Species summary

As in most groups, before considering ancestries of species, before developing hypotheses about species origins, and before exploring evolutionary mechanisms, it is vital to discover accurate boundaries of extant representatives (Sites and Marshall, 2003). If working with poorly circumscribed evolutionary units, it is impossible to reconstruct their evolutionary history, especially when species boundaries are drawn too broadly and when the history of lineages includes episodes of hybridization and allopolyploidy. Perhaps because all of these complications are such prominent elements of the biology of fern and lycophyte species, it is especially important to delimit species accurately. It is becoming clear that the assumptions made about broad species boundaries in ferns and lycophytes should not be followed. Far from being more simple and straightforward than species of seed plants, ferns and lycophytes are constrained by most of the same forces acting on other groups, and, especially given the demonstrated complexities of hybridization and allopolyploidy, fern and lycophyte species should be viewed as dynamic, interactive, and often problematic. Continued emphasis at the species level will only enhance studies of the forces that result in the origin and diversification of ferns and lycophytes.

12.3 Speciation

The origin of species has consistently fascinated and perplexed scientists. Originally considered to be divine creations, Wallace (1858) and Darwin (1859) provided the first scientifically testable hypotheses for mechanisms that could result in new lineages. Since that time, considerable attention has been paid to exploring how natural forces can launch populations of organisms carrying novel combinations of features that become established as distinct from their relatives (Orr and Smith, 1998; Coyne and Orr, 2004). Considerable contemporary research on speciation is focused at the level of genes and on discovering what is involved in isolating unique genetic combinations that are perpetuated as cohesive ancestor-descendant populations. By considering the sequence of bases in the DNA of organisms, it is possible to formulate hypotheses about the ancestors of current species and their closest contemporary relatives, and to consider what genes or gene combinations might have contributed to the isolation of new groups.

Current speciation research is built on a foundation of information and hypotheses concerning the modes and mechanisms of species origins. In most discussions, speciation is a two-stage process. The first stage involves some type of isolating event that separates a prospective new lineage from its progenitor. Central questions then focus on determining as precisely as possible the processes that result in such isolation, such as separation by distance, by habitat modification, or by genetic changes that prevent interbreeding (Hey, 2006). It sometimes seems as though there are nearly as many potential mechanisms as there are new species to consider. Following isolation is usually the development of distinctive features, both molecularly and morphologically. Because the discovery of new species usually requires the availability and recognition of distinctive characteristics, by the time scientists describe a new lineage, the actual point of origin from its ancestor is long past. The challenge, therefore, is to develop hypotheses that (1) incorporate observable evidence and natural situations, (2) are based on experimental data, and (3) explain patterns of diversification.

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