The sporozoans comprise a coherent group of protozoans, with characteristic and complex life cycles, containing 4-5000 species parasitic in invertebrates, particularly annelids and arthropods. and vertebrates. The group is a very successful one but neither its origins nor evolution are well understood. Considerations of traditional life cycles combined with newer molecular methodologies have thrown some light on the evolutionary expansions of the main groups of sporozoans, the gregarines. coccidia, haemosporidians and piroplasms. The sporozoans of economic importance such as the coccidia. malaria parasites and piroplasms have received most attention but the data obtained have also thrown new light on the possible evolution of less well studied groups and it is concluded that conclusions based on simple comparisons of life cycles will have to be modified. It is also clear that humans have played a major part in affecting the distribution and present abundance of many sporozoans of economic significance and probably also those of less importance, and that the rates of evolutionary expansion are much more rapid than previously thought.
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