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Sabine S. Loew
present genetic structure and take the population biology and breeding history of
individuals into consideration wherever possible (Vrijenhoek et al. 1985; Vrijenhoek 1994; Ballou et al. 1995; Avise and Hamrick 1996).
Who Is to Embark on the Ark?
To maintain biodiversity and to identify those taxonomic units worthy of our
protection, we need clearly defined criteria to determine conservation units. Since
the 1973 Endangered Species Act mandated the protection of species, determination of species status has necessarily become of great importance to conservation
management (U.S. House of Representatives 1973).
The species concept, however, has been at the center of an ongoing debate
among evolutionary biologists, and numerous criteria, such as reproductive isolation and ancestral relationships, have been applied to define a species (O’Brien
and Mayr 1991; Crozier 1992; Geist 1992; Rojas 1992). For example, Mayr’s
(1963, 1969) biological species concept defines species as freely interbreeding
populations that are reproductively isolated. Criteria of interbreeding ability, however, are of limited use for clarifying taxonomic relationships of discontinuous
populations because reproductive barriers among allopatric populations are difficult to prove (Cracraft 1983; McKitrick and Zink 1988). Similarly, distinguishing
species on the basis of morphology suffers, in part, from the fact that many
morphological traits are considerably affected by environmental conditions (Geist
1987). Therefore phenotypic differences between populations might reflect temporary local adaptations rather than independent evolutionary histories.
By contrast, phylogenetic analysis based on neutral genetic markers can contribute additional measures of the genetic distinctiveness of taxa and may be more
reflective of their evolutionary history (Avise et al. 1987; Hillis 1987; Dizon et al.
1992; Moritz 1994b; Wayne et al. 1994; Avise and Hamrick 1996; but see Cronin
1993). In general, a variety of genetic markers in combination with morphometric
analysis is preferable to establish the amount of reproductive isolation and phylogenetic uniqueness of a particular taxonomic group. Accordingly, Avise and
Ball (1990) suggest that a suite of phylogenetically concordant characteristics
should be used to define a taxonomic group as a population of individuals that can
be united by one or more derived traits.
Rojas (1992) has pointed out that conservation based on the above typological
approaches to species is problematic for several reasons. If nature reserves are
designed to ensure survival of a representative sample of “types” or species,
unresolved species status will have a major impact on the number of preserved
species and ultimately the level of biodiversity. She points out that “the numbers,
however, are unlikely to be the same if we are considering biological species,
cladistic species, or evolutionary species” (Rojas 1992). In addition, she notes that
conserving species as types ignores the importance of preserving geographic
variation within species.
Sabine S. Loew
present genetic structure and take the population biology and breeding history of
individuals into consideration wherever possible (Vrijenhoek et al. 1985; Vrijenhoek 1994; Ballou et al. 1995; Avise and Hamrick 1996).
Who Is to Embark on the Ark?
To maintain biodiversity and to identify those taxonomic units worthy of our
protection, we need clearly defined criteria to determine conservation units. Since
the 1973 Endangered Species Act mandated the protection of species, determination of species status has necessarily become of great importance to conservation
management (U.S. House of Representatives 1973).
The species concept, however, has been at the center of an ongoing debate
among evolutionary biologists, and numerous criteria, such as reproductive isolation and ancestral relationships, have been applied to define a species (O’Brien
and Mayr 1991; Crozier 1992; Geist 1992; Rojas 1992). For example, Mayr’s
(1963, 1969) biological species concept defines species as freely interbreeding
populations that are reproductively isolated. Criteria of interbreeding ability, however, are of limited use for clarifying taxonomic relationships of discontinuous
populations because reproductive barriers among allopatric populations are difficult to prove (Cracraft 1983; McKitrick and Zink 1988). Similarly, distinguishing
species on the basis of morphology suffers, in part, from the fact that many
morphological traits are considerably affected by environmental conditions (Geist
1987). Therefore phenotypic differences between populations might reflect temporary local adaptations rather than independent evolutionary histories.
By contrast, phylogenetic analysis based on neutral genetic markers can contribute additional measures of the genetic distinctiveness of taxa and may be more
reflective of their evolutionary history (Avise et al. 1987; Hillis 1987; Dizon et al.
1992; Moritz 1994b; Wayne et al. 1994; Avise and Hamrick 1996; but see Cronin
1993). In general, a variety of genetic markers in combination with morphometric
analysis is preferable to establish the amount of reproductive isolation and phylogenetic uniqueness of a particular taxonomic group. Accordingly, Avise and
Ball (1990) suggest that a suite of phylogenetically concordant characteristics
should be used to define a taxonomic group as a population of individuals that can
be united by one or more derived traits.
Rojas (1992) has pointed out that conservation based on the above typological
approaches to species is problematic for several reasons. If nature reserves are
designed to ensure survival of a representative sample of “types” or species,
unresolved species status will have a major impact on the number of preserved
species and ultimately the level of biodiversity. She points out that “the numbers,
however, are unlikely to be the same if we are considering biological species,
cladistic species, or evolutionary species” (Rojas 1992). In addition, she notes that
conserving species as types ignores the importance of preserving geographic
variation within species.