14. Role of Genetics in Conservation Biology
231
past, then new pathogens may pose a significant threat to the future survival of
populations with low MHC variability. Host-parasite coevolution as a selection
mechanism has been supported by studies that have documented a relationship
between MHC haplotypes and disease resistance such as Marek’s disease and
fowl cholera in chicken and malaria in humans (Briles et al. 1977; Lamont et al.
1987; O’Brien and Everman 1988; Hill et al. 1991). Slade (1992) attributed low
MHC variability in Southern Elephant Seals to their evolution in a pathogen-free
marine environment. Reduced genetic diversity at MHC loci has also been linked
to disease susceptibility in Cheetahs (O’Brien et al. 1985).
This evidence and the apparent positive selection for a balanced MHC polymorphism led Hughes (1991) to believe that MHC diversity is paramount to the
survival of small endangered populations. He then made the controversial argument that “all captive breeding programs for endangered vertebrate species should
be designed with the preservation of MHC allelic diversity as their goal” (Hughes
1991). However, Vrijenhoek and Leberg (1991) cautioned that Hughes’ (1991)
management recommendations are based on assumptions that need experimental
evaluation before their general validity is established. Selective breeding for
maintenance of MHC diversity could, for example, increase loss of whole genomic diversity and hence bear the risk of inbreeding depression (Haig et al. 1990;
Falconer and Mackay 1996). In addition, disease protection is not only a consequence of the adaptive immune response of organisms and hence the diversity of
MHC molecules but involves numerous other genes associated with the humoral
immune system (Janeway and Travers 1994). Vrijenhoek and Leberg (1991)
recommended preservation of genetic diversity at the level of the whole genome
instead of selected loci while intensifying efforts to monitor and understand the
relationship between disease susceptibility and MHC variability.
Critics point out that several populations with low MHC variability are viable
and healthy, such as northern European Beaver populations (Ellegren et al. 1993).
However, the apparent well-being of a population with low genetic variability at
either neutral or selected loci may be (1) the result of selection in the past and
survivors represent the fittest genotypes; (2) because low variability is the result
of drift or inbreeding in combination with selection, and although the population
is healthy now, it lacks the potential for future adaptations; or (3) because there is
actually no correlation between fitness and genetic variability. Similarly, a positive correlation between low fitness and low genetic diversity does not necessarily
mean a causal relationship exists. Instead low reproductive success may be explained by nongenetic factors, such as predation, low densities, or disrupted
behavior.
Tools to Assess Genetic Diversity and
Phylogenetic Uniqueness
Molecular tools can contribute to conservation biology by revealing the genetic
structure of the population, evolutionary history, and evolutionary potential of
231
past, then new pathogens may pose a significant threat to the future survival of
populations with low MHC variability. Host-parasite coevolution as a selection
mechanism has been supported by studies that have documented a relationship
between MHC haplotypes and disease resistance such as Marek’s disease and
fowl cholera in chicken and malaria in humans (Briles et al. 1977; Lamont et al.
1987; O’Brien and Everman 1988; Hill et al. 1991). Slade (1992) attributed low
MHC variability in Southern Elephant Seals to their evolution in a pathogen-free
marine environment. Reduced genetic diversity at MHC loci has also been linked
to disease susceptibility in Cheetahs (O’Brien et al. 1985).
This evidence and the apparent positive selection for a balanced MHC polymorphism led Hughes (1991) to believe that MHC diversity is paramount to the
survival of small endangered populations. He then made the controversial argument that “all captive breeding programs for endangered vertebrate species should
be designed with the preservation of MHC allelic diversity as their goal” (Hughes
1991). However, Vrijenhoek and Leberg (1991) cautioned that Hughes’ (1991)
management recommendations are based on assumptions that need experimental
evaluation before their general validity is established. Selective breeding for
maintenance of MHC diversity could, for example, increase loss of whole genomic diversity and hence bear the risk of inbreeding depression (Haig et al. 1990;
Falconer and Mackay 1996). In addition, disease protection is not only a consequence of the adaptive immune response of organisms and hence the diversity of
MHC molecules but involves numerous other genes associated with the humoral
immune system (Janeway and Travers 1994). Vrijenhoek and Leberg (1991)
recommended preservation of genetic diversity at the level of the whole genome
instead of selected loci while intensifying efforts to monitor and understand the
relationship between disease susceptibility and MHC variability.
Critics point out that several populations with low MHC variability are viable
and healthy, such as northern European Beaver populations (Ellegren et al. 1993).
However, the apparent well-being of a population with low genetic variability at
either neutral or selected loci may be (1) the result of selection in the past and
survivors represent the fittest genotypes; (2) because low variability is the result
of drift or inbreeding in combination with selection, and although the population
is healthy now, it lacks the potential for future adaptations; or (3) because there is
actually no correlation between fitness and genetic variability. Similarly, a positive correlation between low fitness and low genetic diversity does not necessarily
mean a causal relationship exists. Instead low reproductive success may be explained by nongenetic factors, such as predation, low densities, or disrupted
behavior.
Tools to Assess Genetic Diversity and
Phylogenetic Uniqueness
Molecular tools can contribute to conservation biology by revealing the genetic
structure of the population, evolutionary history, and evolutionary potential of