Episodic selection as a force in fungal microevolution, with special reference to clonal speciation and hybrid introgression

Episodic selection encompasses any sudden environmental disturbance likely to lead to a significant alteration in a species' population structure. Such disturbances include geographical transposition, a change in substrate availability, exposure to a new host or a new vector, climate change, and pollution stress. Today, such events may often be brought about by man. Their role in the promotion of fungal microevolution is discussed. In some circumstances, episodic selection may result in the emergence of a highly fitted clone from an originally heterogeneous population, and sustained disturbance may lead to clonal speciation. Clonal speciation accompanied by loss of sexual function, whether under episodic selection or under less intensive but analagous environmental conditions, could account for the origin of many of today's imperfect taxa (Deuteromycotina). Geographical transposition, a special form of episodic selection, can lead to hybridization between previously allopatric species. This may result in modifications to existing species via the acquisition of new loci or cytoplasmic elements, in the production of new taxa via secondary speciation, or in the emergence of hybrid swarms. Episodic selection will also favour survival of novel genotypes by providing new habitats for exploitation, so encouraging novel evolutionary development. Key words: episodic selection, fungal speciation, hybridization, introgression.