UV Plumage and Female Mate Choice
Biology 342 Fall 2010
Evolution of Ultraviolet Plumage
Phylogeny refers to the evolutionary history of a group of species, or of a particular trait possessed by a group of species. Looking at phylogeny is often helpful when trying to understand why certain species display a specific character and others don’t. In this section, we will look at first the evolution of the bird class, then the evolution of UV plumage.
Evolution of Birds
The above diagram shows the phylogeny of all vertebrate species; birds evolved from the reptile family and are often considered to be the modern descendents of dinosaurs, along with crocodiles, to whom birds are most closely related. One early bird-like animal, probably an evolutionary link between dinosaurs and modern birds, was Archeopteryx lithographica, shown below as a fossil and as an artist’s rendering of what it probably looked like. Archeopteryx was small and had feathers similar in structure to those of modern birds, but had teeth and a bony tail like dinosaurs.
There are currently about ten thousand species of birds, depending on classification; more are discovered every year. The following tree shows the various kinds of birds and their phylogenetic relationships.
Evolution of UV Plumage
Many different types of birds display feathers that reflect ultraviolet light. For example, in the passerine family Icteridae – New World blackbirds, orioles, grackles, etc. – the possession of UV-reflecting plumage evolved multiple times from an ancestor lacking UV feathers (Eaton 2006). Males typically evolved more patches of UV-reflecting plumage than females, leading to sexual dimorphism in the UV spectrum, aiding in mate choice.
The black lines in this phylogenetic tree of the blackbird family represent species where the male breast feathers have peak reflection in the UV spectrum, meaning that those feathers reflect light more in the UV range than in the visible range. Based on the tree, we can tell that UV plumage evolved multiple times in the blackbird family, rather than once in a single common ancestor (Eaton 2006).
Evolution of UV Perception
The pigment in birds’ eyes that allows them to perceive ultraviolet light was experimentally derived by scientists at Syracuse University simply by changing one amino acid in the sequence coding for a pigment that reacted to visible violet light (Yokoyama et. al. 2000). The ability to see in the UV spectrum therefore could have evolved as the result of a single chance mutation in the DNA of some ancestor only once in the evolution of birds.
Before this mutation occurred, did birds' plumage reflect in the UV even without the corresponding ability to perceive it? This question is akin to the old "if a tree falls in the forest and there's no one to hear it, does it still make a sound?" If a feather reflects UV light and no one can tell, is it still ultraviolet-colored?