GENETICS AND THE
CONSERVATION OF WILD POPULATIONS
Genetic variation is recognized
as one of three fundamental levels of biodiversity (ecosystem, species, genes).
Seminal works in the early 1980s laid the basis for this field (Soule and Wilcox
1980, Frankel and Soule 1981), and since then there has been an explosion of
both the molecular methods for assessing genetic diversity, and the associated
statistical methods (Hillis et al, 1996; Luikart and England, 1999). The new
methods can be applied directly to the genetic level, but there is also a growing
realization that they can be used in studies at the species and even ecosystem
levels. These novel tools for assessing genetic variation and its maintenance
have reopened many fundamental questions in population biology, such as: "What
types of genetic variation contribute to differential survival in the wild,
and differentiation within and between populations?", and "Is there biogeographic
congruence of genetic variation in different species?". However, in the last
20 years there has been little attempt to produce a synthesis of this growing
area of endeavour. Therefore, Moritz and Sherwin are co-authoring a book on
the use of genetic methods for assessing biodiversity at all levels. The book
is aimed at population/evolutionary geneticists with interests in biodiversity.
For this audience, the book integrates conceptual and practical achievements
to date, as well as identifying remaining challenges, thus defining future research
directions. The book is also aimed at conservation biology researchers who are
not geneticists, to help them appreciate the utility of genetics, and its interface
with population demography.
PART 1 Introduction
Chapter 1 History and Scope of Conservation Genetics
2 Tools and Concepts of Genetic Analysis
Chapter 2: Analysis of Molecular Variation
Chapter 3: Genetic Analysis of Fitness
PART 3: Molecular Ecology
Chapter 4: Individuals and their Interactions.
Chapter 5: Rates of Genetic Change
Chapter 6: Population Structure and Connectivity
Chapter 7: Detection and Analysis of Hybridization
PART 4: Conservation of Genetic Variation
Chapter 8: Biodiversity Assessment
Chapter 9: Maintenance and loss of variation in threatened populations
Chapter 10: Changed mating patterns in threatened populations
Chapter 11: Changes to Population Connectivity
Chapter 12: Human-Mediated Selection and Mutation
PART 5: Synthesis
Chapter 13: Prospects for genetics in wildlife conservation
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Graduate Seminar Announcement
PART 1 INTRODUCTION
The book is written to be accessible to graduate students and professional biologists
with an interest in Conservation Biology and to stimulate, perhaps provoke,
those already working in the field.
Chapter 1 History and Scope of Conservation
- Conservation Genetics was born
of concern about erosion of genetic resources and the impact of humans on
the evolutionary process. It has been strengthened by the use of molecular
biology techniques to describe biological diversity and to provide insights
into evolutionary and ecological processes.
- The central concern of conservation
biology is to maximise biological diversity in the short and long term. This
requires that we maintain and restore ecological and evolutionary processes.
- Conservation Genetics contributes
to this goal through two, non-exclusive areas of activity: (i) conservation
of genetic and evolutionary diversity, and (ii) molecular ecology, the use
of molecular techniques to investigate ecological processes. In practice,
Conservation Genetics is most powerful when it complements, rather than replaces,
traditional approaches in systematics and ecology, and is fully integrated
into recovery planning.
Conservation strategies and
management need to recognize the extent to which humans have modified the
evolutionary process and our responsibility as stewards of evolution. Our
approach to conservation of habitats and populations should therefore be informed
by an understanding of historical and current processes that sustain biological
- Genetic diversity can be assessed
via analyses of molecular variation at sampled genes, quantitative genetic
approaches or using surrogates relating to the processes that sustain it (mating
systems, migration, population size). All methods are estimates, typically
crude ones, and should be used only when the questions and management implications
of outcomes are clear.
- Conservation Genetics is at an
exciting stage of its development as principles derived from theory and laboratory
studies are applied to natural systems. Further theoretical and experimental
studies, the latter using both model systems and "management experiments"
conducted as part of recovery actions, are needed for Conservation Genetics
to become a more predictive science with practical benefits.
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PART 2 TOOLS AND CONCEPTS OF GENETIC
Chapter 2: Analysis of Molecular Variation
Chapter 3: Genetic Analysis of Fitness
- The increasingly sophisticated methods for analyzing molecular variation
at several levels within species provide important tools for managing endangered
species. But for most managers and conservation biologists, the methods and
results of molecular genetic surveys are a mystery, bordering on alchemy.
This chapter provides an introduction to the main techniques in use and methods
for analyzing the data, with worked examples. This will provide the basis
for subsequent chapters that discuss the use of the data to make inferences
about population processes and history.
- Chromosomes, proteins, microsatellites, RAPDS and DNA fingerprinting, analyses
of sequence variation. Measures of molecular diversity, partitioning diversity
within and among populations, phylogeography. Organelle and nuclear genes:
strengths & limitations
- Estimates of genetic variation, Pi, Ho, He (call gene diversity), # alleles,
Fis, etc. (?Include Fst, Rst, etc here too? - at the very least forewarn of
their existence, then put in CH6).
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- The aim of conservation genetics is not to conserve every genetic variant,
only those that assist in continued evolution of species in their changing
environment and ecological roles. Thus it is important to choose appropriate
genes or surrogates for management and monitoring. The most appropriate loci
are of course, those whose variation will affect lifetime fitness in the short-or
long-term future, in a changing environment. Although it is difficult to specify
exactly which loci are appropriate, some generalizations can be made. First,
fitness traits tend to be multi-factorial - affected by multiple loci and
- Genetic architecture is the way in which multifactorial traits are controlled.
This includes the number of underlying loci, their physical arrangement (ploidy,
linkages), their biochemical interactions (regulatory, protein-protein, and
environment-gene), and the resulting pattern of additive and non-additive
interactions such as dominance and epistasis. The genetic architecture is
not static, but can alter in response to mutation at any of the contributing
loci, recombination, environmental changes, and random genetic changes. All
of these may be quite high for multifactorial traits, especially under conditions
that threatened species are likely to experience, such as periods of environmental
stress or small population size.
- Natural selection can appear to act at different levels from the individual
locus through to lifetime fitness. The apparent type and direction of selection
may differ at these levels, even in the same system of interacting loci. There
can also be selection for traits to be more or less sensitive to variation
at other loci or in the environment. Limited studies suggest that there is
poor correlation between variation of different types, so the choice of appropriate
suites of surrogates at different levels is an important direction for conservation
- Ideally, conservation managers would monitor and manage the genetic variance
of lifetime fitness, but even this strikes three problems: first, it is difficult
to analyze multifactorial genetic variation in wild populations; second, it
is difficult to analyze total fitness, and finally the analysis would be restricted
to the current population. The first problem may soon be alleviated by use
of molecular techniques, although the current methods are restricted to populations
with an extremely wide range of relatedness values; better methods may come
from molecular reconstruction of wild pedigrees
- In the absence of a generally applicable way of measuring fitness and its
genetic variation, there is a need for judicious choice of a suite of traits
and loci to monitor, as representatives of the types of selection-related
variation to be managed. There have been various suggestions about how to
identify specific traits or loci for monitoring, but none have been validated.
Maybe traits under strong selection could be identified by their insensitivity
to environmental variation. It would also be possible to monitor variation
in suites of single loci chosen for reasons such as their known selective
importance or significance in metabolic networks
- Heritability is important for short-term response to selection, but may
be very low for fitness related traits. Additive genetic variance or the related
measure called evolvability is what conservation genetics strives to maintain,
and is often high for fitness related traits, so its monitoring and maintenance
is a worthwhile exercise.
- Subdivision between populations is expected to increase the effect of linkage
and adaptive divergence, and the small number of studies dealing with both
molecular and multifactorial characters show conflicting results.
PART 3: MOLECULAR ECOLOGY
Chapter 4: Individuals and their interactions.
Chapter 5: Rates of Genetic
- Even the simplest conservation
actions such as a population census can be seriously hampered by incorrect diagnosis
of individual sex and identity, family relationships or mating system. Many
organisms have mixed reproductive strategies with varying ploidy and mixtures
of sexual vs. clonal reproduction - this needs to be recognised in monitoring
and management. More intensive actions such as translocations to wild or captive
populations require detailed attention to these matters. Also, threatening processes
have the potential to affect mating systems dramatically, or may affect certain
family groups more than others. These changes can, in turn, magnify other threats.
- Molecular markers can be used to identify genetically distinct individuals
and their sex This, in turn, can be used for population censuses, monitoring
of sex ratio and tracking individual movements. However physical recaptures
remain necessary to test error rates and provide essential information such
as condition, growth rates etc.
- Highly variable markers are also used in natural
populations to define parentage estimate relatedness among individuals and pedigrees
and analyze the breeding system. Some aspects of population structure, such
as plant mating systems, are very responsive to certain threats. This makes
their molecular analysis a very good indicator of these threats, but the long-term
importance of these changes is debatable, since the system itself is so labile.
- In most cases, molecular analyses are greatly aided by other ecological information,
such as the ages of potential parents and offspring.
Chapter 6: Population structure and Connectivity
- Genetic change underpins evolution and is central to conservation The
likelihood that threatening processes may alter these rates of change, highlights
the importance of estimating the expected rates under different conditions.
Each population, with its particular life-history, population size and other
characteristics, will have a particular rate of change, which can be expressed
as 1/2Ne. Ne is sometimes called the "effective population size", but this name
can be misleading since Ne is a function of the population's entire biology,
not just census size. As a rough approximation, Ne may be thought of as the
number of individuals that are passing their alleles on to the next generation.
Ne is often an order of magnitude smaller than adult population size, especially
if periods of small population size ("bottlenecks") have occurred.
of Ne iare also important to gauge that rate of change at genes affecting survival
and reproduction. This is because the efficiency of selection depends on its
strength, s, relative to that of drift [ i.e s > 1/Ne].
- In the same population,
different genetic processes will not necessarily occur at the same rate, so
there wiil be different Ne values for each estimate of genetic variation, such
as inbreeding (NeI), gene diversity (NeV), and association between alleles at
different loci (genotypic disequilibrium, NeD). The existence of multiple types
of Ne means that conservation biologists must choose carefully which is appropriate
to the particular management concerns.
- There are many ways of determining
Ne and the estimate obtained often relate to different time-scales and stages
of population history. Several methods can be selected to provide complementary
information relevant to management.
- Current Ne can be estimated from appropriate
demographic data, but often these data are difficult to obtain. There have been
attempts to define Ne as a function of adult population size given for particular
life history and ecology, but these methods are debated still. Estimating Ne
over a deeper time-period requires more data, especially fluctuations in population
size. Deeper still, Ne over thousands of generations can be estimated from gene
trees; such estimates are valuable for comparison to those of current Ne, especially
in recently perturbed systems.
- Ne can also be calculated from data on hypervariable
genetic markers. The most obvious method is somewhat circular: to repeatedly
measure genetic change, and from the rate of this change, back-calculate Ne.
Although an important component of ongoing monitoring, this lacks immediate
predictive power, unless molecular analyses of historical collections are possible.
Therefore much attention has been given to molecular methods for Ne estimation
and bottleneck detection. Especially when combined with information on demography
and history from monitoring, these data can be used to estimate simultaneously
both current and historical Ne, providing for more direct comparisons.
scope of the Ne estimation is important. All rates of change are per-generation,
which requires careful definition in species with overlapping generations. Threatened
populations with the same Ne but different generation lengths are likely to
accumulate genetic problems at a different rate. Estimates of long-term Ne such
as analyses of allele genealogies within populations, provide a perspective
on changes in Ne through time. These may provide a yardstick against which current
Ne and population fluctuations can be compared, with large deviations signaling
a need for attention.
- The geographic scope and structure of the population(s)
being managed will affect Ne also, and this will be discussed further in the
Chapter 7: Detection and Analysis of Hybridisation
- Wildlife managers
need to be aware of the geographical scale of populations and the magnitude
and nature of connectivity among populations in order to establish appropriate
sampling schemes for monitoring effects of threatening processes and actions
to ameliorate these.
- Gene flow differs from ecologically measured dispersal
and migration in that it reflects only the net distance between birth and reproduction.
Proper management of populations requires an understanding of both the ecological
and genetic dimensions of dispersal.
- Differences of individual genetic profiles
relative to their geographic separation carry information about local effective
population size, density and dispersal distances within a continuous population
- The geographic structure of populations can, in principle, be discerned by
comparing genetic divergence against geographic separation of populations, although
this can vary within a species across different spatial scales and different
- A Management Unit can be defined as a genetically distinct population,
typically measured by significant differences in allele frequency. This criterion
identifies populations that exchange relatively few individuals and are therefore
likely to be demographically independent. For animals mtDNA generally provides
a particularly sensitive marker of special relevance because it reflects the
behavior of females, which may differ from that of males.
- Levels of gene flow
among subpopulations can be estimated from the distribution of genetic variation
in populations at or near equilibrium and comparison of differently inherited
genetic markers can reveal sex-biased dispersal. However, such estimates are
typically imprecise and many threatened species are likely to be far from equilibrium
conditions because of recent and severe reductions in population size or connectivity,
so that most genetic parameters will refect historical rather than current conditions.
- Comparisons between multilocus genetic profiles of individuals and those of
populations can identify birth siteand thus detect migrants even where populations
are not in equilibrium. Such "assignment tests" provide a powerful way of monitoring
immigration in perturbed or managed systems. Ecological methods can then be
used to follow the fate of immigrants.
- If populations are undergoing local
extinction and recolonization, the genetic parameters may give misleading estimates
of connectivity, but with appropriate comparisons is may be possible to detect
- Genetic data, particularly the distribution of alleles in relation
to their phylogeny (phylogeography; as revealed by DNA sequencing), can provide
unique insights into long-term processes of isolation and range expansions,
against which current population structure can be compared. Major differences
between historical and current structures provides one criterion for prioritizing
conservation actions and targeting ecological studies.
- Fragmentation of habitats
may pose a significant threat to the viability of wildlife populations and the
hope is that genetic data, as a complement to ecological approaches, can provide
an early indication of major changes in connectivity. Whereas it is unlikely
that genetic data can provide realistic estimates of connectivity in recently
fragmented systems, it may be possible to detect major reductions in genetic
population size and, perhaps, increases in inbreeding. Such information could
be used to target intensive ecological studies and focus management strategies.
- Genetic profiles can be used to identify the origins of individuals in harvests
or the relative contributions of separate management units in mixed populations.
These tools can contribute significantly to harvested or migratory species.
- Detection of hybridization between distinct gene pools or species is an
important tool for wildlife managers trying to gauge the effects of deliberate
or inadvertent introductions on existing populations, particularly where the
latter are already under threat.
- It is also important to recognise any role that the process of hybridization
has played in the evolution of species complexes and in maintaining genetic
diversity in small isolated populations.
- Genetic markers derived from both nuclear and organelle genes have been
used extensively to investigate the extent and dynamics of hybridisation in
- These applications are reviewed and illustrated with appropriate case studies,
providing background for the discussion of effects of outbreeding in chapter
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PART 4: CONSERVATION OF GENETIC VARIATION
Chapter 8: Biodiversity Assessment
Chapter 9: Maintenance and loss of variation in threatened populations
- International conventions and national policies call for protection of
biodiversity at multiple levels, including genetic diversity, but how to assess
and prioritize genetic biodiversity within and across species and areas remains
- The assumption the protection of ecosystems and threatened species will
de facto encompass genetic diversity does not hold where current taxonomy
does not reflect evolutionary history. Genetic studies of many groups of organisms
have revealed misaligned species boundaries and morphologically cryptic species.
If undetected, this detracts from species-based assessments of biodiversity
and monitoring and management of individual threatened species.
- Several approaches have been suggested for incorporating phylogenetic information
into biodiversity valuation. One view is that we should maximise the phylogenetic
breadth of species conserved; another holds that rapidly speciating lineages
should have priority. This is the basis of genomic approaches to rapid biodiversity
inventory. Whatever the approach adopted, molecular phylogenies can provide
useful insights into long-term processes of extinction and speciation, against
which current threats can be evaluated.
- How to identify and prioritize intra-specific evolutionary units for conservation
is contentious. Genetic diversity can be considered along two dimensions -
one related to survival and reproduction (natural selection and adaptation)
and the other to historical isolation. Adaptive genetic diversity is, perhaps,
best retained via protection of ecological amplitude rather than protection
of specific phenotypic variants. The diversity due to historical isolation
cannot be recoved if lost and should be given priority in planning to conserve
- We advocate a binary approach to identifying conservation units, recognizing
Evolutionarily Significant Units and, within these, Management Units. These
can be defined from patterns of phylogeography and variation in allele frequencies,
respectively. The former are targets for protection whereas the latter are
the components that must be managed to ensure this
- Listing of geographically peripheral isolates across state or national boundaries
poses a dilemma for management agencies, particularly where the same species
is widespread and secure elsewhere. We argue that protection of such populations
is appropriate where they are historically isolated (ESUs) or occupy environments
that are unique for their species.
- The concepts developed to identify ESUs within species can be extended
to conservation of habitats and areas. Molecular genetic analyses can be used
to identify and, perhaps, rank, areas of habitat where there is a history
of biogeographic isolation, revealed through geographically congruent patterns
of phylogeography. Such rankings can identify sets of areas that maximize
the representation of genetically divergent areas, and through an understanding
of historical processes, also identify areas of potential evolutionary innovation.
Areas of evolutionary significance can be combined with assessments of species
distributions to design conservation strategies that maximize the representation
of current biodiversity and also promote continuing evolution and thus biotic
responses to environmental change. Within these areas it is important to retain
maximum environmental variation in order to protect adaptive genetic diversity.
- Exploitation of genetic resources has achieved increased prominence and,
through the Convention on Biological Diversity, has considerable ramifications
for conservation and the scientific endevour that underlies it. We argue,
from an evolutionary perspective, that nations should be regarded as stewards,
rather than owners, of genetic resources that, thus, that a substantial proportion
of revenue from exploitation of genetic resources should return to conservation
and promotion of sustainable use.
10: Changed mating patterns in threatened populations
- A pre-eminent concern in conservation genetics has been the loss of genetic
diversity within small isolated populations and its likely impact on short
term fitness and the longer-term capacity to respond to environmental change.
Both of these potential impacts are controversial and this chapter reviews
relevant theory and evidence from experiments in laboratory and natural environments.
This chapter discusses the management of two aspects of genetic variation:
gene diversity (He) and allelic diversity (K).
- Populations with higher He are expected to have a higher number of heterozygotes.
These heterozygotes are expected to have higher fitness. This expectation
rests on a variety of studies, mostly of wild or semi-wild populations, which
usually show some advantage to heteroygotes. It is possible that a large number
of studies showing no association between heterozygosity and fitness go unreported,
but very few studies show a negative relationship.
- The fitness effects of heterozygosity may be sufficient to have substantial
influence on short-term population persistence; the scanty data suggest that
can be the case. However, there is a need to better establish the link between
effects on individual fitness and population viability. This is an important
area for further research.
- Long-term effects of genetic variation (He, K) are more difficult to study,
but again theory suggest that more variable populations will be better able
to evolve in the face of changed or fluctuating environmental conditions.
There is some support for this from captive and domestic populations, as well
as the phylogenetic patterns in sexual and asexual species. Both gene and
allelic diversity are expected to be important in long-term evolution. However,
it is not at all clear whether greater responsiveness to selection in the
short-term leads to greater population persistence, as expected from theory.
Again, further research is indicated.
- Aside from loss of heterozygosity, small populations are also prone to
accumulation of mildly disadvantageous alleles as random processes overwhelm
selection. Theory predicts that this can reduce population viability in extremely
small populations or over very long periods of isolation. Direct evidence
for this process is still lacking.
- Loss and maintenance of genetic variation depends the interplay between
random genetic drift, mutation, recombination, mating patterns, dispersal,
and other factors such as genome structure. For any combination of factors,
a population is expected to approach an equilibrium level for each measure
of variation, and then fluctuate about this. Most populations are probably
not at equilibrium. Changed conditions will result in a new target equilibrium.
- Where conditions change as a result of threatening processes, the new equilibrium
level of genetic variation may not be the optimum for short- or long-term
conservation. The response to the new selective regime may be limited in either
speed or total magnitude. Also there may be transitory or permanent changes
in the genetic architecture during bottlenecks or other perturbations.
- Ideally, management of genetic variation will rely on early identification
of problems, through monitoring not only genetic variation itself, but also
surrogates such as population size, dispersal level, and rates of reproduction
and survival. Other suggested predictive tools range from a focus on a single
gene family (MHC) to the use of population history inferred from molecular
studies, but none of these have been adequately explored for general applicability.
- If genetic problems are identified early, then reversal of obvious threats
such as reduced immigration will allow the population to continue its normal
process of adaptation to change.
- More serious genetic management may be required in situations where the
population cannot cope with change, because it has already lost too much variation,
or is too small, or otherwise vulnerable, to cope with the demographic consequences
of selection. In this case, translocations between wild populations or between
wild and ex-situ populations may be required [more on this in chapter 11].
- Ex-situ management may also be instituted for non-genetic reasons, such
as demographic support for the wild population. The utility of this procedure
depends upon the success of reintroduction(s) and the genetic quality of the
reintroduced stock. Ex-situ management usually deals with only small numbers
of individuals, and therefore genetic concerns are very important. Luckily,
the intensive nature of ex-situ management gives considerable scope for genetic
control in most species, but each stage of the process must prioritise genetic,
demographic, and husbandry concerns in a different way. It is important not
to lose sight of the ultimate goal of re-releasing into the wild.
Chapter 11: Changes to Population Connectivity
- Many species have mating patterns which result in considerably more or less
inbreeding than expected in a random mating finite population. Conservation
biologists are concerned when a species' normal level of inbreeding is altered
by threatening processes. Could this worsen the conservation outcome, especially
by interaction with other genetic problems such as lowered variation between
- Inbreeding and other assortative mating cannot directly alter the amount
of genetic variation in the population (He); however, they increase the chance
of identity by descent (IBD#1), and thus decrease heterozygosity below random
mating expectations (positive Fis). Non-random mating patterns can also alter
associations between alleles at multiple loci, giving significant genotypic
- There is abundant evidence from domesticated species, captive breeding
of wild species, and wild plant species, that inbreeding generally reduces
fitness and increases the appearance of unusual traits, or the variance of
traits. This inbreeding depression may be based on single or multiple loci,
and it may be due to the disadvantageous effects of homozygosity for a particular
allele (recessive deleterious mechanism), or to disadvantages for homozygotes
of any kind (overdominance mechanism). The different mechanisms have different
management implications, so it is important to note that the small number
of studies have indicated that all mechanisms can be operating at different
loci in a single species.
- Given that inbreeding reduces fitness, there is concern that it may adversely
affect population persistence, and thus threaten short- or long-term conservation.
There is some supporting evidence from wild and ex-situ populations, especially
from those in stressful environments.
- Monitoring of mating systems can use an array of measures and surrogates.
Even molecular measures must be subjected to scrutiny since some, such as
Fis, could derive from sources other than inbreeding, which may have different
conservation implications. Non-genetic surrogates such as rates of pollinator
movement can also be used, but considerable research is required to show their
relationship to inbreeding.
- As well as monitoring inbreeding levels, it is important to know whether
any increase is associated by adverse changes to the mean and variance of
survival and reproduction- inbreeding depression. It is easy to miss this
association, for a number of reasons: the normal level of inbreeding for the
population may not be known; unless total fitness is measured, the traits
monitored may not be the ones which show depression in that population; the
depression may not be evident in the studied environment, especially if it
is an artificial one; and the effects of inbreeding may be cryptic until a
certain threshold is crossed.
- Management of inbreeding often requires use of surrogates, such as maintenance
or enhancement of normal dispersal mechanisms and Ne. Attempts to produce
general rules for population size or to predict sensitivity from prior Ne
or breeding system have not met with success. It has been suggested that managers
could simply allowing inbreeding to occur and natural selection to purge deleterious
recessive homozygotes, if the population is large enough to withstand the
death and reproductive failure that selection is based upon. However, purging
is not likely to be generally applicable, since the deleterious recessive
mechanism is not universal, and even in cases where there has been some success
at purging, there was not full recovery of pre-inbreeding fitness.
- Removal to an ex-situ facility is usually the only solution that allows
direct control of mating patterns, but this is costly, and the low population
size generally commits the manager to high levels of inbreeding, as well as
other genetic problems. Combined with the cost, this means that ex-situ management
cannot be recommended often.
- Whatever method of management is used, it is important to monitor for the
resultant level of inbreeding and depression, since many studies show that
it is difficult to predict which species will be more prone to depression.
Chapter 12: Human-Mediated Selection and Genetic Modification
- Management of interacting populations requires an understanding of both
demographic and genetic processes and should be conducted in the context of
prior levels of connectivity as inferred from molecular markers.
- Metapopulations may provide for evolutionary innovation, but this does
not mean that unnaturally fragmented systems should be left to their fate
because (i) the evolution of new adaptive forms via this "shifting balance"
process is probably rare and (ii) local population extinctions are unlikely
to be rescued by recolonisation.
- Outbreeding depression, a reduction in mean viability or reproduction,
has commonly been raised as a concern in the conservation literature, but
evidence for its generality in inter-population crosses in nature appears
weak. Nonetheless, wildlife managers should be aware of the different genetic
bases and manifestations of outbreeding depression when managing interacting
populations. The magnitude of outbreeding depression is not generally predicted
by geographic distance between populations, and like inbreeding depression,
may be more apparent in stressful environments.
- Limited genetic exchange between unnaturally isolated populations can reduce
the undesirable effects of inbreeding and loss of genetic variation, without
impeding local adaptation. The level of managed migration needed to achieve
this benefit depends on whether the immigrants are related, the ratio of effective
to census population size, and extinction - colonisation dynamics of the populations
- In practice, wildlife managers should consider the trade-off between the
likelihood of inbreeding vs. outbreeding depression. Outcrossing may have
immediate benefits to survival and reproduction where the populations involved
are inbred and can also greatly enhance levels of within population genetic
diversity. Translocations for conservation purposes should be guided by historical
levels and patterns of gene flow and use of inbred individuals for reintroductions
should be avoided. Guidelines for mixing of populations become particularly
important when in-situ and ex-situ conservation approaches are integrated.
***** More on this from Ch 12.
- With regard to historical processes, translocations can be conducted among
MUs within ESUs, but not among ESUs because there is evidence for recent migration
among the former, but not the latter. However, it remains to be determined
whether difference in local adaptation or outbreeding depression within ESUs
reduce viability of translocated or augmented populations. This is an area
ripe for experimental studies.
- Human-induced selection and genetic modification can often be so intense
as to directly affect conservation management. Impacts on natural evolutionary
processes span all major contributing processes - introduction of genetic
variation, drift, migration, mating system and selection. Here we focus on
the first and last of these.
- Novel genetic variants introduced by humans is often more dramatic than
the naturally-occuring ones. One class of genetic modification is quite deliberate,
being aimed at producing a better domestic species, or improved biological
control. These usually take the form of additions of multiple new alleles
by artificial hybridization with a different strain or species, or insertion
of an entire new locus by molecular manipulation. Also, the introduction of
new species continues apace, and this opens the way for untold novel hybridizations.
- None of the mutational processes is completely controlled. Nor in general,
do we know what range of phenotypes can result from interaction of novel alleles
or loci with the genetic background of the host population and environment-dependent
expression. In the case of hybridization, we do not know exactly what the
genetic changes are. In the case of molecular manipulation, we usually do
not yet have the expertise to insert the locus in the part of the genome where
we can predict the way that it will function, nor to provide all the appropriate
regulatory loci. The products of these rapid and somewhat random genome reorganization
are usually screened in experimental conditions to eliminate those which do
not have the desired new traits, or even show undesirable traits. One thing
we have learnt from conservation genetics is that the benign lab environment
is a poor predictor of outcomes in natural, stressful environments. In principle,
this screening could be extended to include any trait which concern conservation
biologists, and any environment, but the expense could be prohibitive.
- The conservation concerns from genetic modification are that the organism
itself may become a pest, or that the novel genetic material may be passed
on to other species, altering their conservation significance.
- It could be debated that the release of only a modified version of a species
already present in the environment is less hazardous than an entirely new
species. In theory, all additions to the wild should be subjected to the same
controls that are applied to deliberate releases of entire new species, as
in for biological control. Taking this point of view behoves us to improve
the screening of new species, to the point where there are never errors like
the "escape" of control agents. Precautions are particularly important where
it is proposed to release the new organism or genetic material into the environment
at large, as in attempts to infect pest populations with genetically modified
biological control agents, or to genetically improve harvested wild populations.
- Human alteration of selection is even more noteworthy. Whenever humans
activities favour one genotype at the expense of others, then there is deliberate
or inadvertent artificial selection. If a high proportion of the population
is affected, then this selection can result in major changes to the genetic
and ecology of the species. Examples include changes in life history following
selective harvests, and control programs. The more efficient the harvest or
control is, then the faster will be the selection for the less harvestable
or more resistant genotypes. It is sometimes stated that there cannot be buildup
of resistance to certain types of control methods, but a full appreciation
of the biology of the species usually shows points where genetic resistance
could develop. The build-up of less-desirable genotypes can be slowed by integrating
multiple methods with different selectivity.
- As well as affecting the loci under selection, such intense selection has
the potential to affect other loci in two ways: by a selective sweep in which
alleles at linked loci are driven to fixation, and by reducing Ne dramatically,
because only a small proportion of the population survives selection each
generation. Additional, side effects are expected in which selection for one
phenotype will cause disadvantageous changes at genetically correlated traits.
All these can have adverse effects on loci which are important for long-term
evolution, which is particularly important in populations which are harvested
but endangered, or those which are controlled as a pest in one part of their
native range, but conserved elsewhere.
- Addition of new species or genetic material can also affect selective pressures
on other species, especially in closely interacting systems like host and
parasite, where coevolution may occur.
- Human activities can change the selective regimes also, as in the case
of global warming and acid rains. Species which do not have appropriate genetic
variants to adapt, or the ability and opportunity to move to unaffected areas,
are expected to decline, as seen in notable examples in severely polluted
areas. It is possible that broad swathes of conserved areas across latitudinal
and altitudinal gradients would allow some species to move in response to
these threats, but others may need to be artificially moved, or the pollution
ameliorated at the site.
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PART 5: SYNTHESIS
Chapter 13: Prospects for genetics in wildlife conservation
- Conservation genetics should not be seen as an abstract science removed
from ecological approaches or as one making inflexible recommendations to
be adopted uncritically by wildlife managers. Increasingly, efforts to conserve
threatened species are guided by multidisciplinary recovery teams including
both research and management perspectives. As the barriers between theory
and practice are eroded, both the application and theory of conservation genetics
will be improved.
- Major challenges for the future include:
- moving from conservation of entities to process
- putting genetics into an ecological timeframe
- combining quantitative and molecular approaches
- extrapolating from laboratory to natural environments - the nexus
between adaptive management & cons. Genetics.
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