Butterfly wing color-patterns are highly diverse. Many color-patterns or their elements are ecologically and behaviorally functional in mating, mimicry and camouflage, but it has been thought that not all of them have clearly identifiable functions. How such non-functional, sometimes extravagant color-pattern traits evolved has been enigmatic. To illustrate this process, a model for the color-pattern evolution of the Admiral butterflies (the genus Vanessa sensu stricto) is proposed. It is first assumed that ancestral populations of Vanessa were geographically isolated in high altitude regions where temperatures fluctuate widely. In this model, pupae produce the coldshock hormone (CSH) to protect differentiating cells in response to the temperature fluctuation. This hormone has an opportunistic ability to modify the wing color-patterns as a side effect or pleiotropic effect, revealing phenotypic plasticity of the ancestral populations. The modified phenotypes are rarely adaptive; they are in most cases either not favorable in mating or simply neutral with no adverse or adaptive effects. The modified phenotypes are nonetheless canalized in a population through a process of genetic assimilation accompanying natural selection for high hormonal activity, resulting in diverse orange areas on the wings in the present Vanessa species living in high altitude regions. Thus, the CSH physiologically acts as a mediator of genetic assimilation of the non-functional or neutral traits. In other words, a non-functional or neutral trait can be assimilated in a population as a "parasite" of a functional trait. Following the evolution of mating preferences, premating reproductive isolation might be established, as in the classical mechanism for allopatric speciation. This "physiological side-effect model" may explain the non-functional or neutral wing color-pattern diversity of Vanessa and other butterflies.
View full abstract