One of the most dramatic statements in a recent review of polyploidy by Soltis et al. (2003) is "A major discovery of the past decade is the extent and rapidity of genome reorganization in polyploids" (p. 183). It is now becoming clear that change can occur quickly in polyploid lineages, and can result in what have been called "tertiary" species (Haufler, 1989). Given that (1) homosporous ferns and lycophytes contain a higher percentage of polyploid species than other plant groups (Otto and Whitton, 2000), (2) the nuclear genome in these lineages appears unique in having diploid genetic expression despite large basal chromosome numbers, and (3) the many questions that remain unanswered about the fate of polyploid species (see Chapter 7), it is unfortunate that there is little research exploring the mechanisms that resulted in this combination of features. When Werth and Windham (1991) proposed that reciprocal gene silencing within polyploid lineages could result in genetically (but probably cryptically) isolated populations, polyploids were generally considered to be static entities whose bloated, polygenic systems prevented rapid evolutionary change. It seems clear that if we are to develop a synthesis of the evolutionary modes and mechanisms of ferns and lycophytes, we must consider the fate of polyploids, and the role that they play in generating biodiversity.
Contemporary analyses are providing a progressive illumination of species boundaries, and an improved perspective on how ferns and lycophytes evolve. More widespread application of molecular methods will continue the trend of discovering cryptic species, especially within those currently considered to be polymorphic and widespread. It is critical, however, that we carry this technology into tropical regions, as it is clear that the bulk of fern and lyco-phyte species occur there. The power of these tools has been well demonstrated in the many studies completed on temperate species complexes, and we need to build on this foundation.
As presented in Chapter 4, changes in our understanding of fern and lycophyte breeding systems (from primarily inbreeding to an emphasis on outcrossing) have promoted a shift from considering ferns and lycophytes (with their easily wind-dispersed propagules) as unlimited in dispersal capacity to a combination of dispersal and vicariance-driven mechanisms of biogeographical change (Wolf et al., 2001; Haufler, 2007). As a consequence, we have a better perspective on the shift from a relatively small number of polymorphic, widely dispersed species to many, more narrowly distributed, often cryptic species. These populationlevel discoveries extend to speciation and a consideration of a range of mechanisms including changes in genetic composition, geographic barriers, ecology, and ploidal level. As presented in Chapter 9, it is critical that we learn more about the ecology of the gametophyte generation as these colonizers are likely the agents that are selecting and defining habitat specificities of many groups, especially in species-rich tropical regions.
Studies of speciation in the ferns and lycophytes remain based primarily on circumstantial evidence. Our definition of monophyletic groups continues to improve (see Chapters 13, 15, and 16), and, therefore, so does the accuracy of comparing levels of diversity within and among lineages. With accurate assessment of the amount of change that is associated with the origins of species, we can consider more confidently the events and/or mechanisms that have caused that change. As it has now been demonstrated that ferns and lycophytes have breeding systems that are directly comparable to those of seed plants and as sophisticated molecular tools provide even more fine-grained interpretations of the genetic variation in populations, it will be possible to draw on the many models and hypotheses that have been proposed in work with other groups of plants. Even now it is possible to develop hypotheses that explain populationlevel mechanisms for maintaining and modifying genetic variation (see Chapter 4), providing a much improved foundation for understanding speciation mechanisms. Just as with other plant groups, combining mechanisms operating in populations with regional and global patterns of biogeography and vicariance will generate robust hypotheses to explain and model speciation in ferns and lycophytes.
At the same time as the basis for establishing a solid understanding of primary speciation is developed, current interest in and understanding of poly-ploidy as a powerful force in generating opportunities for radical reorganization of genomes and rapid change in lineages should extend to the ferns and lycophytes. Especially considering the unique combination in the homo-sporous groups of high chromosome numbers and genetic diploidy, ferns and lycophytes should yield important perspectives on this emerging and exciting element of the genetics of organisms. Clearly, polyploidy is an important agent of genetic change and without exploring the fern and lycophyte genomes, a full understanding of the history of evolutionary progress will not be obtained (see Chapter 7).
Tertiary speciation through polyploid genetic revolutions or through passive silencing of reciprocal loci expands the realm of possibilities for explaining the origins of biodiversity. Especially among widespread polyploid species, considering the patterns of variation within and among populations will yield a superior picture of (1) multiple origins of polyploids, (2) how polyploids change as they migrate, and (3) whether polyploids differ from diploids in how and how fast they become modified over time and distance.
The time is ripe for building on the advances we have made in proposing accurate hypotheses about the phylogenetic trees of relationships among fern and lycophyte lineages (see Chapters 15 and 16). These studies are generating a clear picture of the patterns of change. We should use this phylogenetic foundation to propose and test hypotheses about the processes that resulted in the patterns. These studies also provide robust hypotheses about major lineages, but often the circumscription of the elements of these major lineages requires additional investigation. Especially in tropical regions, where it appears that changes may be rapid and tied to the complex ecology of such areas, our understanding of evolutionary mechanisms is limited at best. Targeting efforts at these regions is critical, and should yield valuable insights and perspectives.
Adjie, B., Masuyama, S., Ishikawa, H., and Watano, Y. (2007). Independent origins of tetraploid cryptic species in the fern Ceratopteris thalictroides. Journal of Plant Research, 120, 129-138.
Barrington, D. S. (1990). Hybridization and allopolyploidy in Central American Polystichum: cytological and isozyme documentation. Annals of the Missouri Botanical Garden, 77, 297-305.
Barrington, D. S. (1993). Ecological and historical factors in fern biogeography. Journal of Biogeography, 20, 275-279.
Barrington, D. S., and Conant, D. S. (1989). Breeding system, genetic distance, and hybridization in Alsophila. American Journal of Botany, 76, Supplement, 201 (abstract).
Barrington, D. S., Haufler, C. H., and Werth, C. R. (1989). Hybridization, reticulation and species concepts in the ferns. American Fern Journal, 79, 55-64.
Benham, D. M. and Windham, M. D. (1992). Generic affinities of the star-scaled cloak ferns. American Fern Journal, 82, 47-58.
Bennert, W., Lubiensky, M., Korner, S., and Steinberg, M. (2005). Triploidy in
Equisetum subgenus Hippochaete (Equisetaceae, Pteridophyta). Annals of Botany, 95, 807-815.
Berlin, B., Breedlove, D. E., and Raven, P. R. (1966). Folk taxonomies and biological classification. Science, 154, 273-275.
Blasdell, R. F. (1963). A monographic study of the fern genus Cystopteris. Memoirs of the Torrey Botanical Club, 21, 1-102.
Bruce, J. G. (1975). Systematics and morphology of subgenus Lepidotis of the genus Lycopodium (Lycopodiaceae). Unpublished Ph.D. Thesis, University of Michigan, Ann Arbor, MI.
Butters, F. K. (1917). Taxonomic and geographic studies in North American ferns. I. The genus Athyrium and the North American ferns allied to Athyrium filix-femina. Rhodora, 19, 169-207.
Carlson, T. J. (1979). The comparative ecology and frequencies of interspecific hybridization of Michigan woodferns. Michigan Botanist, 18, 47-56.
Conant, D. S. (1990). Observations on the reproductive biology of Alsophila species and hybrids (Cyatheaceae). Annals of the Missouri Botanical Garden, 77, 290-296.
Conant, D. S. and Cooper-Driver, G. (1980). Autogamous allohomoploidy in Alsophila and Nephelea (Cyatheaceae): a new hypothesis for speciation in homoploid homosporous ferns. American Journal of Botany, 67, 1269-1288.
Coyne, J. A. and Orr, H. A. (2004). Speciation. Sunderland, MA: Sinauer.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London: J. Murray.
De Queiroz, K. (1998). The general lineage concept of species, species criteria, and the process of speciation. In Endless Forms: Species and Speciation, ed. D. J. Howard and S. H. Berlocher. Oxford: Oxford University Press, pp. 57-75.
Diamond, J. M. (1966). Zoological classification system of a primitive people. Science, 151,1102-1104.
Flora of North America Editorial Committee (eds.) (1993). Flora of North America North of Mexico, Vol. 2. New York: Oxford University Press.
Gastony, G. J. (1983). The Pellaea glabella complex: electrophoretic evidence for the derivations of the agamosporous taxa and a revised taxonomy. American Fern Journal, 78, 44-67.
Gastony, G. J. (1986). Electrophoretic evidence for the origin of fern species by unreduced spores. American Journal of Botany, 73, 1563-1569.
Gastony, G. J. and Windham, M. D. (1989). Species concepts in pteridophytes: the treatment and definition of agamosporous species. American Fern Journal, 79, 65-77.
Gentry, A. H. (1988). Changes in plant community diversity and floristic composition on environmental and geographical gradients. Annals of the Missouri Botanical Garden, 75, 1-34.
Grant, V. (1981). Plant Speciation. New York: Columbia University Press.
Haufler, C. H. (1987). Electrophoresis is modifying our concepts of evolution in homosporous pteridophytes. American Journal of Botany, 74, 953-966.
Haufler, C. H. (1989). Toward a synthesis of evolutionary modes and mechanisms in homosporous pteridophytes. Biochemical Systematics and Ecology, 17, 109-115.
Haufler, C. H. (1996). Species concepts and speciation in pteridophytes. In Pteridology in Perspective, ed. J. M. Camus, M. Gibby, and R. J. Johns. Kew: Royal Botanic Gardens, pp. 291-305.
Haufler, C. H. (1997). Modes and mechanisms of speciation in pteridophytes. In
Evolution and Diversification of Land Plants, ed. K. Iwatsuki and P. H. Raven. Tokyo: Springer-Verlag, pp. 291-308.
Haufler, C. H. (2002). Homospory 2002: an odyssey of progress in pteridophyte genetics and evolutionary biology. Bioscience, 52, 1081-1093.
Haufler, C. H. (2007). Genetics, phylogenetics, and biogeography: considering how shifting paradigms and continents influence fern diversity. Brittonia, 59, 108-114.
Haufler, C. H. and Ranker, T. A. (1995). rbcL sequences provide phylogenetic insights among sister species of the fern genus Polypodium. American Fern Journal, 85, 359-372.
Haufler, C. H. and Windham, M. D. (1991). New species of North American Cystopteris and Polypodium, with comments on their reticulate relationships. American Fern Journal, 81, 6-22.
Haufler, C. H., Windham, M. D., Britton, D. M., and Robinson, S. J. (1985). Triploidy and its evolutionary significance in Cystopteris protrusa. Canadian Journal of Botany, 63, 1855-1863.
Haufler, C. H., Windham, M. D., and Ranker, T. A. (1990). Biosystematic analysis of the Cystopteris tennesseensis complex. Annals of the Missouri Botanical Garden, 77, 314-329.
Haufler, C. H., Windham, M. D., and Rabe, E. W. (1995a). Reticulate evolution in the Polypodium vulgare complex. Systemetic Botany, 20, 89--109.
Haufler, C. H., Soltis, D. E., and Soltis, P. S. (1995b). Phylogeny of the Polypodium vulgare complex: insights from chloroplast DNA restriction site data. Systematic Botany, 20, 110-119.
Haufler, C. H., Hooper, E. A., and Therrien, J. P. (2000). Modes and mechanisms of speciation in pteridophytes: implications of contrasting patterns in ferns representing temperate and tropical habitats. Plant Species Biology, 15, 223-236.
Hauk, W. D. and Haufler, C. H. (1999). Isozyme variability among cryptic species of Botrychium subgenus Botrychium (Ophioglossaceae). American Journal of Botany, 86, 614-633.
Hey, J. (2006). Recent advances in assessing gene flow between diverging populations and species. Current Opinion in Genetics and Development, 16, 592-596
Hickey, R. J. (1984). Chromosome numbers in neotropical Isoetes. American Fern Journal, 74, 9-13.
Hickey, R. J., Taylor, W. C., and Luebke, N. T. (1989). The species concept in
Pteridophyta with special reference to Isoetes. American Fern Journal, 79, 78-89.
Hooper, E. A. and Haufler, C. H. (1997). Genetic diversity and breeding system in a group of neotropical epiphytic ferns (Pleopeltis; Polypodiaceae). American Journal of Botany, 84, 1664-1674.
Jermy, A. C. and Walker, T. G. (1985). Cytotaxonomic studies of the ferns of Trinidad. Bulletin of the British Museum (Natural History), Botany, 13, 133-276.
Kato, M. (1993). Biogeography of ferns: dispersal and vicariance. Journal of Biogeography, 20, 265-274.
Kelloff, C., Skog, J., Adamkewicz, L., and Werth, C. R. (2002). Differentiation of two taxa of eastern North American Athyrium: evidence from allozymes and spores. American Fern Journal, 92, 185-213.
Kluge, J. and Kessler, M. (2006). Fern endemism and its correlates: contribution from an elevational transect in Costa Rica. Diversity and Distributions, 12, 535-545.
Kurihara T., Watano, Y., Takamiya, M., and Shimizu, T. (1996). Electrophoretic and cytological evidence for genetic heterogeneity and hybrid origin of Athyrium oblitescens. Journal of Plant Research, 109, 29-36.
Lang, F. A. (1971). The Polypodium vulgare complex in the Pacific Northwest. Madrono, 21, 235-254.
Lieberman, D., Lieberman, M., Peralta, R., and Hartshorn, G. S. (1996). Tropical forest structure and composition on a large-scale altitudinal gradient in Costa Rica. The Journal of Ecology, 84, 137-152.
Lloyd, R. M. and Lang, F. A. (1964). The Polypodium vulgare complex in North America. British Fern Gazette, 9, 168-177.
Lovis, J. D. (1977). Evolutionary patterns and processes in ferns. In Advances in Botanical Research, Vol. 4, ed. R. D. Preston and H. W. Woolhouse. London: Academic Press, pp. 229-415.
Manton, I. (1950). Problems of Cytology and Evolution in the Pteridophyta. Cambridge: Cambridge University Press.
Marcon, A. B., Barros, I. C. L., and Guerra, M. (2005). Variation in chromosome numbers, CMA Bands and 45S rDNA sites in species of Selaginella (Pteridophyta). Annals of Botany, 95, 271-276.
Martens, P. (1943). Les organes gladuleux de Polypodium virginianum (P. vulgare var. virginianum). I. Valeur systematique et repartition géographique. Bulletin du Jardin Botanique de L'État, Bruxelles, 17, 1-14.
Masuyama, S. (1979). Reproductive biology of the fern Phegopteris decursive-pinnata. I. The dissimilar mating systems of diploids and tetraploids. Botanical Magazine (Tokyo), 92, 275-289.
Masuyama, S. and Watano, Y. (2005). Cryptic species in the fern Ceratopteris thalictroides (L.) Brongn. (Parkeriaceae). II. Cytological characteristics of three cryptic species. Acta Phytotaxonomica et Geobotanica, 56, 231-240.
Masuyama S., Yatabe, Y., Murakami, N., and Watano, Y. (2002). Cryptic species in the fern Ceratopteris thalictroides (L.) Brongn. (Parkeriaceae). I. Molecular analyses and crossing tests. Journal of Plant Research, 115, 87-97.
Mayden, R. L. (1997). A hierarchy of species concepts: the denouement in the saga of the species problem. In Species: The Units of Biodiversity, ed. M. F. Claridge, A. H. Dawah, and M. R. Wilson. London: Chapman and Hall, pp. 381-424.
Mayr, E. (1942). Systematics and the Origin of Species. New York: Columbia University Press.
Mayr, E. (1969). The biological meaning of species. Biological Journal of the Linnean Society, 1, 311-320.
Mishler, B. D. and Donoghue, M. J. (1982). Species concepts: a case for pluralism. Systematic Zoology, 31, 491-503.
Moran, R. C. and Smith, A. R. (2001). Phytogeographic relationships between neotropical and African-Madagascan pteridophytes. Brittonia, 53, 304-351.
Mullenniex, A., Hardig, T. M., and Mesler, M. R. (1998). Molecular confirmation of hybrid swarms in the fern genus Polystichum (Dryopteridaceae). Systematic Botany, 23, 421-426.
0llgaard, B. (1987). A revised classification of the Lycopodiaceae s. lat. Opera Botanica, 92, 153-178.
Orr, M. R. and Smith, T. B. (1998). Ecology and speciation. Trends in Ecology and Evolution, 13, 502-506.
Otto, S. P. and Whitton, J. (2000). Polyploid incidence and evolution. Annual Review of Genetics, 34, 401-437.
Paris, C. A. and Windham, M. D. (1988). A biosystematic investigation of the Adiantum pedatum complex in eastern North America. Systematic Botany, 13, 240-255.
Paris, C. A., Wagner, F. S., and Wagner, W. H. (1989). Cryptic species, species delimitation, and taxonomic practice in the homosporous ferns. American Fern Journal, 79, 46-54.
Parks, C. R., Wendel, J. F., Sewell, M. M., and Qiu, Y.-L. (1994). The significance of allozyme variation and introgression in the Liriodendron tulipifera complex (Magnoliaceae). American Journal of Botany, 81, 878-889.
Phipps, C. J., Taylor, T. N., Taylor, E. L., Cuneo, N. R., Boucher, L. D., and Yao, X. (1998). Osmunda (Osmundaceae) from the Triassic of Antarctica: an example of evolutionary stasis. American Journal of Botany, 85, 888-895.
Pryer, K. M. and Haufler, C. H. (1993). Isozymic and chromosomal evidence for the allotetraploid origin of Gymnocarpium dryopteris (Dryopteridaceae). Systematic Botany, 18, 150-172.
Rabe, E. W. and Haufler, C. H. (1992). Incipient polyploid speciation in the maidenhair fern (Adiantum pedatum; Adiantaceae). American Journal of Botany, 79, 701-707.
Ranker, T. A. (1992a). Genetic diversity of endemic Hawaiian epiphytic ferns: implications for conservation. Selbyana, 13, 131-137.
Ranker, T. A. (1992b). Genetic diversity, mating systems, and interpopulational gene flow in neotropical Hemionitis palmata L. (Adiantaceae). Heredity, 69, 175-183.
Ranker, T. A. (1994). Evolution of high genetic variability in the rare Hawaiian fern Adenophorus periens and implications for conservation management. Biological Conservation, 70, 19-24.
Ranker, T. A., Floyd, S. K., Windham, M. D., and Trapp, P. G. (1994). Historical biogeography of Asplenium adiantum-nigrum (Aspleniaceae) in North America and implications for speciation theory in homosporous pteridophytes. American Journal of Botany, 81, 776-781
Ranker, T. A., Gemmill, C. E. C., Trapp, P. G., Hambleton, A., and Ha, K. (1996).
Population genetics and reproductive biology of lava-flow colonising species of Hawaiian Sadleria (Blechnaceae). In Pteridology in Perspective, ed. J. M. Camus, M. Gibby, and R. J. Johns. Kew: Royal Botanic Gardens, pp. 581-598.
Raven, P. H. (1976). Systematics and plant population biology. Systematic Botany, 1, 284-316.
Richardson, B. A., Richardson, M. J., Scatena, F. N., and McDowell, W. H. (2000). Effects of nutrient availability and other elevational changes on bromeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico. Journal of Tropical Ecology, 16, 167-188.
Rieseberg, L. H. and Burke, J. M. (2001). The biological reality of species, gene flow, selection, and collective evolution. Taxon, 50, 47-67.
Sarvella, J. (1978). A synopsis of the fern genus Gymnocarpium. Annales Botanici Fennici, 15, 101-106.
Sarvella, J. (1980). Gymnocarpium hybrids from Canada and Alaska. Annales Botanici Fennici, 17, 292-295.
Schneider, H., Schuettpelz, E., Pryer, K. M., Cranfill, R., Magallon, S., and Lupia, R. (2004). Ferns diversified in the shadow of angiosperms. Nature, 428, 553-557.
Schneller, J. J. (1979). Biosystematic investigations on the lady fern (Athyrium filix-femina). Plant Systematics and Evolution, 132, 255-277.
Schneller, J. J. (1981), Evidence for intergeneric incompatibility in ferns. Plant Systematics and Evolution, 137, 45-56.
Schneller, J. J. (1989). Remarks on hereditary regulation of spore wall pattern in intra- and interspecific crosses of Athyrium. Botanical Journal of the Linnean Society, 99, 115-123.
Sciarretta, K. L., Arbuckle, E. P., Werth, C. R., and Haufler, C. H. (2005). Patterns of genetic variation in southern Appalachian populations of Athyrium filix-femina var. asplenioides (Dryopteridaceae). International Journal of Plant Science, 166, 761-780.
Shivas, M. G. (1961). Contributions to the cytology and taxonomy of Polypodium in Europe and America. I. Cytology. Botanical Journal of the Linnean Society, 58, 13-25.
Simpson, G. G. (1961). Principles of Animal Taxonomy. New York: Columbia University Press.
Sites, J. D. and Marshall, J. C. (2003). Delimiting species: a Renaissance issue in systematic biology. Trends in Ecology and Evolution, 18, 462-470.
Soltis, D. E. and Rieseberg, L. H. (1986). Autopolyploidy in Tolmiea menziezii
(Saxifragaceae): evidence from enzyme electrophoresis. American Journal of Botany, 73, 1171-1174.
Soltis, D. E. and Soltis, P. S. (1988). Estimated rates of intragametophytic selfing in lycopods. American Journal of Botany, 75, 248-256.
Soltis, D. E. and Soltis, P. S. (1989). Polyploidy, breeding systems, and genetic differentiation in homosporous pteridophytes. In Isozymes in Plant Biology, ed. D. E. Soltis and P. S. Soltis. Portland, OR: Dioscorides Press, pp. 241-258.
Soltis, P. S., Soltis, D. E. and Wolf, P. G. (1990). Allozymic divergence in North American Polystichum (Dryopteridaceae). Systematic Botany, 15, 205-215.
Soltis, D. E., Soltis, P. S., and Tate, J. A. (2003). Advances in the study of polyploidy since Plant Speciation. New Phytologist, 161, 173-191.
Somers, P. and Buck, W. R. (1975). Selaginella ludoviciana, S. apoda and their hybrids in the southeastern United States. American Fern Journal, 65, 76-82.
Taylor, W. C. and Hickey, R. J. (1992). Habitat, evolution, and speciation in Isöetes. Annals of the Missouri Botanical Garden, 79, 613-622.
Taylor, W. C., Luebke, N. T., and M. B. Smith. (1985). Speciation and hybridization in North American quillworts. Proceedings of the Royal Society of Edinburgh, 86B, 259-263.
Terborgh, J. (1985). The vertical component of plant species diversity in temperate and tropical forests. The American Naturalist, 126, 760-776.
Trewick, S. A., Morgan-Richards, M., Russell, S. J., Henderson, S., Rumsey, F. J., Pinter, J. A., Barrett, J. A., Gibby, M., and Vogel, J. C. (2002). Polyploidy, phylogeography and Pleistocene refugia of the rockfern Asplenium ceterach: evidence from chloroplast DNA. Molecular Ecology, 11, 2003-2012.
Tryon, R. M. (1955). Selaginella rupestris and its allies. Annals of the Missouri Botanical Garden, 42, 1-99.
Tryon, R. (1971). The process of evolutionary migration in species of Selaginella. Brittonia, 23, 89-100.
Tryon, A. F. and Britton, D. M. (1958). Cytotaxonomic studies on the fern genus Pellaea. Evolution, 12, 137-145
Tuomisto, H., Poulsen, A. D. O., and Moran, R. C. (1998). Edaphic distribution of some species of the fern genus Adiantum in western Amazonia. Biotropica, 30, 392-399.
Tuomisto, H., Ruokolainen, K., Poulsen, A. D., Moran, R. C., Quintana, C., Canas, G., and Celi, J. (2002). Distribution and diversity of pteridophytes and Melastomataceae along edaphic gradients in Yasuni National Park, Ecuadorian Amazonia. Biotropica, 34, 516-533.
Tuomisto, H., Ruokolainen, K., Aguilar, M., and Sarmiento, A. (2003). Floristic patterns along a 43-km long transect in an Amazonian rain forest. Journal of Ecology, 91, 743-756.
Vogel, J. C., Russell, S. J, Barrett, J. A., and Gibby, M. (1996). A non-coding region of chloroplast DNA as a tool to investigate reticulate evolution in European Asplenium. In Pteridology in Perspective, ed. J. M. Camus, M. Gibby, and R. J. Johns. Kew: Royal Botanic Gardens, pp. 313-327.
Wagner, W. H., Jr. (1954). Reticulate evolution in the Appalachian aspleniums. Evolution, 8, 103-118.
Wagner, D. H. (1979). Systematics of Polystichum in western North America, north of Mexico. Pteridologia, 1, 1-64.
Wagner, F. S. (1992). Cytological problems in Lycopodium sens. lat. Annals of the Missouri Botanical Garden, 79, 718-729.
Walker, S. (1955). Cytogenetic studies in the Dryopteris spinulosa complex I. Watsonia, 3, 193-209.
Walker, T. G. (1958). Hybridization in some species of Pteris L. Evolution, 12, 82-92.
Walker, S. (1961). Cytogenetic studies in the Dryopteris spinulosa complex. II. American Journal of Botany, 48, 607-614.
Walker, T. G. (1962). Cytology and evolution in the fern genus Pteris L. Evolution, 16, 27-43.
Walker, T. G. (1984). Chromosomes and evolution in pteridophytes. In Chromosomes in Evolution ofEukaryotic Groups, Vol. 2, ed. A. K. Sharma and A. Sharma. Boca Raton, FL: CRC Press.
Wallace, A. R. (1858). On the tendency of varieties to depart indefinitely from the original type. Journal of the Proceedings of the Linnean Society (Zoology), 3, 53-62.
Watkins, J. E., Cardelus, C., Colwell, R. K., and Moran, R. C. (2006). Species richness and distribution of ferns along an elevational gradient in Costa Rica. American Journal of Botany, 93, 73-83.
Werth, C. R. (1989). The use of isozyme data for inferring ancestry of polyploid species of pteridophytes. Biochemical Systematics and Ecology, 17, 117-130.
Werth, C. R. (1991). Isozyme studies on the Dryopteris "spinulosa" complex, I: the origin of the log fern Dryopteris celsa. Systematic Botany, 10, 184-192.
Werth, C. R. and Windham, M. D. (1991). A model for divergent, allopatric speciation of polyploid pteridophytes resulting from silencing of duplicate gene expression. American Naturalist, 137, 515-526.
Werth, C. R., Guttman, S. I. and Eshbaugh, W. H. (1985). Electrophoretic evidence of reticulate evolution in the Appalachian Asplenium complex. Systematic Botany, 16, 446-461.
Whittemore, A. T. (1993). Species concepts: a reply to Mayr. Taxon, 42, 573-583.
Wilce, J. H. (1965). Section Complanata of the genus Lycopodium. Nova Hedwigia, 19, 1-233.
Wiley, E. O. (1978). The evolutionary species concept reconsidered. Systematic Zoology, 27, 17-26.
Wiley, E. O. and Mayden, R. (2000). The evolutionary species concept. In Species Concepts and Phylogenetic Theory: A Debate, ed. Q.D. Wheeler and R. Meier. New York: Columbia University Press, pp. 70-89.
Windham, M. D. (1987). Argyrochosma, a new genus of cheilanthoid ferns. American Fern Journal, 77, 37-41.
Windham, M. D. and Yatskievych, G. (2003). Chromosome studies of cheilanthoid ferns (Pteridaceae: Cheilanthoideae) from the western United States and Mexico. American Journal of Botany, 90, 1788-1800.
Wolf, P. G., Schneider, H., and Ranker, T. A. (2001). Geographic distributions of homosporous ferns: does dispersal obscure evidence of vicariance? Journal of Biogeography, 28, 263-270.
Yatabe Y., Masuyama, S., Darnaedi, D., and Murakami, N. (2001). Molecular systematics of the Asplenium nidus complex from Mt. Halimun National Park, Indonesia: evidence for reproductive isolation among three sympatric rbcL sequence types. American Journal of Botany, 88, 1517-1522.
Yatskievych, G. and Moran, R. C. (1989). Primary divergence and species concepts in ferns. American Fern Journal, 79, 36-45.
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