Sperm transfer in the scorpion Androctonus australis involves a sclerotized spermatophore, which is formed in the paraxial organs of the male reproductive system. The right paraxial organ produces the right half of the spermatophore (hemispermatophore), and the left paraxial organ the left hemispermatophore. The hemispermatophores are mirror images of each other, and are always produced one pair at a time.
During mating activities, a sexually mature male locates a receptive female, grasps her pedipalps, and proceeds to drag her along in his search for a suitable substrate onto which the spermatophore will be glued. This search may last for several hours. When the male finds a suitable substrate, the hemispermatophores slide out of their respective paraxial organs, move past the genital atrium (where the hemispermatophores are glued together), and the spermatophore is attached to the substrate.
The spermatophore contains the sperm mass in a concealed vesicle so that sperm aren't dessicated. The male then pulls the female over the spermatophore, guiding her so that her genital opening reaches a position directly above the spermatophore. A brief struggle ensues and the female's rocking motion helps to engage the spermatophore with her genital operculi. Continued rocking by the female triggers spermatophore opening, and the sperm mass is transferred into her genital pore. The transfer requires a pressurized vesicle that forces the sperm into her genital pore.
The spermatophore originated in aquatic organisms, but it has undergone extensive modification for use in terrestrial organisms like scorpions. Which evolutionary developments caused the transition from an aquatic deployment of the spermatophore to a terrestrial deployment involving the various interdependent behavioral and physiological elements described above? Since the behavioral elements are invariant, they must be encoded in the genome just as the physiological elements are. A rigorous answer to this question necessitates the elucidation of a series of intermediate steps, each of which was driven by natural selection, and each of which involved a set of mutations whose overall probability was not prohibitively low. If one or more of the steps was driven by genetic drift, then we must take into account the fact that, although the time for fixation of a neutral mutation is shorter in small populations (such as occurs during a population bottleneck), there is less opportunity for mutations to appear in small populations.