Color Vision

Evolution and selection of trichromatic

vision in primates

Alison K. Surridge1, Daniel Osorio2 and Nicholas I. Mundy3

1School of Biological Sciences, University of East Anglia, Norwich, UK NR4 7TJ

2Biological Sciences, University of Sussex, Brighton, UK BN1 9QG

3Department of Zoology, University of Cambridge, Cambridge, UK CB2 3EJ

Trichromatic colour vision is of considerable importance

to primates but is absent in other eutherian mammals.

Primate colour vision is traditionally believed to

have evolved for finding food in the forest. Recent work

has tested the ecological importance of trichromacy to

primates, both by measuring the spectral and chemical

properties of food eaten in the wild, and by testing the

relative foraging abilities of dichromatic and trichromatic

primates. Molecular studies have revealed the

genetic mechanisms of the evolution of trichromacy,

and are providing new insight into visual pigment gene

expression and colour vision defects. By drawing

together work from these different fields, we can gain a

better understanding of how natural selection has

shaped the evolution of trichromatic colour vision in primates

and also about mechanisms of gene duplication,

heterozygote advantage and balancing selection.

Among eutherian mammals, primate vision is unique.

Across primate taxa, colour vision is remarkably diverse

and studies of how and why such vision has evolved span

many fields in ecology and evolution. Although vision is

clearly of interest to those studying primate behaviour and

forest ecology, the underlying genetics of colour vision

provide important insights into polymorphism and population

genetics and also into the evolution of gene function

and regulation.

Our knowledge of the genetic basis for colour vision now

enables us to understand how it evolved, but much work

still focuses on why colour vision evolved. Such theories

are often controversial, and can be difficult to test. One

long-standing hypothesis is that enhanced colour vision in

primates evolved to detect ripe fruit on a dappled background

of leaves [1,2]. In the past few years however, this

has been challenged by the idea that foraging for young

leaves, which are often red in colour, explains such

adaptive variation [3,4]. A recent finding indicates that

primate colour vision might have evolved much earlier

than was previously thought, and provides some evidence

that an ancestral primate was possibly diurnal rather than

nocturnal [5]. However, paleontologists and taxonomists

tend to disagree [6]. Interestingly, in some primate taxa,

the variety of colours that can be discriminated differs,

even among individuals of the same species and sex. This

polymorphism enables a direct comparison of individual

primates with a variety of colour vision phenotypes to be

made, and links differences in a single gene to changes in

behaviour. Emerging experiments can now test models of

natural selection that were first proposed some 20 years

ago [7]. These studies follow the behaviour of individual

animals whose genotype and phenotype are known, and

provide an exciting way forward for understanding the

mechanisms of selection.

Seeing colours

Vertebrate colour vision requires both the presence of

PHOTORECEPTOR (see Glossary) cells, called CONE CELLS, in

Glossary

Allelic trichromacy: trichromatic vision achieved through the presence of

multiple alleles at the single X-linked opsin locus. Only females can be