Testing Hypotheses of Evolutionary Relationships Among Parasites: The Digeneans of Crocodilians

Abstract

Parasite systematists may question whether or not all the parasites of a given host taxon share a common evolutionary history, especially one which also includes the host taxon. This study presents a method for testing hypotheses of such faunal coevolution using crocodilians and their digeneans as a model host-parasite system. First, phylogenetic analysis, using numerical methods when large data bases require such handling, produces estimates of genealogical relationships among members of various taxa represented. The estimates, in the form of branching diagrams (cladograms), can each be compared with host genealogy and with geographic genealogy for the areas in which the parasites occur. Congruence of such patterns suggests a common evolutionary history and corroborates an hypothesis of faunal coevolution; incongruence suggests an invasive or colonizing fauna and falsifies an hypothesis of faunal coevolution. The digeneans of crocodilians apparently have coevolved as a faunal unit with their host group, at least since the end of the Mesozoic.