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Sabine S. Loew
taxa (Ashley 1999). This information can serve to detect past, present, and possibly future population declines, besides establishing evolutionary uniqueness. Natural populations of threatened species are managed to conserve genetic diversity
to enhance individual fitness and maintain the evolutionary potential for future
adaptations. Such genetic management generally requires information on demography (e.g., migration rates, population size). The amount and distribution of
genetic variation within and between populations can provide relatively quick
indirect estimates of migration, population subdivisions, and isolation, which can
serve to define the appropriate scale for short- and long-term management (Moritz
1994b). Furthermore, characterization of the genetic make-up (i.e., the pedigree)
of captive individuals can help to determine appropriate breeders to maximize
offspring survival through inbreeding avoidance and genetic compatibility (Ryder
1986; Garner and Ryder 1992).
The proliferation of genetic markers in recent years has facilitated reliable
estimates of some forms of genetic diversity and phylogenetic analysis at several
taxonomic levels. The genetic variation detected by these molecular markers
differs quantitatively and qualitatively as a result of the kind and number of
genomic sites they assay. Protein electrophoresis uncovers putative genetic variation associated with protein coding regions, whereas DNA-DNA hybridization,
restriction fragment length polymorphism (RFLP), and DNA sequencing analysis
reveal differences at the DNA level for coding as well as noncoding regions. DNA
sequencing data generally provide genetic distance estimates among taxa based
on single locus comparisons whereas the other methods reveal variation at several
loci. Resolution power of any molecular marker depends on the number of independent linkage groups (sites) it assays and the evolutionary rate of change
associated with those groups. Animal and plant mitochondrial DNA (mtDNA),
for example, usually is maternally inherited as one nonrecombining linkage
group. Although the rate of sequence evolution varies along the mtDNA molecule, it is generally higher than the rate of single-copy nuclear DNA (scnDNA)
(Moritz et al. 1987). The intramolecular variability of the rate of evolution makes
mtDNA ideally suited to resolve taxonomic differences at various levels of
divergence. In addition maternal inheritance uniquely qualifies mitochondrial
markers for tracing of maternal genealogies. Lack of recombination, however,
renders all mitochondrial genes part of one linkage group that are subjected to the
same stochastic (e.g., random lineage extinction) and deterministic events (e.g.,
selective sweeps). Therefore they do not provide independent estimates of evolutionary change.
Choice of the appropriate marker for a particular task has to be based on the
resolution power necessary to determine genetic differences and similarities
among the individuals, populations, or other taxonomic units of interest. In addition, quality and quantity of the DNA source as well as to the required sample size
have to be taken into consideration as these factors affect the applicability and
cost-efficiency of a particular molecular tool. In general, rapidly evolving DNA
(e.g., mini- and microsatellite DNA) generates genetic differences among individuals and populations that provide information on individual identity, paternity
Sabine S. Loew
taxa (Ashley 1999). This information can serve to detect past, present, and possibly future population declines, besides establishing evolutionary uniqueness. Natural populations of threatened species are managed to conserve genetic diversity
to enhance individual fitness and maintain the evolutionary potential for future
adaptations. Such genetic management generally requires information on demography (e.g., migration rates, population size). The amount and distribution of
genetic variation within and between populations can provide relatively quick
indirect estimates of migration, population subdivisions, and isolation, which can
serve to define the appropriate scale for short- and long-term management (Moritz
1994b). Furthermore, characterization of the genetic make-up (i.e., the pedigree)
of captive individuals can help to determine appropriate breeders to maximize
offspring survival through inbreeding avoidance and genetic compatibility (Ryder
1986; Garner and Ryder 1992).
The proliferation of genetic markers in recent years has facilitated reliable
estimates of some forms of genetic diversity and phylogenetic analysis at several
taxonomic levels. The genetic variation detected by these molecular markers
differs quantitatively and qualitatively as a result of the kind and number of
genomic sites they assay. Protein electrophoresis uncovers putative genetic variation associated with protein coding regions, whereas DNA-DNA hybridization,
restriction fragment length polymorphism (RFLP), and DNA sequencing analysis
reveal differences at the DNA level for coding as well as noncoding regions. DNA
sequencing data generally provide genetic distance estimates among taxa based
on single locus comparisons whereas the other methods reveal variation at several
loci. Resolution power of any molecular marker depends on the number of independent linkage groups (sites) it assays and the evolutionary rate of change
associated with those groups. Animal and plant mitochondrial DNA (mtDNA),
for example, usually is maternally inherited as one nonrecombining linkage
group. Although the rate of sequence evolution varies along the mtDNA molecule, it is generally higher than the rate of single-copy nuclear DNA (scnDNA)
(Moritz et al. 1987). The intramolecular variability of the rate of evolution makes
mtDNA ideally suited to resolve taxonomic differences at various levels of
divergence. In addition maternal inheritance uniquely qualifies mitochondrial
markers for tracing of maternal genealogies. Lack of recombination, however,
renders all mitochondrial genes part of one linkage group that are subjected to the
same stochastic (e.g., random lineage extinction) and deterministic events (e.g.,
selective sweeps). Therefore they do not provide independent estimates of evolutionary change.
Choice of the appropriate marker for a particular task has to be based on the
resolution power necessary to determine genetic differences and similarities
among the individuals, populations, or other taxonomic units of interest. In addition, quality and quantity of the DNA source as well as to the required sample size
have to be taken into consideration as these factors affect the applicability and
cost-efficiency of a particular molecular tool. In general, rapidly evolving DNA
(e.g., mini- and microsatellite DNA) generates genetic differences among individuals and populations that provide information on individual identity, paternity