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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
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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