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The species problem in protozoa revisited

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The biological species concept as coined by Ernst Mayr is not applicable to many protists which reproduce by inbreeding or asexually. An extended concept supplementing the biological species concept was suggested by T. M. Sonneborn after intensive studies on differently reproducing species of the Paramecium aurelia complex. In his concept based on the hypothesis that inbreeding or asexually reproducing taxa also evolve as discrete units, he suggested that a species should be recognized as an evolving entity that has undergone a threshold of minimum evolutionary divergence. However, Sonneborns idea was poorly received. We examine different morphological and molecular characters discovered and applied in taxonomy since Sonneborn developed his hypothesis. We conclude that there is now an abundance of objective characters to arrive at sound judgement about the complexity of the genetic differences necessary to delimit species in Sonneborns sense when the biological species concept is not applicable. In addition, combined morphological and molecular studies reveal that, although many free-living protists may be globally distributed, geographical patterns and local distribution also occur.

Document Type: Research Article


Affiliations: 1: Institut für Zoology, Universität Leipzig, Talstraße 33, 04103 Leipzig, Germany;, Email: 2: Institut für Biologie II, RWTH Aachen, Kopernikusstraße 16, 52056 Aachen, Germany

Publication date: December 1, 2003

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