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vation unit (CU). In other words, we need to define what we wish to conserve before we can take measures to conserve it. Although this may seem trivial, the evolutionary process is often murky enough to lead to great difficulty in defining CUs, particularly when we are dealing with closely related species that have been thrown together recently by human-induced changes in the environment or those which were never isolated fully from one another reproductively (that is, hybridization has occurred). In these cases, careful examination of the biological characteristics of an organism that are most likely to carry evolutionary historical information (a field known as biological systematics)—such as features of anatomy, behavior, and genetics—often yields the clues necessary to place a series of populations and species on a synthetic family tree (also known as a phylogenetic tree). These clues even can be used to determine whether a group of organisms can be defined as a single evolutionarily significant unit (ESU), which may be but is not necessarily synonymous with what we usually refer to as a species.

Molecular-generic approaches to biological systematics have emerged as one of the most exciting new areas of biological research (Hillis and others 1996). A wide array of technical and analytical methods has been used to address issues of evolution and conservation at all levels of organization, ranging from genes within populations through the process of speciation to the reconstruction of the tree of life itself (Avise 1994). Of particular importance here are the contributions of molecular techniques to the identification and phylogenetic placement of rare and endangered species. Knowledge about diversity at the molecular level can be used to reconstruct the evolutionary history of an endangered organism (Avise and Hamrick 1996) and to identify the ESUs on which to focus our attention for conservation (Moritz 1994, 1995). Because much of conservation planning depends on taxonomic or species assignments (Taberlet 1996), identifying systematically based CUs aids considerably in developing management plans and in evaluating priorities for conservation (Smith and Wayne 1996).

Case Studies

Molecular-systematic studies can help clarify taxonomic issues at three different levels. First, we can identify cases of “oversplitting”, that is, when distinct morphological forms are considered different evolutionary entities but are in fact genetically indistinguishable. This implies that gene flow still may be occurring between the different forms and that they therefore should not be considered evolutionarily distinct. For example, the now-extinct dusky seaside sparrow (Ammodramus maritimus nigrescens) of Florida, originally described as a distinct species, was redefined later as a subspecies when it was shown to be genetically indistinguishable from other populations of seaside sparrows (Avise and Nelson 1989). Moreover, it was shown that all populations of seaside sparrows on the Atlantic coast (including the dusky seaside sparrow) are genetically more similar to each other than they are to populations of seaside sparrows that are found along the Gulf of Mexico. Clearly, molecular data supported the inclusion of the dusky seaside sparrow in the seaside-sparrow species and suggested that its loss, although regrettable, had little or no effect on the long-term evolutionary course of the entire species.