In many species, exaggerated secondary sexual characters are used for both competition with rivals and courtship. One possible explanation for this association is the pre-existing trait hypothesis – ornamental characters initially evolve for male contests, but become co-opted to be evaluated by female choice. Comparative studies of a variably decorated group of animals can test the applicability of this hypothesis. Here I apply a phylogenetic approach to study the evolution of enlarged dorsal fins in male poeciliid fish of the subgenera Mollienesia and Limia, which have repeatedly developed a sailfin phenotype and use an erect-fin display for both intrasexual and intersexual purposes. These subgenera include species that show considerable male-male aggression without any elaborate courtship displays toward females, as well as species that show frequent male displays to both sexes. In contrast to prior categorical assessments of poeciliid adornments, I propose that dorsal fin size can be measured as a continuous index of ornamentation. I construct a new phylogeny to examine relationships between multiple subgenera of Poecilia, with Bayesian inference and two maximum likelihood methods robustly supporting the sister grouping of Limia and Pamphorichthys to the exclusion of Mollienesia, in accord with previous studies. Bayesian tests provide strong evidence for correlated evolution of the ornamentation index and several behavioral and morphological traits. The results of phylogenetic logistic and generalized least-squares regressions indicate that a high ornamentation index is significantly associated with the presence of exaggerated traits and component postures of courtship displays, but not with sexual dichromatism or variability in male mating tactics. Larger species with male-biased sexual size dimorphism have increased dorsal fin height relative to body length, body depth, and dorsal fin length. Ancestral state reconstructions and correlation analyses based on Bayesian Markov chain Monte Carlo simulations reveal that variability in male reproductive behavior has often appeared in courting species, and that two components of the courtship display have evolved together. The aggressive form of the display was present near the base of Poecilia well before the appearance of the mating display, suggesting that this trait originated for male-male competition and has become co-opted for use in courting females.