Let’s say you’ve just found out something new about a species of bird, a certain turaco that is a very dark cryptic green, in the forests of the Congo. The species hasn’t been studied much, and you’ve discovered that the male birds have a special pigmentation only visible in the ultraviolet spectrum, and this pigment is found in a barred pattern across their undersides, which they display to females and other males in their dense colonies. Based on certain test results, you can positively claim that the pigment is easy and inexpensive to produce, and you’ve also ascertained that the bars of ultravioletly-pigmented feathers are closely correlated with the bird’s growth—that is, older birds have wider bars. But apart from these few facts, you know very little about the turaco species, and your job is to wonder how and why the ultraviolet barring and its display evolved.

The most compelling and widely held explanation for why a species has developed particular morphological features and common behaviors is natural selection. Either sexual or predatory forces have acted upon individuals of the species until certain traits, such as unique plumage, become genetically ingrained in the species as a whole. The turaco’s special pigmentation, bar pattern, and display almost certainly evolved in such a fashion. How did the bars, and their display, evolve? Given what little is known about this new discovery—for example, that the pigment is visible only in the ultraviolet spectrum, and that males intentionally display their bars to females and other males—you can make certain conjectures and conclude that barring in male turacos probably evolved as a result of sexual selection. In the next series of posts I will walk through a couple theories that might prove different biological hypotheses.