Two Little Drops of Sunshine

Here's a couple of familiar sights, here in Lake County (though not quite yet):

Clouded Sulphur (Colias philodice), Illinois Beach SP, 10/4/2011

Orange Sulphur (Colias eurytheme), Illinois Beach SP, 7/11/2012

The first shot's a Clouded Sulphur (Colias philodice) and the second one is an Orange Sulfur (C. eurytheme). Caterpillars of both feed on legumes, both native ones and introduced things like Vetch and, especially, Alfalfa. This last host means that they can be serious pests, and as a result, we know quite a bit about them.

These two species are very closely related, and hybridize quite a bit.  Even without hybridization, they both vary so much in color that identification in the field is often impossible. (The Illinois Butterfly Monitoring Network combines the two species for it's surveys, in fact.) In addition to variation in melanin concentrations and the amount of orange in the wing, females of both species have a normal yellow/orange morph and a white one. Yet despite all this, they manage to remain distinct. Clearly, they're using something more than the color and pattern. Turns out they're using pheromones. (This shouldn't be a surprise if you're into bugs.) (1)

But where does this color variation come from? Males do use wing color in mate choice, which normally reduces this sort of variation, and if hybridization isn't sufficient to drive the populations together, it must not be driving that variation. Ellers & Boggs found that in C. philodice populations in Colorado, male mate choice for brighter females was countered by natural selection for darker wings at higher elevations. Since the populations aren't actually isolated, this has the effect of maintaining variation throughout the population. (2)

In addition to this, however, Sappington & Taylor found that sexual selection by itself can maintain this variation. They studied the chemical makeup of the pheromones in C. eurytheme, and found it to be surprisingly variable. They then examined female mate preferences for different mixes, and found that white morph females prefer one form of the pheromones, while yellow morph females prefer another. (3) I haven't found any similar work on C. philodice, but given how closely related they are, it wouldn't be a surprise to find something similar going on with them.

The nature of variation in a population and how it changes over time is the essence of evolutionary biology. Whenever our days finally warm up and allow these little bits of sunshine out, stop for a moment and think about them as living critters, making their way through history the best that they can.

(1) TAYLOR, O. R. (1973). Reproductive isolation in Colias eurytheme and C. philodice (Lepidoptera: Pieridae): use of olfaction in mate selection. Annals of the Entomological Society of America, 66(3), 621-626.

(2) Ellers, J., & Boggs, C. L. (2003). The evolution of wing color: male mate choice opposes adaptive wing color divergence in Colias butterflies. Evolution, 57(5), 1100-1106.

(3) Sappington, T. W., & Taylor, O. R. (1990). Disruptive sexual selection in Colias eurytheme butterflies. Proceedings of the National Academy of Sciences,87(16), 6132-6135.