14. Role of Genetics in Conservation Biology
229
individuals, mating among relatives will result in a population with less fit individuals, irrespective of its mating history. By contrast, inbreeding should no
longer result in depression of individual fitness if recessive deleterious alleles
underlie inbreeding depression and can be purged from populations with regular
inbreeding (Lande 1988; Hedrick 1994; Fu et al. 1998). Byers and Waller (1999)
reviewed evidence for the purging hypothesis among plant populations and concluded that “purging appears neither consistent nor effective enough to reliably
reduce inbreeding depression in small and inbred populations.”
Lande (1995) suggested that inbreeding depression is the result of segregation
of deleterious alleles and that there is little evidence to support the overdominance
hypothesis. He concluded that gradual inbreeding will only reduce the risk of
inbreeding depression due to lethal and sublethal mutations but will not succeed in
purging mildly deleterious alleles (Hedrick 1994; Lande 1995). This prediction is
consistent with Ballou’s (1995) results that a long history of inbreeding in 25
captive populations resulted in a slight reduction but not complete removal of
inbreeding depression.
Experimental studies of inbreeding effects in wild populations are rare and have
produced mixed results. Brewer and co-workers (1990) tested the prediction of
the dominance hypothesis that small isolated populations of White-footed Mice
with low genetic diversity would show less inbreeding depression than large
central populations. The severity of fitness depression in inbred litters did not
correlate with genetic diversity, and deleterious effects did not diminish through
several generations of inbreeding (support for overdominance hypothesis). However, maternal care was only depressed in inbred mothers from genetically diverse
populations (support for dominance hypothesis). Brewer and associates (1990)
concluded that “overdominance of fitness traits probably contributed as much to
the genetic load as did deleterious recessive alleles.”
Keane (1990a, b) bred wild-caught mice from a single population. Whereas
significant inbreeding depression was documented for full-sibling and halfsibling matings, mating among cousins resulted in high reproductive success, and
there was potential for outbreeding depression (fitness reduction due to matings
among genetically distant individuals). Jimenez and colleagues (1994) documented severe inbreeding depression in White-footed Mice when they reintroduced inbred offspring of wild-caught mice into natural habitat. Inbred mice
showed continual weight loss and suffered higher mortality than noninbred mice
after release. These deleterious effects of inbreeding were much more severe in
the natural environment than in captivity. In a “natural experiment” involving a
Song Sparrow population, outbred individuals had a higher probability of survival
during severe population crashes. Keller and associates (1994) concluded that
environmental and genetic effects on survival interact and that inbreeding depression among the Song Sparrow population was expressed when the population
experienced environmental stress (i.e., severe winter weather). These studies
reiterate a point made by Hedrick and Miller (1992) who caution that inbreeding
depression may be greater in natural populations than in laboratory animals due to
more severe environmental conditions.
229
individuals, mating among relatives will result in a population with less fit individuals, irrespective of its mating history. By contrast, inbreeding should no
longer result in depression of individual fitness if recessive deleterious alleles
underlie inbreeding depression and can be purged from populations with regular
inbreeding (Lande 1988; Hedrick 1994; Fu et al. 1998). Byers and Waller (1999)
reviewed evidence for the purging hypothesis among plant populations and concluded that “purging appears neither consistent nor effective enough to reliably
reduce inbreeding depression in small and inbred populations.”
Lande (1995) suggested that inbreeding depression is the result of segregation
of deleterious alleles and that there is little evidence to support the overdominance
hypothesis. He concluded that gradual inbreeding will only reduce the risk of
inbreeding depression due to lethal and sublethal mutations but will not succeed in
purging mildly deleterious alleles (Hedrick 1994; Lande 1995). This prediction is
consistent with Ballou’s (1995) results that a long history of inbreeding in 25
captive populations resulted in a slight reduction but not complete removal of
inbreeding depression.
Experimental studies of inbreeding effects in wild populations are rare and have
produced mixed results. Brewer and co-workers (1990) tested the prediction of
the dominance hypothesis that small isolated populations of White-footed Mice
with low genetic diversity would show less inbreeding depression than large
central populations. The severity of fitness depression in inbred litters did not
correlate with genetic diversity, and deleterious effects did not diminish through
several generations of inbreeding (support for overdominance hypothesis). However, maternal care was only depressed in inbred mothers from genetically diverse
populations (support for dominance hypothesis). Brewer and associates (1990)
concluded that “overdominance of fitness traits probably contributed as much to
the genetic load as did deleterious recessive alleles.”
Keane (1990a, b) bred wild-caught mice from a single population. Whereas
significant inbreeding depression was documented for full-sibling and halfsibling matings, mating among cousins resulted in high reproductive success, and
there was potential for outbreeding depression (fitness reduction due to matings
among genetically distant individuals). Jimenez and colleagues (1994) documented severe inbreeding depression in White-footed Mice when they reintroduced inbred offspring of wild-caught mice into natural habitat. Inbred mice
showed continual weight loss and suffered higher mortality than noninbred mice
after release. These deleterious effects of inbreeding were much more severe in
the natural environment than in captivity. In a “natural experiment” involving a
Song Sparrow population, outbred individuals had a higher probability of survival
during severe population crashes. Keller and associates (1994) concluded that
environmental and genetic effects on survival interact and that inbreeding depression among the Song Sparrow population was expressed when the population
experienced environmental stress (i.e., severe winter weather). These studies
reiterate a point made by Hedrick and Miller (1992) who caution that inbreeding
depression may be greater in natural populations than in laboratory animals due to
more severe environmental conditions.