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The role of introgression in plant evolution has been the subject of considerable discussion since the publication of Anderson's influential monograph, Introgressive Hybridization (Anderson, 1949). Anderson promoted the view, since widely held by botanists, that interspecific transfer of genes is a potent evolutionary force. He suggested that" the raw material for evolution brought about by introgression must greatly exceed the new genes produced directly by mutation"(1949, p. 102) and reasoned, as have many subsequent authors, that the resulting increases in genetic diversity and number of genetic combinations promote the development or acquisition of novel adaptations (Anderson, 1949, 1953; Stebbins, 1959; Rattenbury, 1962; Lewontin and Birch, 1966; Raven, 1976; Grant, 1981). In contrast to this" adaptationist" perspective, others have accorded little evolutionary significance to introgression, suggesting instead that it should be considered a primarily local phenomenon with only transient effects, a kind of" evolutionary noise"(Barber and Jackson, 1957; Randolph etal., 1967; Wagner, 1969, 1970; Hardin, 1975). One of the vociferous doubters of a significant role of hybridization in plant evolution was Wagner (1969, p. 785), who commented that the" ultimate contributions made by hybrids must be very small or negligible." Wagner's frequently expressed opinion appears to be based on ecological and compatibility arguments, which were encapsulated as follows:" In the rare cases that two well differentiated species happen to be interfertile enough to produce fertile progeny, their hybrids will usually have to fit into some hybrid niche. Such …