When studying biodiversity, a fundamental question that emerges is, "Do species exist?" Why is the variety of life on earth subdivided into a set of discontinuous and distinct groups rather than existing as a seamless series of intergrading populations? Although this appears to be a central question for biologists to answer, prominent authorities consider it to be "one of the most intriguing unsolved problems of evolutionary biology" (Coyne and Orr, 2004). How do the clearly observable distinctions between the groups we label species arise, and what maintains separate ancestor-descendant lineages through time and space? According to some scientists (including Charles Darwin), species may be arbitrary human constructs erected for our convenience (see also Raven, 1976; Mishler and Donoghue, 1982). On the other hand, we can all detect and give names to non-overlapping distinctions among natural populations of organisms. Supporting this view of clearly discernable boundaries among natural groups,
Biology and Evolution of Ferns and Lycophytes, ed. Tom A. Ranker and Christopher H. Haufler. Published by Cambridge University Press. © Cambridge University Press 2008.
in studies comparing scientific classifications to the names that native peoples apply to animals and plants, a remarkably close correspondence was discovered between the numbers of species and their boundaries as recognized by both approaches (Berlin et al., 1966; Diamond, 1966). Further, as scientists explore the characteristics and origins of species, evidence continues to accumulate demonstrating that some clusters of populations represent coherent, cohesive, interactive biological units that deserve recognition, and that these units are isolated by a remarkable variety of mechanisms from other such clusters (Sites and Marshall, 2003). Thus, it is widely accepted that species actually do constitute real components of the biosphere (Rieseberg and Burke, 2001).
If we accept that species are fundamental components of biodiversity, what is the best way to characterize their basic features? What makes species different from other clusters of organisms such as varieties or populations? Beginning with Darwin (1859), scientists considered species from the perspective of history and formulated hypotheses about how species originate. These are two separate but linked components of a successful and accurate framing of the nature of species. Thus, in discussing what species are, I separate "concepts" from "definitions." A concept can be defined as "an abstract idea" whereas a definition is "a statement of the exact meaning of a word" and by separating these two similar but different perspectives on characterizing species, it may be possible to explore broad, fundamental elements of species as well as providing precision in applying evidence to studies of species and speciation.
If a concept is to capture important components of what species are, it should include historical as well as contemporary elements. Species are lineages of ancestor-descendant individuals that (1) originate, (2) develop to occupy unique morphological, geographical, and ecological space, (3) persist as a genetically cohesive set of populations, and ultimately (4) decline and become extinct. An inclusive species concept, therefore, should recognize and be broad enough to include these stages and dimensions (Mayden, 1997; De Queiroz, 1998). The concept that fits these best is the "evolutionary species" concept. As articulated by Wiley (1978; Wiley and Mayden, 2000), and adapted by him from Simpson (1961), "a species is a single lineage of ancestor descendant populations which maintains its identity from other such lineages and has its own evolutionary tendencies and historical fate." This concept accommodates both sexual and asexual lineages, recognizes the importance of history, and includes the many aspects of contemporary isolation from other species by referring to "maintaining its identity" from other species. If we knew enough about the organisms on earth to apply this grand concept to all of them, there would be little left for evolutionary biologists to accomplish. However, to delineate species accurately using this concept is a daunting task. We simply do not have the evidence necessary for all species to show a progression of ancestor-descendant organisms that constitute lineages. The fossil record provides benchmarks, but the many gaps in it must be inferred from a combination of comparative morphology, ecology, and biogeography of contemporary species as well as DNA sequencing studies. Even when applying increasingly sophisticated algorithms, the mass of information necessary to develop robust hypotheses of relationships is available for only a small percentage of species.
Building up to the mass of evidence necessary to characterize "evolutionary species," a series of "species definitions" can be considered as provisional hypotheses. "Morphological" species (also called "taxonomic" species) are those based on a consideration of observable structural differences between clusters of individuals. When explorers encounter new variants during a floristic survey, their initial descriptions of new species are based on morphological evidence. Careful diagnoses indicate how the new discovery may be differentiated from its closest relative based on a novel feature or combination of features. Shared physical characteristics are used to demonstrate common ancestry between the new discovery and presumably related taxa. Such morphological evidence constitutes a first hypothesis and opens the door to further analysis and experimentation. Most fern and lycophyte species conform to morphological species expectations and can be distinguished based on suites of unique traits.
In 1950, Irene Manton published a book that launched a revolution in the study of ferns and lycophytes. She and her students initiated the era of experimental investigations of species boundaries by incorporating studies of chromosome number, meiotic behavior, and artificial hybridization. These studies focused attention on cryptic but fundamental aspects of species limits and demonstrated that some polymorphic "species" were actually species complexes harboring diploid species, allopolyploid derivative species, and hybrid back-crosses. Based on morphological evidence alone, the combination of interactions among the entities in such complexes resulted in an apparently continuous intergradation of morphological features without the discontinuities necessary to circumscribe discrete units. However, when the allopolyploids and hybrid backcrosses are removed, the diploids emerge as reasonably well demarcated independent lineages. As reviewed by Lovis (1977), after Manton's innovative introduction, scientists worldwide began incorporating chromosomal techniques in revisionary studies of ferns and lycophytes. The following are just some of the important contributions that built on Manton's insightful foundation (see Yatskievych and Moran, 1989 for a more inclusive list): Asplenium
(Wagner, 1954); Athyrium (Schneller, 1979); Cystopteris (Blasdell, 1963); Dryopteris (Walker, 1955, 1961), Gymnocarpium (Sarvella, 1978, 1980); Isoetes (Hickey, 1984; Taylor et al., 1985); Lycopodium (Wilce, 1965; Bruce, 1975); Pellaea (Tryon and Britton, 1958); Polystichum (Wagner, 1979). The perspectives gained through this work brought studies of species biology among ferns and lycophytes to the "biological" species definition era of understanding. Ernst Mayr developed this definition (1942,1969) as a way to capture the important elements that provide meaningful criteria and an experimental basis for studying species limits. Centered on reproductive biology, Mayr's definition considers species as "groups of interbreeding natural populations that are reproductively isolated from other such groups" (Mayr, 1969). Considering species boundaries to be controlled by processes that take place within and between populations, and involving active participation by individual members of the species, the pros and cons of Mayr's definition have been debated ever since he proposed it. Although widely embraced by zoologists (e.g., Coyne and Orr, 2004), botanists have had more difficulty considering this as a universal definition of species (e.g., Whittemore, 1993). Further, because the biological species definition focuses on contemporary interactions between extant populations, it is difficult to apply when considering species as lineages with histories. It is also not possible to apply this definition to clonal species or asexual species. Nonetheless, by raising the importance of reproductive behavior and by considering species as interactive, cohesive sets of populations, and when combined with Manton's emphasis on genetic aspects of fern and lyco-phyte species, a link was forged between defining species and a mechanism of speciation. Species are perpetuated as integrated units when their individuals maintain gene flow by sexual reproductive interaction; speciation can occur when gene flow is interrupted.
Building on the foundation laid by Manton and others, fern and lycophyte species were studied in the 1980s and 1990s using enzyme electrophoresis to analyze isozyme variants (Haufler, 1987, 1996, 2002). By developing unique molecular profiles for each lineage, these studies revealed details that were unavailable through earlier approaches. Even in some of the most convoluted species complexes, isozymes were able to clarify the boundaries of species. Isozymes also helped to look within species and provide details of population structure and breeding behavior (see Chapter 4) and, as discussed below, yield insights for understanding aspects of speciation.
By linking the approaches available, it had become possible to delineate species boundaries quite accurately, to separate diploid and polyploid lineages, and, in many cases, to define the details of reticulate patterns of species origins and the pedigrees of polyploid species (reviewed in Barrington et al., 1989; Werth, 1989). Still lacking was clarity concerning the history of each species and their trajectories of ancestor-descendant relationships. Although the databases discussed above merge to build reliable hypotheses for delimiting contemporary species, none of the available lines of evidence yields a firm historical perspective. It is difficult to extract a clear phylogenetic signal from morphology because ecological influences can result in convergent similarities; changes in chromosome complement are quite dynamic and difficult to interpret; isozyme variants cannot be tracked accurately over long periods of time. Developing through the 1990s and continuing into the twenty-first century, the application of DNA sequencing techniques has allowed researchers to chart the genealogy of species, to coordinate fossil benchmarks with molecular clocks, and to formulate evolutionary species definitions for some ferns and lycophytes. We now have species that are firmly rooted in a solid phylogenetic foundation and we are beginning to have an accurate appreciation for the influence that geography, ecology, and breeding system dynamics have on species. By combining fossil and molecular data, we now know that some fern species are remarkably old (e.g., Osmunda species date to the Triassic, about 225 mybp (Phipps et al., 1998)) and others are surprisingly young (e.g., epiphytic species in Polypodium diversified along with and in response to angiosperms (Schneider et al., 2004)). Such data truly do yield species that conform to evolutionary species definitions. They are well circumscribed and embedded in an extensively documented phylogenetic framework. In many cases we are able to assert confidently that our species hypotheses delineate the boundaries of contemporary species and place the lineage into historical context accurately. As we develop this level of confidence with more and more groups, we will build classifications that reflect phylogenies and that can be used to predict trends and mechanisms of biogeography and speciation.
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