Endemic Areas and Geographic Speciation in Tropical American Ferns

Abstract

The species of continental tropical American ferns are concentrated in three primary geographic areas, the Mexican, the Andean and the Brazilian, and two secondary areas, the Central American and Guayanan. These regional centers contain about 90 percent of the continental fern flora, and about 60 percent of these species are endemics. The intervening areas between the centers are relatively poor in species and especially in endemics. The regional centers are distinctive floristic-geographic regions. Their location and environmental history have had a major role in the development of the tropical American fern flora by the processes of geographic speciation: regional isolation, long-distance migration, and peripheral divergence. The spores of ferns have a high dispersal capacity, but geographic isolation may be effective at distances within the dispersal range especially if enforced by ecological isolation. Speciation by peripheral divergence has been especially important in the development of the high numbers of species and endemics that characterize the primary regional centers. THE SIZE AND location of species ranges and of dosely related (vicarious) species provide the basis for many of the aspects of floras pertinent to biogeography. These aspects include the numbers of species and the degree of endemism in an area and the degree and kind of relationship with other areas, for example, the number of species in common and the number of vicarious species shared by two areas. They also include aspects of the geography of evolutionary groups: the distribution of species-groups or genera (the ranges of the component species) and their geographic centers of speciation. The species and its distribution and the group of vicarious species and their distributions are thus basic elements in biogeography. These elements derive their characteristics from the biogeographic processes that control the distribution of species and from the processes of geographic and hybrid (amphiploid) speciation. A preliminary objective in modern biogeography is the analysis of geographic speciation (the evolutionary divergence of separate geographic elements of a species) so that the process can be profitably studied within the current framework of evolutionary thought. In a previous paper (R. Tryon 1970) some aspects of geographic speciation in ferns on isolated islands were considered; the present paper is concerned with the process on continents. Detailed studies of the process of speciation have mostly been made on the flowering plants (Grant 1963, 1971 for examples and references). The fewer studies of fern speciation have largely been devoted to hybrid speciation (Chiarugi 1960, Fabbri 1963, 1965 provide references). The process of geographic speciation in ferns has received little attention. It merits detailed studies, however, because of the wide and nearly equivalent dispersal capacity of fern spores. The ferns of isolated islands demonstrate that dispersal may occur for distances of 2000 miles (and more) and that relatively frequent dispersal may take place at distances of about 500 miles. Studies of fern geography can largely eliminate dispersal capacity as a variable, or as a limiting factor, and attention can be focused on other aspects of geographic and evolutionary processes. One fundamental aspect that has not been sufficiently studied to permit a definitive discussion is the role of the reproductive biology of ferns (Klekowski 1972, Voeller 1971 and references) in the speciation processes. Many of the conclusions drawn from the study of the geography of ferns may be applied to geographic problems in the flowering plants. Ferns and angiosperms show many geographic similarities in species ranges, species disjunctions, and centers of speciation. Most of the flowering plants have a lower dispersal capacity than ferns, and the reproductive biology of the flowering plants provides, through pollination and pollinators, special opportunities for natural selection that are not present in the ferns. However, these are not basic differences, but merely ones of degree. The role of adaptation to environment is dominant in the determination of the ranges of vascular plants. Both lower dispersal capacity and greater selective opportunities may reduce the geographic scale upon which rangeformation and speciation may operate without changing the processes that are involved. BIOTROPICA 4(3): 121-131 1972 121 1 I wish to acknowledge constructive comments on the manuscript by Dr. Ernst Mayr, Dr. Verne Grant, and Dr. Alice Tryon. This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 05:35:30 UTC All use subject to http://about.jstor.org/terms