Interspecific Hybridization in Pinus: a Summary Review

The pines and other groups of temperate forest trees often fail to conform to widely accepted principles of plant evolution that are based primarily on observations of herbaceous plants. For example, interpreters of plant evolution tend to equate the fertility or sterility of inter specific hybrids with the magnitude of the genetically controlled reproductive barriers limiting the production of hybrids. This tendency has been articulated by Grant (1958, p. 353): "This is assuming that incompatibility barriers and sterility barriers, two distinct phenomena in the physiological sense, tend to run parallel in most groups, an assumption which despite a few exceptions is generally supported by the facts.11 The common corollary to this assumption is that vigorous, fertile interspecific hybrids reflect the absence of well-developed barriers to hybridization. In Pinus9 neither Grantfs assumption nor its corollary is supported by the facts summarized in this paper.