A phylogenetic tree is a way to represent the genetic relatedness among species in an evolutionary context (see Baum et al 2005). When we construct a phylogenetic tree for monkeys, based on morphological, behavioral, or DNA sequence characteristics, we find that the old world monkey species (trichromats) form a cohesive monophyletic group (or clade) on one side of the tree while the new world monkey species (dichromats) form a clade on the other side of the tree. The presence of these two clades indicates that the new world monkeys separated from the old world monkeys at some point in their history. Our observation that old world monkeys have trichromatic vision and new world monkeys have dichromatic vision implies that trichromatic vision evolved in the old world monkeys after this separation event.
Given what we know about geology, paleontology and ancient primate history we can start to map other events onto the phylogenetic tree.
- The first primate species evolved sometime between 70 and 80 million years ago (e.g. Chatterjee et al 2009).
- There were primate species that lived throughout the contiguous new world and old world landmasses around 55 million years ago (see: Oldest North American Primate Fossil Discovered).
- The new and old world landmasses separated sometime around 50 million years ago.
- The successful gene duplication and mutation events of the MWS opsin that resulted in a function LWS opsin happened sometime around 40 million years ago in an ancient old world primate lineage Surridge et al 2003).
You may notice that the above phylogenetic tree does not include all primate species but is instead a representative tree of three different primate groups. The dichromate phylogenetic group (i.e. cluster of branches) the right shows the new world monkeys (platyrrhines). The trichromatic phylogenetic groups to the left show the apes (hominoids) and old world monkeys (cercopithecids). While this case study has focused on the old world monkeys, the evolution of trichromatic vision actually occured before old world monkeys and apes were phylogenetically distinctive groups.