14. Role of Genetics in Conservation Biology
245
Such infraspecific variation, however, is potentially crucial for the long-term
survival of a species. Conserving species as evolutionary units addresses the
importance of variation within species to preserve the evolutionary potential of
the protected organisms and ultimately minimize the extinction probability across
their range (Vrijenhoek 1989; Rojas 1992). Although reserve management based
on evolutionary units is less subject to “bean counting” of rare types, clear
delimiting of taxonomic groups is still essential to avoid inadvertent hybridization
and outbreeding depression.
Amendments to the Endangered Species Act (ESA) extended protection of
species to include distinct population segments that interbreed when mature. This
broader definition of species protection allowed flexibility to preserve species in
only part of their range without resolving their taxonomic status (Pennock and
Dimmick 1997). As ESA provided no clear definition for “distinct population
segments,” criteria for identifying fragile populations were highly varied, including uniqueness based on geographic isolation and morphological and genetic
differences (Pennock and Dimmick 1997).
The concept of evolutionary significant units (ESU) was developed, in part, to
provide biological guidelines for determining conservation units below the species level (Ryder 1986; Vane-Wright et al. 1991; Waples 1991). This attempt to
apply the concept of ESUs to identify “distinct population segments” for conservation has resulted in ongoing debates that focus on the biological merits (Dizon
et al. 1992; Moritz 1994a; Vogler and DeSalle 1994; Waples 1998) and the
fulfillment of the ESA mission (Rohlf 1994; Pennock and Dimmick 1997; Waples
1998). Rojas’s (1992) conclusion that “considering the species problem more
critically may result in recognizing the limitations of the taxonomic information
used; it may also contribute to the refinement of the concepts and methods” still
holds, and we continue to benefit from the interaction between systematists and
conservation biologists to determine conservation units.
Conclusions
Which species are most at risk of extinction may depend on many interactive
aspects of their biology, such as their ecology (e.g., specialists), mating or migratory behavior (e.g., threatened breeding or wintering grounds), demography (e.g.,
low birth rate), and population genetics (e.g., small effective population size).
Populations that are dwindling in size are generally marked by high mortality or
low birth rates and eventually may show changes in gene frequencies due to
increased population subdivisions and reduced gene flow. Hence, ecologists make
use of alarming demographic trends such as low growth rates and conservation
geneticists focus on changes in genetic variation (e.g., loss of heterozygosity) to
screen for possible threatened populations. Population viability analyses that estimate extinction risks of populations due to both demographic and genetic stochasticity provide powerful monitoring tools that will help to guide the allocation of
245
Such infraspecific variation, however, is potentially crucial for the long-term
survival of a species. Conserving species as evolutionary units addresses the
importance of variation within species to preserve the evolutionary potential of
the protected organisms and ultimately minimize the extinction probability across
their range (Vrijenhoek 1989; Rojas 1992). Although reserve management based
on evolutionary units is less subject to “bean counting” of rare types, clear
delimiting of taxonomic groups is still essential to avoid inadvertent hybridization
and outbreeding depression.
Amendments to the Endangered Species Act (ESA) extended protection of
species to include distinct population segments that interbreed when mature. This
broader definition of species protection allowed flexibility to preserve species in
only part of their range without resolving their taxonomic status (Pennock and
Dimmick 1997). As ESA provided no clear definition for “distinct population
segments,” criteria for identifying fragile populations were highly varied, including uniqueness based on geographic isolation and morphological and genetic
differences (Pennock and Dimmick 1997).
The concept of evolutionary significant units (ESU) was developed, in part, to
provide biological guidelines for determining conservation units below the species level (Ryder 1986; Vane-Wright et al. 1991; Waples 1991). This attempt to
apply the concept of ESUs to identify “distinct population segments” for conservation has resulted in ongoing debates that focus on the biological merits (Dizon
et al. 1992; Moritz 1994a; Vogler and DeSalle 1994; Waples 1998) and the
fulfillment of the ESA mission (Rohlf 1994; Pennock and Dimmick 1997; Waples
1998). Rojas’s (1992) conclusion that “considering the species problem more
critically may result in recognizing the limitations of the taxonomic information
used; it may also contribute to the refinement of the concepts and methods” still
holds, and we continue to benefit from the interaction between systematists and
conservation biologists to determine conservation units.
Conclusions
Which species are most at risk of extinction may depend on many interactive
aspects of their biology, such as their ecology (e.g., specialists), mating or migratory behavior (e.g., threatened breeding or wintering grounds), demography (e.g.,
low birth rate), and population genetics (e.g., small effective population size).
Populations that are dwindling in size are generally marked by high mortality or
low birth rates and eventually may show changes in gene frequencies due to
increased population subdivisions and reduced gene flow. Hence, ecologists make
use of alarming demographic trends such as low growth rates and conservation
geneticists focus on changes in genetic variation (e.g., loss of heterozygosity) to
screen for possible threatened populations. Population viability analyses that estimate extinction risks of populations due to both demographic and genetic stochasticity provide powerful monitoring tools that will help to guide the allocation of