In Platyrrhines, only a small population is trichromatic (e.g. Howler monkey) due to a specific polymorphic allele of a gene. Some females in this parvorder are trichromatic, whereas most males are dichromatic. Nocturnal owl monkeys are monochromatic and see only black, white, and intermediate greys. Marmosets, tamarins, and spider monkeys only see blues and greens and are dichromatic. Complete details on colour vision in this parvorder are lacking as only small number of studies have been carried out till date. Humans, apes, and most of the Old World monkeys, which are diurnal species, are routinely trichromatic. It is generally assumed that the ancestors of all monkeys were prosimians who were monochromatic or dichromatic. The Old and New World monkeys became separated 30-40 million years ago and had different evolutionary phases after this separation. It is likely that mutations in the X chromosome gene or genes that provide the ability to see red colour occurred after this separation. As trichomacy occurs in both the New and Old Worlds, it is probable that the mutation for it occurred more than once. However, the switch to trichomacy was clearly more complete among Old World monkeys than the New World ones. Colour vision requires the presence of special photoreceptors or pigmented cones located inside the eyes. These photoreceptors or cones are encoded by genes. Most mammals have two cone pigments, S cone pigments that are maximally sensitive to blue colour and M cone pigments to green or yellow. These apparently diverged from a common ancestral cone pigment gene about five hundred million years ago. Many marine mammals and a few nocturnal rodents, carnivores, and primates have lost the S cone pigment and became monochromatic. This was an adaptive behavior that originated as there was no need to discern more colours in the dark Nocturnal Owl monkeys are monochromatic, Humans and apes are routinely dichromatic conditions that they started living in. Many diurnal primates, on the other hand, have acquired a third cone pigment, the L cone pigment, which is maximally sensitive to the longer visible wavelengths (red). Many recent studies have hypothesized that appearance of this L cone pigment (and consequently trichromacy) in primates was an adaptive trait that arose due to the need to identify either coloured fruits against the green foliage background or nutritious young green leaves from mature leaves. Such a selection would aid in propagation of their generations. It has been suggested that trichromacy in primates and the reflectance functions of certain tropical fruits are the result of a co-evolved seed-dispersal system: primate colour vision. These systems have been shaped by the need of primates to find coloured fruits amongst foliage. The fruits themselves, in turn, have evolved to be coloured to secure dissemination of their seeds. Other studies indicate that primate trichromacy could have evolved not only for foraging but also as an adaptation for many other visual tasks. The traits that are similar due to shared ancestry are known as homologous traits, whereas traits that are similar due to convergent evolution (but not inherited from a common ancestor) are called analogous traits. Study of homologous as well as analogous traits is very useful in establishing phylogentic (study of evolutionary relatedness among groups of organisms) relationships. Many biological tools can be used to establish the phylogenetic relationships of a trait. Older methods include comparison of structural features of the fossils with the modern day organism. Behavioural functions are also used in some cases to study phylogeny e.g. behavioural patterns of ants. Developmental studies during 39 the embryonic stage of organisms are also useful in understanding phylogentic relationships as these studies compare the developmental processes of different organisms. Organisms having similar embryonic developmental stages may have close ancestral relationships. Contemporary methods involve determining similarities and differences between the DNA and protein sequence of two organisms and using them to reveal the phylogentic relationships. In the case of colour vision trait, even the remote ancestors of vertebrates such as fish, birds and reptile are trichromatic. An ancestral group (most of the mammals) down the evolutionary track lost the threecolour vision as they no longer required it. However, another group comprising of primates acquired it to survive in a changing environment. Since there is a common ancestor involved in this trait, therefore, the colour vision trait is a homologous trait. Both morphological and genetic evidence corroborate this. Nucleotide sequencing of two New World monkeys has been done and suggests that L/M allele divergence has arisen from gene duplication in Old World monkeys. Comparative studies of mammalian eyes indicate that primates are the only placental mammals that have in their retina a pre-existing neural machinery capable of utilizing the signals of an additional spectral type of cone. Thus, the failure of non-primate placental mammals to evolve trichromacy can be explained by constraints imposed due to morphologic differences. Dr. Geetanjali Singh is an Assistant Professor in the Department of Veterinary Physiology and Biochemistry, College of Veterinary and Animal Sciences, CSK HP Agriculture University, Palampur, Himachal Pradesh-176062 SCIENCE REPORTER, JANUARY 2013