The Phylogenetic Relationships among Requiem and Hammerhead Sharks: Inferring Phylogeny When Thousands of Equally Most Parsimonious Trees Result

Abstmct-Protein variation among 37 species of carcharhiniform sharks was examined at 17 presumed loci. Evohttionary trees were inferred from these data using both cladistic character and a distance Wagner analysis. Initial cladistic character analysis resulted in more than 30 000 equally parsimonious tree arrangements. Randomization tests designed to evaluate the phylogenetic information content of the data suggest the data are highly significantly different from random in spite of the large number of parsimonious trees produced. Different starting seed trees were found to influence the kind of tree topologies discovered by the heuristic branch swapping algorithm used. The trees generated during the early phases of branch swapping on a single seed tree were found to be topologically similar to those generated throughout the course of branch swapping. Successive weighting increased the frequency and the consistency with which certain clades were found during the course of branch swapping, causing the semi-strict consensus to be more resolved. Successive weighting also appeared resilient to the bias associated with the choice of initial seed tree causing analyses seeded with different trees to converge on identical final character weights and the same semi-strict consensus tree. The summary cladistic character analysis and the distance Wagner analysis both support the monophyly of two major clades, the genus Rhizopionodon and the genus Sphyma.. The distance Wagner analysis also supports the monophyly of the genus Gzrcharhinus. However, the cladistic analysis suggests that Grrcharhinus is a paraphyletic group that includes the blue shark Rionnc~ +Lcn.