The evolution of copulation in water mites: a comparative test for nonreversing characters

Although copulation is a widespread behavior with multiple origins, hypotheses about selective forces behind its evolution have not been tested by the comparative method. Because copulation has arisen many times in the water mites (Acari: Parasitengona), they are good subjects for a comparative study of copulation. I determined that copulation evolved 91 times in the 343 extant genera. There was no evidence of reversals to noncopulation; therefore, Ridley's (1983) contingency-table comparative test was not appropriate. I designed a comparative test for instances in which there is no loss of the derived trait of interest. This test determines whether independent evolutions of a trait cooccur with a predictor more often than expected by chance. Two hypotheses were tested: 1) that copulation would be selected for in running-water habitats because of disruption of pheromonal communication; 2) that copulation would be selected for in swimming mites because females are less likely to contact spermatophores deposited on a substrate. Independent evolutions of copulation among running-water mites were not more frequent than expected by chance (P > 0.4), but there were more evolutions of copulation than expected among swimming mites (P < 0.005). Endoparasitism, secondary loss of swimming hairs, production of spermatophore fields, courtship, and benthivorous habits may explain why some mites copulate but do not swim while others swim but do not copulate.