Hybridization runs rampant between these, making phylogenetic analyses exclusively utilizing one type of data (especially mtDNA sequences) unreliable.

According to studies, these ultraviolet reflecting wing scales found in males also contain pterin pigments that absorb wavelengths below 550 nm.

[5] Further studies have found that the ultraviolet reflectance signal is brightest within a wing beat cycle when viewed from directly above the male.

This supports the idea that male wing color should be able to be readily distinguished from that of females and the visual background that consists mostly of UV-absorbing vegetation.

The X-chromosome carries most of the genes controlling production of 13-methyl heptacosane, the main component of pheromones involved in sexual selection, and the ultraviolet wing reflectance pattern.

Expression of the ultraviolet wing reflectance pattern found in male C. eurytheme is controlled by a recessive allele on the X-chromosome.

Upon mating, male C. eurytheme donate a nutritious spermatophore to the female, which will erode over time as nutrients are extracted for egg production and somatic maintenance.

[10] Due to the widespread cultivation of the alfalfa, the host plant for C. eurytheme and C. philodice, the species was able to expand their ranges across most of North America.

These two species of sulphur butterflies have retained a large degree of genetic compatibility that allows them the produce viable and fertile offspring.

[12] As a result of the recent sympatry and possible hybridization between these two species of sulphur butterflies, numerous studies have been conducted on intraspecific and interspecific mating.

[13] In terms of mating under natural conditions, the males do not discriminate between the species, but females maintain nearly complete reproductive isolation.

[9] The brightness of ultraviolet reflectance and pheromone descriptors, both important factors in mate selection, are also negatively correlated with age.