240
Sabine S. Loew
is four times smaller than that of nuclear markers. Consequently, small populations may show genetic diversity at nuclear DNA loci but have no mtDNA
variation (Avise 1994, 1995). Consequently, ambiguities in phylogenetic relationships and the genetic structure of natural populations are best resolved by using a
combination of nuclear and mtDNA markers (Moritz 1994b; Avise 1995). The
value of this approach has been demonstrated in studies revealing hybridization in
the history of the endangered Red Wolf (Wayne and Jenks 1991; Roy et al. 1994)
and the Florida Panther (O’Brien et al. 1990), as well as in determining the
phylogeographic history and infraspecific taxonomy of Leopards (Miththapala et
al. 1995).
“Ups and Downs” of Genetic Diversity
Generating Diversity
The amount of genetic diversity present at any point in time in an individual or
population is the result of opposing forces that have affected allele frequencies in
the past. For example, mutation events, recombination, and immigration are important evolutionary forces that introduce additional alleles and polymorphic sites
into a population, whereas selection and inbreeding, as well as random genetic
drift, generally homogenize gene pools (for further details, see Hartl and Clark
1989; Loeschcke et al. 1994).
Only mutations can generate novel alleles in entirely monomorphic populations. However, mutations occur rarely and are often deleterious or neutral and
therefore cannot be relied on as a major source of genetic variation for short-term
genetic management. Immigration offers mixing of gene pools and rapid infusion
of new genes into genetically homogeneous populations. The fragmentation of
natural habitats, however, decreases natural rates of migration and dispersal and
results in population subdivision and eventual isolation.
Conservation biologists can increase gene flow by either encouraging successful dispersal among subpopulations through dispersal corridors or by translocating new genetically distinct breeders to isolated populations. In addition, wild
populations can be supplemented with captive-bred individuals through reintroduction programs (for further discussions, see Gibbs 1991; Olney et al.
1994). Numerous threatened or endangered populations have benefited from these
management strategies, and consequently, dispersal corridors (e.g., natural areas
associated with rivers) and translocation have become an integral part of reserve
design (Beier 1993; Dunning et al. 1995; Madsen et al. 1996; Beier and Noss
1998).
Nevertheless, there is a cost to helping gene flow along through human-made
corridors or translocations (Simberloff and Cox 1987). Although such population
management tools allow the influx of new genetic material into isolated populations, they might also create new problems, such as facilitation of the spread of
Sabine S. Loew
is four times smaller than that of nuclear markers. Consequently, small populations may show genetic diversity at nuclear DNA loci but have no mtDNA
variation (Avise 1994, 1995). Consequently, ambiguities in phylogenetic relationships and the genetic structure of natural populations are best resolved by using a
combination of nuclear and mtDNA markers (Moritz 1994b; Avise 1995). The
value of this approach has been demonstrated in studies revealing hybridization in
the history of the endangered Red Wolf (Wayne and Jenks 1991; Roy et al. 1994)
and the Florida Panther (O’Brien et al. 1990), as well as in determining the
phylogeographic history and infraspecific taxonomy of Leopards (Miththapala et
al. 1995).
“Ups and Downs” of Genetic Diversity
Generating Diversity
The amount of genetic diversity present at any point in time in an individual or
population is the result of opposing forces that have affected allele frequencies in
the past. For example, mutation events, recombination, and immigration are important evolutionary forces that introduce additional alleles and polymorphic sites
into a population, whereas selection and inbreeding, as well as random genetic
drift, generally homogenize gene pools (for further details, see Hartl and Clark
1989; Loeschcke et al. 1994).
Only mutations can generate novel alleles in entirely monomorphic populations. However, mutations occur rarely and are often deleterious or neutral and
therefore cannot be relied on as a major source of genetic variation for short-term
genetic management. Immigration offers mixing of gene pools and rapid infusion
of new genes into genetically homogeneous populations. The fragmentation of
natural habitats, however, decreases natural rates of migration and dispersal and
results in population subdivision and eventual isolation.
Conservation biologists can increase gene flow by either encouraging successful dispersal among subpopulations through dispersal corridors or by translocating new genetically distinct breeders to isolated populations. In addition, wild
populations can be supplemented with captive-bred individuals through reintroduction programs (for further discussions, see Gibbs 1991; Olney et al.
1994). Numerous threatened or endangered populations have benefited from these
management strategies, and consequently, dispersal corridors (e.g., natural areas
associated with rivers) and translocation have become an integral part of reserve
design (Beier 1993; Dunning et al. 1995; Madsen et al. 1996; Beier and Noss
1998).
Nevertheless, there is a cost to helping gene flow along through human-made
corridors or translocations (Simberloff and Cox 1987). Although such population
management tools allow the influx of new genetic material into isolated populations, they might also create new problems, such as facilitation of the spread of