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Western Australian populations of Mecyclothorax punctipennis (MacLeay) exhibit chiral polymorphism for male genitalic asymmetry. The plesiomorphic genitalic enantiomorph, wherein the male aedeagal median lobe is left side superior when retracted in the abdomen, is rotated 180° to a right side superior position in 23% of males from Western Australia. Conversely, population samples from eastern Australia are monomorphic for the plesiomorphic left side superior condition. Western Australian population samples are significantly heterogeneous for the percentages of chirally reversed males, with right side superior frequencies ranging 0–58%. Conversely, asymmetry of the M. punctipennis female reproductive tract, wherein the apex of the bursa copulatrix is distally expanded toward the right side of the individual, is shown to be monomorphic within the species. Based on the vast disparity in frequencies of left versus right enantiomorphs among populations of Western Australian M. punctipennis, we hypothesize that population demographic factors related to very small population size and differential gene sampling via genetic drift could interact to establish populations fixed for the novel form. When such chiral genitalic substitutions are coupled with speciation, subsequent diversification stemming from that common ancestor would result in monophyletic lineages characterized by genitalic inversion. This hypothesis is corroborated by the sporadic occurrences of individual males with chirally inverted genitalia throughout the Carabidae, and the known occurrence of eight carabid taxa — individual species to diverse lineages — that are monomorphically characterized by male genitalic inversion.

In: Insect Systematics & Evolution