Morphology and Phytogeography: The Classical Approach to the Study of Disjunctions

Abstract

Disjunctions are evident in the distribution of almost every species of plant, for it is obvious that no species is composed of completely continuous populations. Discontinuities, however, range from small to very large. Although formerly widely distributed, the giant redwoods or big trees (Sequoiadendron giganteum) now occur in groves (populations) of varying size scattered over a distance of 250 miles along the western slopes of the Sierra Nevada of California. The gaps between groves may be insignificant or up to 50 miles wide, but disjunctions of this size do not draw particular attention, except as showing that the intervening habitats presumably are now unfavorable for the establishment and growth of Sequoiadendron. If, however, a grove of big trees were to be found in some part of the world as remote from California as the Himalayas of Nepal or the Smoky Mountains of North Carolina and Tennessee, the find might be more exciting than the discovery of living Metasequoia in central China. To the inquisitive mind such a large gap in distribution would demand explanation, and, unsurprisingly, disjunctions of this size are the ones that have most intrigued biologists. A disjunction in the distribution of a single species or genus, even a very wide one as in the hypothetical example above, may make one wonder, but when numerous species in the flora of a given region show the same general disjunction, the curious scientist who realizes that there is a common pattern can hardly resist searching for the circumstances behind it. Patterns of disjunction between eastern North America and eastern Asia, between Europe and North America, between eastern and western North America, and between North and South America are among those that have proved to be especially intriguing (at least to biologists of the continents involved). Almost from Linnaeus onward, botanists have sought and advanced explanations as diverse as double creation of species, long-distance dispersal, disruption (by a variety of factors) of formerly continuous ranges, drifting of continents, and the either conscious or unintentional activities of man. A great deal of effort has gone into the study of disjunct distributions, and in all of the published material that has resulted, it is evident that comparative morphology is basic. Morphology is basic to the study of disjunctions (and to all of phytogeography, for that matter), because it is basic to taxonomy. Without a taxonomic frame of reference for each disjunct taxon comparisons of almost any sort become meaningless. As taxonomic ideas about a taxon change, so must conclusions based upon the earlier taxonomy. The new data brought into taxonomy by biosystematics have not upset the fundamental taxonomic framework built on morphology but have generally confirmed its soundness. The new data have, moreover, served to broaden the parameters of taxonomy, at the same time