Biological Systematics: Principles and Applications by Randall T. Schuh & Andrew V. Z. Brower

Author:Randall T. Schuh & Andrew V. Z. Brower [Schuh, Randall T. & Brower, Andrew V. Z.]
Language: eng
Format: epub
Tags: Science, Life Sciences, Biology, Zoology, General, Botany
ISBN: 9781501717017
Google: an8yDwAAQBAJ
Publisher: Cornell
Published: 2011-04-15T13:19:53+00:00

Figure 7.1. A cladogram of four species (and an outgroup). The stars represent synapomorphies or autapomorphies for various clades or terminal taxa, respectively. Note that species C is diagnosed by the presence of synapomorphy 5 and the absence of autapomorphy 6—“a unique combination of character states.”

The practical aspects of studying the natural world at times confound the seemingly simple issue of connecting different life stages and reproductive communities together. For example, in certain groups of wasps, virtually independent classifications exist for the males and the females because current knowledge does not allow for unequivocal association of the two sexes of the same species. Species discrimination among clonal organisms or suspected hybrids can also be problematic. In fungal systematics, there are formally separate nomenclatures for anamorph (asexual) and teleomorph (sexual reproductive) life stages, reflecting the long-standing historical incapacity to associate asexual vegetative hyphae with their sexual counterparts, “mushrooms” (ascocarps and basidiocarps). Indeed, the solutions to species delimitation adopted by specialists in one group of organisms may be quite different from those applied in another. In some cases the problem will be evident and therefore subject to resolution before detailed systematic studies commence; in other cases solutions to such problems will be clarified only through careful systematic study.

According to the Nixon and Wheeler, PSC species can be recognized by some unique combination of character states, although many may also possess derived attributes unique to themselves. At the level of more inclusive taxa, such as genera or families, groups cannot be diagnosed by this method because any and all possible combinations would thereby become diagnosable. This would produce arbitrary classifications, potentially including paraphyletic taxa. Instead, monophyletic groups of two or more species are recognized and diagnosed on the basis of shared possession of derived character states—synapomorphies—that do not also occur in other groups (but see discussion of homoplasy in Chapter 4).

Across the community of systematists and other kinds of biologists who may adhere to a diversity of species concepts and criteria, the circumscription of species—deciding what breadth of morphological, geographical, or temporal variability falls within a species boundary (sometimes referred to in the taxonomic literature as “splitting” versus “lumping”)—is not always clear-cut and may take on a subjective quality (see Sidebar 16). For example, early in the twentieth century, ornithologists recognized some 20,000 species of birds, whereas toward its end, they recognized about 9000 species (Haffer 1997). Presumably, birds have neither speciated nor despeciated during this short period of evolutionary time. What changed, instead, was the “concept” of the minimal diagnosable unit, the criterion by which species were delimited—and the consequent proliferation of subspecies. Particularly owing to the influence of Mayr and his preference for the BSC, with its criterion of interbreeding, many geographically differentiated populations of birds formerly considered specifically distinct, such as the Baltimore and Bullock’s Orioles (Icterus galbula and I. bullockii), were consolidated into single species based on the existence of geographical hybrid zones between them, where some interbreeding takes place and individuals with characters of both parental forms may be encountered.

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