The diversity of parasites

Parasitism is one of the most successful modes of life displayed by living organisms, as measured by how often it evolved and how many parasitic speci es are presently in existence. Studying the diversity of parasites is parti cularly relevant because sympatric diversification may be important in some parasite taxa, and because of the opportunity for independent tests of evo lutionary hypotheses in the many separate lineages in which parasitism evol ved. Our incomplete knowledge of existing parasite species-the result of a range of phenomena that includes inadequate sampling effort or the lumping of different cryptic species under one name-is not always a major obstacle for the study of parasite diversity. Patterns in the diversity of parasites may be associated with either host or parasite characteristics. The distri bution of parasite diversity among host taxa does not simply reflect the sp ecies diversity of the hst taxa themselves; life history and ecological tra its of hosts appear to play important roles. These may determine the likeli hood that hosts are colonized by parasite species over evolutionary time. I t is not yet clear whether some host traits also favor intrahost speciation and diversification of parasites, and the formation of new parasite specie s. Certain features of parasites may also be associated with speciation and diversification. Only parasite body size has received much attention; the patterns observed are not greatly different form those of free-living speci es, with small-bodied parasite taxa being more speciose than related large- bodied taxa. Epidemiological parameters such as the basic reproductive rate of parasites, or R-0, can also generate predictions regarding the distribu tion or evolution of parasite diversity. For instance, parasite taxa charac terized by high R-0 values may be more speciose than related taxa with lowe r values of R0; such predictions remain untested. Large-scale biogeographic al patterns of diversity have only been well studied for metazoan parasites of marine fish; for these parasites, latitudinal have only been well studi ed for metazoan parasites of marine fish; for these parasites, latitudinal patterns can be explained by effects of temperature on speciation rates and epidemiological variables, though other causes are possible. The emphasis for future research must shift from pattern description to the elucidation of the processes responsible for the structure and diversity of parasite fa unas. A better integration of ecological and historical (or phylogenetic) a pproaches to the study of parasite diversity should make this objective pos sible.