Estimation of present day diversity of organism and understanding their diversity forms new cornerstones of conservation biology, evolutionary biology and ecology. Maintaining the totality of all texa from past to present and classifying into groups reflect how they have changed over the period of times. Necessarily, phylogenetics and evolutionary study of a particular group help out in knowing pattern of occurrence and relationships between two distantly related class or families and their descendents.
They may be a group of parasites of virus, bacteria, fungi or helminthes or even normally occurring species like animal and plants may provide a suitable platform for such analysis. Comparatively, parasites have been more sophisticated for studying morphological changes with respect to ecological and evolutionary pattern since they represent rapid rate of adaptation after being developed into new form, species. They are also supposed to mutate with nucleic acids and exhibit by morphological pattern, pretending great sensitivity towards changing ecological conditions. Strictly, Monogenea, parasite on fishes and amphibians, have been and ideal parasite taxon for investigations of past diversification and present diversity for three reasons.
First, monogeneans represent a diverse group, with several thousand species currently described since they are the only clade among the parasitic flatworms to have undergone an adaptive radiation. Monogeneans are diverse not only in terms of numbers but also with respect to their morphology and ecology. Monogeneans have expanded to colonize internal as well as external organs of a range of living aquatic vertebrates, and now display a variety of designs.
Second, the phylogenetic study on monogenean family is very well resolved. And the elucidation of within-family relationships is well under way. A good knowledge of the relationships among members of a clade, especially among higher taxa such as families, is essential for the study of diversity using modern comparative methods.
Third, monogeneans tend to be host specific, i.e. each monogenean species infects only one or very few host species. This is not obligatory the outcome of strict co-speciation between the parasites and their hosts, although this has been marked and clearly documented in many monogeneans. There is also evidence of host switching during the co-evolution of monogeneans and their hosts, from studies both at the level of sister species and among more basal branches in the monogenean phylogeny.