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Trends Parasitol. 2006 Nov;22(11):509-15. Epub 2006 Sep 12.

The great escape: do parasites break Dollo's law?

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Bio-Protection and Ecology Division, Lincoln University, PO Box 84, Lincoln, Canterbury 7647, New Zealand.


A long-held assumption in evolutionary studies is that a character that changes from a complex to a simple state is unlikely to return to the same complex state. The extreme version of this assumption has been codified as Dollo's law. Unfortunately, this paradigm has supported the idea that simple and complex traits are qualitatively different, when it is more sensible to suggest that there is a quantitative difference. Dollo's law has been the predominant paradigm in parasitology, where a move from a free-living state to parasitism has been considered a unidirectional pathway or 'one-way trip' because organisms lose the structures required to return to the free-living state. Several recent studies have suggested that complex structures can be regained from simple traits, and we suggest that this is also possible for parasites.

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