At least twice, once in print,observers of pitviper mating behavior have reported use of both hemipenes simultaneously--but in at least one of those cases and I feel pretty sure both, they mistook the deeply forked large organ as being a single hemipenis.
Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population. The females lost their hemipenes as they got close to hatching.
An example of positive frequency-dependent selection is the mimicry of the warning coloration of dangerous species of animals by other species that are harmless. On the other hand, the tuatara—a reptile that resembles a lizard—has no penis at all.
Physiology of Insecta : 9— Search for:. Researchers investigated females of Phymaturus and Liolaemus lizards and discovered a structure that was previously undescribed. Asymmetry in hemipenis size and usage in gartersnakes Thamnophis sirtalis.
Learning Objectives Contrast stabilizing selection, directional selection, and diversifying selection. The hemiclitoris was observed to be smaller than a hemipenis,  and the organs were consistently observed in the lizards in the study. Stabilizing, Directional, and Diversifying Selection Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.
Download as PDF Printable version. This is one hypothesized reason for males having two penises instead of one: as each hemipenis is associated with one testis and only one side can be used during mating, having a second hemipenis functions as a "backup" and ensures that mating can continue even if one side were to run out of sperm.
Mice that carry alleles that make them slightly lighter or slightly darker will stand out against the ground and will more probably die from predation. Sexual selection, the selection pressure on males and females to obtain matings, can result in traits designed to maximize sexual success.
Because penises don't fossilize well, we don't know very much about the anatomy of ancient snakes and lizards, but it's safe to assume that the common ancestor of all squamates had hemipenes.