Fluctuations in population size and gene flow of maladaptive alleles can potentially produce inbreeding depression and outbreeding depression, both of which can reduce the fitness of a wild population.
This is the exposure of the individuals in a population to the
effects of deleterious recessive genes through matings between
close relatives. For a given locus, some alleles will confer
more fitness on an individual than other alleles. Within the
"other" class of alleles are rare deleterious recessive
alleles, which when appearing as a homozygous genotype in an individual
because of mating between relatives, greatly reduces the fitness
of the individuals carrying them. Deleterious alleles arise constantly
through mutation, so they are always present in a population at
low frequencies. Suppose we have two alleles, A and a,
where A is a normal allele and a is a deleterious
allele. The homozygous genotype aa of the deleterious
allele is rare in a large population, because with random mating
the expected frequency of a homozygote is the square of the allelic
frequency p2, and for a low-frequency allele this is a
small value. AA individuals are the most fit of the three
possible genotypes. Aa individuals have the same fitness
as AA individuals if the A allele is dominant over
the a alleles, or they may have some intermediate level
of fitness if the effects of the alleles are more additive. Lastly,
aa individuals show some recessive deleterious trait that
reduces their fitness.
In a large population where the a allele occurs at a low
frequency, the a allele appears chiefly in the heterozygous
state Aa, and heterozygous individuals will almost always
mate AA individuals. The offspring of an AA X Aa
mating will be AA or Aa, and the effects of the
recessive deleterious allele are masked. On the other hand, if
mating occurs between relatives in which both relatives have a
copy of the deleterious allele in the heterozygous state, an Aa
X Aa mating, one-fourth of the offspring of the mating
are expected to have the deleterious aa genotype. Mating
between relatives "unmasks" the effects of recessive
deleterious alleles that would otherwise occur only in heterozygous
So far, we have considered only a single deleterious allele at
a single locus. However, extrapolating from lower organisms and
plants (Lynch et al. 1995), about 100 deleterious alleles are
present in individuals of higher organisms when we look across
all genetic loci (see Lynch and Gabriel 1990). The problem is
therefore not trivial when all of the loci are considered. Most
of these deleterious mutations produce only a small reduction
in fitness of about 2%, when the alleles are made homozygous.
If all of the loci in an individual are made homozygous through
mating between relatives, the reduction in fitness would be on
the order of 200%, enough to "kill" the individual two
times over. This, essentially, is inbreeding depression.
The converse of inbreeding depression is outbreeding enhancement,
which is often referred to as hybrid vigor or heterosis. An example
of outbreeding enhancement is the use of hybrid strains of corn,
which greatly outperform inbred strains. From the standpoint
of deleterious recessive genes, hybrid vigor is nothing more than
the reverse of inbreeding depression; that is, it is the masking
of recessive deleterious alleles by crossing individuals from
different populations. Typically, different populations of the
same species harbor different recessive deleterious alleles, so
hybrid offspring between parents from the two populations will
not be homozygous for the same deleterious alleles. The offspring
are fitter than either parent because the effects of the deleterious
alleles have been masked. If the hybrid offspring are allowed
to mate randomly in subsequent generations, the deleterious alleles
will segregate out because of the mechanics of Mendelian inheritance
and produce individuals homozygous for the same deleterious allele,
which will have reduced fitness. But the mean level of fitness
in the population will still be higher than the level in either
parental population, because the frequency of each deleterious
allele has been reduced by mixing.
In summary, consider a hypothetical population in which an individual
mates at random with an unrelated individual in the same population.
Other individuals may mate with a sibling, a cousin, or other
close relative, and as this mating between relatives continues
we begin to see the effects of inbreeding depression. The more
closely related two mated individuals are, the greater the depression
in fitness that is expected to appear in their offspring. On
the other hand, matings of unrelated individuals from genetically
diverged populations of the same species may produce outbreeding
enhancement, if different deleterious mutations have accumulated
in the two populations.
If inbreeding depression and outbreeding enhancement were the
only genetic mechanisms we had to consider and matings between
individuals could be controlled, obviously the best strategy would
be always to mate individuals from different populations. However,
things are not so simple. So far we have considered only single-locus
effects, but typically alleles at different loci interact so that
complexes of genes co-evolve in a population, acting harmoniously
with one another to produce a high level of fitness. Different
isolated populations may evolve different complexes of genes that
interact well within a particular population, but poorly when
the genes are mixed through cross-population matings. This reduction
in fitness in the offspring is called outbreeding depression.
This phenomenon can occur in two ways. One way is by the "swamping"
of locally adapted genes in a wild population by straying from,
for example, a hatchery population. In this case, adaptive gene
complexes in wild populations are simply being displaced by the
immigration of genes that are adapted to the hatchery environment
or to some other locality. For example, selection in one population
might produce a large body size, whereas in another population
small body size might be more advantageous. Gene flow between
these populations may lead to individuals with intermediate body
sizes, which may not be adaptive in either population. A second
way outbreeding depression can occur is by the breakdown of biochemical
or physiological compatibilities between genes in the different
populations. Within local, isolated populations, alleles are
selected for their positive, overall effects on the local genetic
background. Due to nonadditive gene action, the same genes may
have rather different average effects in different genetic backgrounds--hence,
the potential evolution of locally coadapted gene complexes.
Offspring between parents from two different populations may have
phenotypes that are not good for any environment. It is important
to keep in mind that these two mechanisms of outbreeding depression
can be operating at the same time. However, determining which
mechanism is more important in a particular population is very
Interaction between mechanisms
Figure 1 shows the theoretical effects of outbreeding depression,
relative to outbreeding enhancement and inbreeding depression.
Both outbreeding depression and outbreeding enhancement may be
occurring at the same time in a population receiving immigrants.
As individuals in a local population mate with individuals that
are genetically more and more different, outbreeding depression
builds up because of the mechanisms we just mentioned. But notice
that outbreeding enhancement, because of the masking of deleterious
recessive alleles, may also be occurring at the same time that
outbreeding depression is occurring. If you average these divergent
effects, small amounts of outbreeding may lead to an increase
in fitness over that in a local, randomly mating population; however,
at higher levels of outbreeding, outbreeding depression may exceed
the beneficial effects of outbreeding enhancement. One of the
key questions is to determine at what genetic distance the detrimental
effects of outbreeding depression exceed the beneficial effects
of outbreeding enhancement. If populations have not diverged
for a long enough time to acquire separate, co-evolved gene complexes,
then it is unlikely that outbreeding depression will occur. The
degree that outbreeding enhancement occurs is not predictable
and must be determined experimentally.
It is also possible for a population to suffer from both outbreeding
depression and inbreeding depression at the same time. Suppose
we have two populations, a wild and a hatchery population, that
are each fixed for two kinds of alleles for each locus because
of local inbreeding (Fig. 2 [below]). The wild population has good (A)
and bad (A') alleles at locus A, is fixed for a bad allele
(B') at locus B, but is fixed for a good allele (D)
at locus D. On the other hand, a hatchery population has good
(A) and bad (A') alleles at locus A, only good alleles
(B) at locus B, but only bad alleles (D') at locus
D. Suppose too that alleles A' and D' are particularly
deleterious when combined in the same individual. These populations
are then mixed and the hybrid population is allowed to go through
several generations. Since most wild populations are small, it
also undergoes inbreeding over this time. Eventually, the population
may become fixed for the bad A' allele at locus A, for
the good B at locus B, and for the bad D' allele
at locus D. The alleles at the A and D loci therefore produce
inbreeding depression. Also notice that alleles from the two
different populations have become fixed in the hybrid population
so that outbreeding depression has also become fixed. Two forms
of genetic depression are piled on top of each other.
|Wild population||Hatchery population|
|1 A' B D||1 A B D'|
|Genotype||2 A B D||X||2 A' B D'|
|of fish||3 A' B D||3 A' B D'|
|4 A B D|| 4 A B D'
|1 A' B D|
|2 A' B D|
|3 A' B D|
|4 A' B D|
Figure 2. Possible outcome of breeding between hatchery
fish and wild fish in small populations. The prime mark (') indicates
a recessive deleterious allele.
Yet another genetic mechanism can lead to problems in wild populations,
especially populations of endangered species. We have assumed
that the effective sizes of populations that we have discussed
are on the order of only a few individuals or a few tens of individuals
at most. We know from empirical results from several organisms
that deleterious mutations, mild as they may be in their individual
effects, appear at a fairly high rate. About one deleterious
mutation appears per individual per generation. That means that
on average each fish has one deleterious mutation that was not
present in either parent.
As we said, the average reduction in fitness when one of these
mutations is made homozygous is only about 2%. Earlier speakers
noted that the amount of random genetic drift is inversely proportional
to population size, 1/2Ne. If 1/2Ne is larger than
the selection coefficient, the efficiency of selection against
new mutations is less than the force of random drift for that
population size. The result is that the "noise" of
random drift will overwhelm natural selection and the new deleterious
alleles will accumulate in the populations as though they were
neutral alleles, even though they have deleterious effects on
the individuals that carry them. Thus, if the selection coefficient
is 2%, the effect will be important in populations with effective
sizes of 50, or with adult census sizes of a few hundred fish.
A rule of thumb is that, in small populations, new, mildly deleterious
mutations will accumulate in the population at a rate that is
half the mutation rate at the genomic level. Even in the absence
of inbreeding depression and outbreeding depression, this accumulation
of deleterious mutations will lead to a reduction in fitness of
about 1% each generation. Since the effective sizes of many endangered
populations of salmon are on the order of 50 or smaller, this
is a major potential source of long-term genetic deterioration.
First of all, virtually every trait that has been examined in
a wide variety of species can exhibit inbreeding depression, such
as by full-sib matings or by self-fertilization in the case of
some plants. Some traits are more susceptible to inbreeding than
others, but the fact remains that inbreeding depression occurs
in all complex genetic characters. A linear decline in mean fitness
with the inbreeding coefficient has been observed in a diverse
array of organisms including fruit flies, flour beetles, and many
species of mammals (including humans). Because inbreeding depression
is linear with the inbreeding coefficient, we can extrapolate
to future generations if we know the effects of inbreeding depression
in the first few generations of inbreeding.
The second point of particular importance for economically important
traits in salmon is that traits most closely related to fitness
are the ones that exhibit the most inbreeding depression. Again,
this has been observed in numerous species, but the data for fruit
flies illustrate this principle very well. Table 1 [below] shows a summary
of several studies of fruit flies. For morphological characters,
the effects of inbreeding are relatively mild. The greatest
changes are observed for primary fitness components, such as reproductive
capacity, viability, competitive ability, and so on, and not for
characters only remotely related to fitness.
The final point with respect to inbreeding depression is that
all the studies presented here were done in the laboratory to
ensure that observable results were acquired at the end of the
experiment (reviewed in Lynch and Walsh 1997). When parallel
studies were done in the laboratory and in the field under natural
conditions, the effects of inbreeding were typically much greater
under natural conditions. The message here is that the assertions
about the negative effects of inbreeding outlined above are conservative.
Evidence for outbreeding depression is much less extensive than
evidence for inbreeding depression, but outbreeding depression
is nevertheless a general genetic phenomenon. One problem in
studying outbreeding depression is the number of generations that
may occur before outbreeding depression reveals itself. The effects
of outbreeding enhancement due to the masking of deleterious alleles
and outbreeding depression due to hybrid breakdown may cancel
each other in the first generation after crossing individuals
from two populations. So the effects of outbreeding depression
may not be apparent for a few generations. A few experiments
have been done in which reciprocal transplants have been made
between plants separated by as little as tens or hundreds of meters.
In a study of plants separated by various distances, progeny
of crosses between plants separated by 10-30 meters showed greater
fitness than plants separated by smaller or larger distances (Wasser
and Price 1989). Many of these studies show that populations
are locally adapted and that outbreeding depression occurs between
genetically divergent individuals. Comparable studies in animals
are rare, but it is likely that similar results occur in animals.
Experiments on marine copepods in intertidal pools show that
hybrid individuals between populations some tens of kilometers
apart show breakdowns in salinity tolerance, prolonged development
and so on (Burton 1987, 1990). In another study, clones of the
microcrustacean Daphnia in the same lake show hybrid breakdown
(Lynch and Deng 1994). The overwhelming evidence is that these
genetic effects occur in every group of organisms studied, and
although not much research has been done on salmon, there is no
reason to believe that the genetics of salmon are any different.
Table 1. Inbreeding depression (I.D.) in laboratory populations
of Drosophila. I.D. = 1-(zr/zo), where zo
and zr are means of the random mating base, and the completely
inbred population (obtained by linear extrapolation), respectively.
Results marked with an asterisk were obtained from studies of
only one or two chromosomes; in these cases, I.D. for the entire
genome was extrapolated by assuming that each chromosome arm constituted
20% of the genome, and that the effects were multiplicative across
chromosomes. Negative values imply an increase in character value
|Character||I.D. (various studies)
|Competitive ability||0.84, 0.97
|Egg-to-adult viability||0.57, 0,44, 0.66*, 0.48*, 0.06
|Female fertility||0.81, 0.18, 0.35
|Female rate of reproduction||0.81, 0.56, 0.96, 0.57
|Male mating ability||0.52*, 0.92, 0.76
|Male fertility|| 0.00*, 0.22*
|Male weight||0.07, 0.10
|Abdominal bristle number|| 0.05, 0.06, 0.00
|Sternopleural bristle number||-0.01, 0.00
|Wing length||0.03, 0.01
A question that is often raised is how to obtain information on
the genetic consequences of inbreeding and outbreeding in salmon.
Many managers would like to have harder evidence that these are
real issues with salmonids. The only way of getting this evidence,
however, is by doing experiments with salmon themselves. Demonstrating
inbreeding depression is straightforward and is done by monitoring
the performance of offspring from full-sib matings, because these
matings are genetically the closest possible in a sexually reproducing
species. Such experiments, however, represent a substantial investment
and may take a decade or so. Since the decline in fitness is
approximately linear with the degree of inbreeding, useful extrapolations
to small natural populations could be made from the results of
Experiments to demonstrate outbreeding depression are also conceptually
straightforward, but the work needed to complete the experiments
is not trivial. To understand the effects of hatchery straying
on wild populations, hatchery and wild fish would be crossed to
make first generation hybrids, which would then be released for
normal ocean migration. Second generation offspring would be
made from returning hybrid individuals, which may represent only
a small fraction of those released. The effects of outbreeding
depression, however, may not be apparent in these early generations,
so the crosses of further generations are required. A hybridization
between odd- and even-year pink salmon made with cryopreserved
sperm yielded only a small amount of evidence about outbreeding
depression after several years of work (Gharrett and Smoker 1991).
The bottom line is that any kind of quantitative results would
take several years of hard work to generate.
Proceeding without results for salmon
Since the empirical evidence of inbreeding depression and outbreeding
depression in salmonids will not be available for some time, what
is the best way to proceed? The first concern for any stock,
whether it is a hatchery stock or a wild stock, is with its effective
population size. One way of looking at this question in an objective
manner is to ask how big a population would have to be to make
it behave genetically as an infinitely large population. In other
words, at what point would a further increase in size fail to
increase the level of genetic variability beyond that maintained
in an effectively infinite population? To make a population genetically
"secure" requires an effective population size of several
hundred fish, or a census size of about 1,000 reproductive fish.
This number is one or two orders of magnitude larger than many
populations of salmon that have dwindled to only a few individuals.
The results from other species and population genetic theory can
be used to make recommendations that would reduce the likelihood
of outbreeding depression in salmonids. One of the most important
precautions would be to minimize the degree of interbreeding between
hatchery and wild stocks. The effects of outbreeding depression
are not likely to appear for at least a couple of generations
after outbreeding occurs. If the progeny of an out-crossed stock
appear to be fine in the first few generations, this does not
necessarily mean that outbreeding depression will not occur later.
After genes have been mixed from two populations, it is then
impossible to eradicate the potential difficulties with outbreeding
depression. At that point, the only way out is to allow natural
selection to sort things out, but how long this might take is
Relevant data for determining the potential effects of inbreeding
depression and outbreeding depression in natural populations of
salmonids is not yet available. However, theoretical studies
and empirical results for other species show that both inbreeding
depression and outbreeding depression can lead to the decline
in fitness of natural populations. Both of these effects, however,
may take several generations to become apparent. At this point,
prevention may be better than waiting to implement corrective
management policies until empirical evidence demonstrates these
effects in salmon.
Burton, R. S. 1987. Differentiation and integration of the genome
in populations of the marine copepod Tigriopus californicus.
Burton, R. S. 1990. Hybrid breakdown in developmental time in
the copepod Tigriopus californicus. Evolution 44:1814-1822.
Gharrett, A. J., and W. W. Smoker. 1991. Two generations of
hybrids between even- and odd-year pink salmon (Oncorhynchus
gorbuscha): A test for outbreeding depression? Can. J. Fish.
Aquat. Sci. 48:1744-1749.
Lynch, M., J. Conery, and R. BÅrger. 1995. Mutational
accumulation and the extinction of small populations. Am. Nat.
Lynch, M., and H.-W. Deng. 1994. Genetic slippage in response
to sex. Am. Nat. 144:242-261.
Lynch, M., and W. Gabriel. 1990. Mutation load and the survival
of small populations. Evolution 44:1725-1737.
Lynch, M., and J. B. Walsh. In press. The biology and analysis
of quantitative traits. Sinauer, Sunderland, MA.
Wasser, N. M., and M. V. Price. 1989. Optimal outcrossing in
Ipomopsis aggregata: seed set and offspring fitness. Evolution
Question: Ed Crateau: In the experimental hatchery X
wild salmon crosses that you mentioned to demonstrate these effects,
don't you also need control experiments to show that the hatchery
X wild salmon offspring are worse off in either the hatchery or
natural environments? These results, however, would not show
whether the problem was adaptation to another environment or outbreeding
Answer: Mike Lynch: Yes. One of the big problems is
to determine which mechanism is responsible for declines in fitness.
To show that outbreeding depression--the breakdown of intrinsic
coadaptation--was the mechanism for reduced fitness, a researcher
would have to show that fitness was reduced in all environments.
With the rapid habitat changes that are occurring, it is not
clear which environment will be relevant several years from now.
Perhaps, if fitness begins to decline in a wild population because
of a breakdown in local adaptation, stopping gene flow from non-native
stocks may allow the local population to recover. Such a recovery
would still take several generations.
Question: Dolph Schluter: Since the experiments you described
take so long, is there any way of predicting the amount of outbreeding
depression that might occur in salmonids from the results of studies
of other species? Is it possible to use the amount of time the
stocks have been separated from each other or the genetic distance
between them to make such predictions?
Answer: Mike Lynch: Few studies of outbreeding depression
exist, and in these studies, the degree of outbreeding depression
has not been correlated with any kind of molecular marker. So
it is difficult to make statements about the time of separation
or the degree of genetic differentiation in the characters affected
by outbreeding from molecular markers. For the flower, Delphinium,
outbreeding depression occurred between plants in the same field.
Molecular markers could be used simply to monitor changes in
genotypic frequencies in the offspring over time, to estimate
mortalities in the different populations of fishes. If the different
groups identified by the molecular markers show different levels
of fitness, then you can be sure something is happening. If there
is no differential fitness in these groups in the first few generations,
however, you still cannot be sure that there is not a problem.
Question: Audience: If a hatchery stock is only one or
two generations away from a local stock, does this change the
likelihood of outbreeding depression in hatchery X wild crosses
because of hatchery releases?
Answer: Mike Lynch: If hatchery-reared fish are only
one or two generations removed from wild populations, outbreeding
depression is unlikely to be a problem. If the hatchery is being
used to ensure the survival of large numbers of fry, and if the
brood stock is continually taken from wild populations, outbreeding
depression is unlikely to occur.
Question: Richard Carmichael: I would like to clarify
the kind of experiment that would be needed to show outbreeding
depression. For example, someone is proposing to enhance a wild
population with a non-local stock, and we want to understand if
outbreeding depression might occur. We would first need to know
how the hatchery stock performed in the natural environment by
itself in the absence of wild salmon. Then we would need to know
the productivity of the wild population apart from the hatchery
stock. Finally, we need to measure the productivity of the hybrid
population. Is that correct?
Answer: Mike Lynch: Yes. But in addition you should
follow the hybrid population for at least two generations.
Question: Richard Carmichael: Does the size of the wild
population affect the outcome? For example, would a very large
population of thousands show more outbreeding depression than
a small population where outcrossing may cover inbreeding through
Answer: Mike Lynch: Population size is important. Inbreeding
is measured on a 0-1 scale, and the rate of increase in inbreeding
is roughly equal to one over twice the effective population size,
1/2Ne. If the effective size of a population is five fish,
the rate of increase in inbreeding due to random mating is 1/10
or 10%. Since some of the five fish may be related, the rate
of increase in inbreeding may be more. The point is that small
populations become inbred very quickly.
It is really difficult to make specific predictions about outbreeding
depression. The common observation from line-cross analysis in
agronomy and in animal breeding is an increase in productivity
traits in the first generation of a cross between two lines, followed
by outbreeding depression in crosses between F1 or later hybrids.
The explanation is that hybrid vigor in the first generation
results from the masking of deleterious recessive alleles in the
two lines. In subsequent generations, adapted combinations of
alleles break down, and this leads to outbreeding depression.
The breakdown can be due to the loss of ecological adaptation
or to a loss of the favorable interactions among genes. If, in
fact, the migration rates are exceedingly large, on the order
of 50-70% as suggested in some of the talks today, then outbreeding
depression is probably not occurring. This level of flushing
would simply lead to the replacement of the wild population with
the hatchery populations.
Comment: Robin Waples: First, the high rates of straying
of non-native fish in the Grande Ronde Basin and the Umatilla
River precipitated this workshop. However, a more general issue
involves a wide range of straying rates and population sizes.
Second, the theoretical treatments of migration and population
size are in terms of individuals per generation, whereas fish
biologists often state the number of fish returning to spawn each
year. So even though 10, 20, or 50 fish may return in 1 year,
a whole generation may be 4 or 5 years. The population size per
generation is then the number of fish returning per year times
the number of years per generation. This number is not quite
so small as the returns per year mentioned earlier.
Question: Audience: How reversible is inbreeding after
a population grows quickly, say from 100 to 2,000?
Answer: Mike Lynch: This point arises frequently with
captive and endangered populations. Some researchers argue that
inbreeding and selection could purge a population of its deleterious
mutations. If the population survives, it will be better off.
This strategy has been used in the captive breeding program of
Spekes gazelle, which was started with four individuals. The
cost of this procedure is that most lines or populations go extinct,
so that in laboratory experiments with mice, for example, only
about 5% of the lines survive. Replicate lines cannot usually
be established for an endangered population, so that means that
a population has only about a 5% chance of surviving an episode
of such intense inbreeding. Also keep in mind that even if all
deleterious mutations have been purged, they will eventually return
to the population, because the per individual mutation rate to
deleterious genes is about one per generation. If a previously
inbred population grows and then experiences another reduction
in population size, inbreeding depression would occur again, because
of the accumulation of recessive deleterious mutations.
Question: Audience: After a population experiences inbreeding
because of a strong reduction in size and grows again, you are
saying that you have lost the diversity contained in the various
lines of descent in the population. Is that correct?
Answer: Mike Lynch: Mutation can eventually bring new
useful mutations into a population at a good rate as it grows.
So if a population declines to a small size and experiences inbreeding,
but its numbers recover, the population may recover genetically.
This can, however, take several dozens of generations. What
is critical is the transient phase when the population is small
and demographic extinction is a possibility.
Question: Audience: How arbitrary is the effective population
size of 1,000 individuals?
Answer: Mike Lynch: From population genetics theory, an effective population size of between 500 and 1,000 individuals is the point that, for quantitative characters (e.g., morphology), the genetic variation maintained by a balance between input by mutation and loss by genetic drift is about the same as would be expected in an effectively infinite population.
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