key: cord-0040248-6ajzhojy
authors: Woodruff, David S.
title: Populations, Species, and Conservation Genetics
date: 2004-11-28
journal: Encyclopedia of Biodiversity
DOI: 10.1016/b0-12-226865-2/00355-2
sha: 6799518f158be9b62f5cac77e43940f7ba04fe33
doc_id: 40248
cord_uid: 6ajzhojy

nan

with a particular genotype relative to another individual with a different genotype. Natural selection typically favors ''survival of the fittest'' and thus facilitates the future evolvability of a population. genetic enhancement Management actions taken to increase the genetic variability and viability of a population; includes translocations and reintroductions. genetic erosion A population viability-threatening process in small, isolated populations whereby random genetic drift and inbreeding diminish the population's innate genetic variability. Increasingly, it is associated with habitat fragmentation. N e Symbol for the genetic effective size of a population, an important indicator of future evolvabilty or extinction risk; usually much less than the observed census size (N). variation The abundant normal genetic differences between individuals in a population. Such variation is ultimately due to changes at the DNA base pair sequence level but is typically monitored at higher levels of expression. Overall genetic variation is associated with population viability and future evolvability. Conservation genetics is focused mainly on the protection and maintenance of genetic variation.

CONSERVATION GENETICS is concerned with population genetic variation, population viability, and the future evolution of species. Conservation genetics, ecol-ogy, and habitat management together provide the technical underpinnings of conservation biology, a crisisoriented science of biodiversity management. Still in its infancy, conservation genetics focuses on the characterization of variation in populations and species and on the management of innate levels of variation in evolutionarily significant units in nature and in their captive or managed analogs. Conservation genetic methods are borrowed from evolutionary biology and molecular genetics and are under development. Although some genetic management principles flow directly from current evolutionary theory, several key scientific problems remain to be solved before we can effectively deal with the issues presented by the biodiversity crisis. Although single-species ecological methods have dominated conservation management practice, it is clear that maintaining the future evolvability of species will require greater genetic intervention in the future. Conservation genetics is thus a cornerstone of biodiversity conservation.

Until the 1960s, it was widely thought that genetic variation was unusual. Biologists regarded populations as being composed of many very similar ''wild-type'' individuals and a few rare mutant individuals. Following the introduction of allozyme electrophoresis it quickly became clear that most plant and animal populations were highly variable. Subsequently, it has become possible to examine DNA sequences directly and large numbers of base pair mutations have been discovered in functional genes of most living organisms studied. Even higher levels of variation occur in the introns and highly repetitive stretches of noncoding ''junk DNA'' that lie between eukaryote genes. High levels of variability characterize most natural populations of plants and animals and this variability is thought to be both a product of evolutionary agents and a determinant of the future evolvability of a population. Genetics became a cornerstone of conservation science in 1981 with the publication of Frankel and Soulé's Conservation and Evolution. Since then, geneticists have studied the effects of the ongoing global reduction of genetic variability on population viability and species persistence. Key reviews of the progress made include Schonewald-Cox et al. (1983) , Loeschcke et al. (1994) , Olney et al. (1994) , Frankham (1995) , Avise and Hamrick (1996) , Smith and Wayne (1996) , and Landweber and Dobson (1999) .

Estimates of variability in a population vary with the feature examined and analytical method used. Karyotypic variation is usually low within a species, allozyme variation is high, and DNA sequence variation may be very high in some parts of the genome. The principle of high natural variability is perhaps best illustrated by the published surveys of allozyme patterns in plants and animals (Schonewald-Cox et al., 1983) . Geneticists typically sample a population of 20 individuals and determine each individual's genotype at 10-20 loci. This allows them to calculate the mean number of alleles per locus (A), the mean individual heterozygosity (H), and the proportion of loci in the population that are polymorphic (P). A few thousand species have been examined to date and variation is typically in the following range: H ϭ 0.05-0.15 and P ϭ 0.20-0.50. Still higher levels of natural variation are found at the DNA sequence level, especially in areas of the genome which are apparently free to mutate. Short repeat sequences (microsatellites) of nuclear DNA, for example, are often 10 times more variable than allozyme loci. A polymorphic allozyme may have 2 or 3 alleles segregating in a population, whereas a dinucleotide repeat microsatellite may have 10-15 alleles (differing in the number of times the motif is repeated) in the same individuals. Biochemical and molecular genetics have shown that most living organisms are richly variable.

Much of the cryptic variation in natural populations that has been discovered in the past 30 years appears to be selectively neutral or near neutral in its effect on the phenotype. That is, the individuals carrying these various allelic variants appear perfectly healthy. It is rare to find a single locus genetic trait in which a deleterious condition such as albinism is controlled by a single allelic variant. In fact, there is strong circumstantial evidence that much of this genetic variation is actually beneficial. Although the relationship between genetic variability and individual fitness is not well understood, it is clear that variability is associated with viability. Experiments and field observations with a few species have shown that there is a positive relationship between genetic variability and individual growth rate, size at maturity, symmetry of body parts, fecundity, and health as measured by parasite load. Extrapolating from the individual level to the population level, it is a fundamental maxim of evolutionary biology that genetic variation is positively related to adaptability or evolvability. Fisher's fundamental theorem holds that additive genetic variation in fitness is positively related to a population's ability to respond to natural selection. Evolutionary success, the ability of a species to persist despite changes in climate and exposure to new diseases, predators, and competitors, is somehow related to innate genetic variability. In a world of unpredictable change, alleles that are selectively neutral for thousands of generations may suddenly become lifesavers for the individuals that carry them. If evolution is largely dependent on genetic variability, then the conservation of species will sooner or later depend on the conservation of their innate genetic variability (Woodruff, 1989) .

Of course, there are many apparently successful genetically invariant plants and animals. There are many known clonal organisms whose descendents are genetically identical to their mothers. Such clonal plants, snails, fish, and lizards are genetically invariant even though they may exhibit great ecological success. However, they are more likely to go extinct when their environment changes than are closely related sexually reproducing species. Although meiosis ensures the maintenance of high variability in the sexual species, their asexually reproducing daughter species gradually lose their initial low variability and over time become evolutionary dead ends.

There are many different ways of measuring genetic variability, including the examination of karyotypes and single-locus markers (allozymes, mitochondrial and chloroplast DNA restriction fragment length polymorphisms), DNA minisatellite fingerprints, random amplified polymorphic DNA, mtDNA sequences, nuclear DNA sequences, and nDNA microsatellites (simple sequence repeats). Benirschke and Kumamoto (1991) and Smith and Wayne (1996) provide numerous examples of the application of karyotypes and molecular genetic markers, respectively, to conservation. The various methods differ in their resolution of pedigree, population, and species-level questions and there is no single correct technique. One method, allozyme electrophoresis, is informative with mammals but frustratingly uninformative for birds. The methods also differ greatly in their cost and technical difficulty. Some relatively inexpensive methods are satisfactory for one-time analyses but the results cannot be built on in subsequent studies. Other methods, in contrast, provide genotypes which can be archived in expandable permanent databases such as GenBank for future comparative study of samples collected at other times or places. Although a given method will give comparable results in a study of closely related populations or species, results across unrelated taxa may not be comparable.

Currently, nuclear and mitochondrial sequence data are the most informative methods for characterizing variability at or above the level of populations. For studies of variation within populations, hypervariable nuclear microsatellite loci are ideal markers. Sequence data and microsatellite genotypes can now be determined directly from minute DNA samples amplified many times by the polymerase chain reaction. Both methods require fully equipped laboratories, trained personnel, and considerable time and money for developing the synthetic DNA primers to amplify the gene sequences of interest in species that have never been studied before.

Noninvasive genotyping methods involving the extraction of DNA from shed hair and feathers were introduced in 1989 and are now widely used (Morin and Woodruff, 1996) . Noninvasive (shed tissues, feces, urine, and scent markings) and nondestructive (toe, tail, and ear clips and fish scales) genotyping permits the study of wildlife populations that previously were almost impossible to sample. Not surprisingly, the DNA extracted from some types of samples may be degraded and very difficult to work with. Nevertheless, with technical care and patience it is possible to genotype some animals without actually seeing or handling them.

Conservation geneticists can also genotype museum specimens and determine patterns of variability over periods of decades and sometimes centuries. Several species that have gone extinct recently, including the dodo, moa, thylacine, and quagga, have been characterized genetically. DNA can also be extracted and sequenced from some fossil remains, but DNA degradation rates are such that fossils more than a few million years old are unlikely to yield reliable sequences. Although enormously interesting to evolutionary biologists, such ancient DNA cannot be used to recreate extinct organisms. Conservation geneticists must concentrate on saving existing biodiversity; they cannot fall back on the idea of being able to recreate extinct species from the tiny fragments of DNA (typically less than 0.00001% of an organism's genome) currently under scrutiny.

Genetic data, once acquired, are used by conservation geneticists to quantify within-and between-population variability. Variability within populations is used to establish pedigree relations, mating system, sex ratio, and genetic effective population size (N e ). Interpopulation comparisons reveal spatial structuring and historical patterns of gene flow. Geographic variation is normal within a species and the new field of phylogeography uses genetic information to infer the historical relationships among populations. Single and multilocus genetic differences between kin, populations, subspecies, and species are expressed as genetic distances. These metrics and their interpretations are beyond the scope of this review, but it is important to note that the absolute values of genetic distances will vary between different groups of plants and animals and increase over geological time. Within a group of related species a major difference in the genetic distances observed within and between taxa can be used to define species and other evolutionarily significant units for conservation management purposes.

Although the previous discussion focused on molecular genetic variation, the growing field of quantitative trait genetics promises to provide a new means of measuring evolutionarily significant population variables (Storfer, 1996) . Quantitative genetics is concerned with characters such as morphology, behavior, parasite resistance, and physiology that are controlled by several to many genes that work additively, in dominance/recessive relationships, or epistatically. Such oligogenic and polygenic control, involving quantitative trait loci (QTL), is characteristic of many traits of interest to conservationists-those that effect long-term population persistence and evolvability. QTLs affecting body size, hatching date, and predator avoidance behavior (escape speed), for example, are ecologically important and are arguably of greater significance to conservationists than allozyme polymorphism. Conservationists are therefore interested in the heritability of such traits that have a direct impact on fitness. High heritabilities of a QTL indicate that a population has great potential for trait evolution and low heritabilities indicate a more limited ability to respond to environmental change. Unfortunately, such heritability is difficult to measure because it requires pedigree-level studies conducted over several generations or long-term manipulative experiments such as controlled garden plots. Heritability (h 2 ) is the ratio of the variance of the genetically inherited proportion of a trait (the additive genetic variance, V A ) to the total phenotypic variance V P measured in the population. V A is one component of genetic variance (V G ) which also includes nonadditive genetic variance due to dominance (V D ) and epistasis (V I ); only V A responds to directional selection. Estimating V A is further complicated by the need to estimate variance due to the environment (V E ) as well as the other genetic components. Nevertheless, the preparation of studbooks for captive populations of endangered animals and the comparison of laboratory-raised seedlings of rare plants to their parents in the wild have provided opportunities for the first limited applications of QTL analyses to conservation. Future technical advances may permit the inclusion of QTL in the conservation geneticist's tool kit of predictors of a population's risk of extinction.

A major issue requiring resolution is the validity of the widely held relationship between molecular genetic variation and ecological viability and evolutionary po-tential. Are single gene markers (those typically surveyed by geneticists) useful indicators of variation at quantitative traits? Are multilocus allozyme surveys unreliable as predictors of a population's viability since heterozygosity may be only weakly correlated with the additive genetic variation associated with QTLs? In desert topminnows of the genus Poeciliopsis, Vrijenhoek (in Avise and Hamrick, 1996, pp. 367-397) found that rapid losses of heterozygosity in small, isolated populations were associated with a decline in fitness that was manifested as poor competitive ability, growth rate, developmental stability, and resistance to parasites. O'Brien (1994) , in a remarkable series of genetic studies of captive and wild cheetahs (Acynonyx jubatus), demonstrated a powerful association between low levels of genetic variability and susceptibility to viral diseases.

There is no ''normal'' level of variation for a population-even as determined with a specific method. Cheetah, northern elephant seal (Mirounga angustirostris), and European badger (Meles meles) are ecologically successful despite low absolute levels of genetic variation. Different types of genetic variability will respond differently to natural selection, inbreeding, population collapse, and range fragmentation. Conservation geneticists can identify cases in which variation has or is being lost, establish the causes of the loss, and make recommendations to counter its ultimate effects. Because currently available genetic markers are only proxies for fitness determinants, this underscores the importance of using different markers. It is not the purpose of this review to discuss the technical problems associated with each method or the applicability of the different methods to different groups of plants and animals, but it is important to emphasize that each method has unresolved technical problems (e.g., null alleles, allelic dropout, pseudogenes, and nonreplicable patterns). Until molecular and QTL genotyping become routine, conservation geneticists must guard against the natural tendency to overstate the statistical power of their results.

Long before the term conservation genetics was coined, the phrase ''genetic conservation'' was introduced to describe the science of managing specific genes or phenotypic traits in crop plants, land races and cultivars, bacteria and fungi used in food production, and domes-ticated animals. Genetically modified organisms (GMOs) are simply special cases that require even more intensive management for their perpetuation. The methods of gene discovery and artificial selection developed for managing microorganisms, plants, and animals are relevant to the far more broadly focused field of conservation genetics, but very few wild species have received such intensive effort.

Discussions of the need to save this or that desired gene in a population are in fact arguments for saving a particular allelic form (variant) of a gene and not the gene itself. Some alleles are common and others are rare. Deleterious alleles (e.g., alleles responsible for albinism or other genetic ''defects'') are typically very rare and have frequencies of less than 0.0001. Conservation geneticists are often asked to devise breeding plans that will further reduce or even eliminate such alleles from a population. On the other hand, it has been argued that conservation geneticists should strive to save rare alleles in threatened populations because they may prove vital for a population's adaptation to future environmental changes. Although this argument is reasonable, the maintenance of desirable rare alleles, even if they were identifiable, requires very large population sizes (N e Ͼ 5000) and is simply not possible in most management programs. Rare alleles contribute very little to variation in fitness among individuals and are less likely than alleles at relatively high frequency to be the basis of adaptive response to environmental change.

It has been suggested that conservationists should focus on saving diversity at major histocompatibility complex (MHC) genes because they play a role in recognition of infectious agents, disease susceptibility, and defense. This recommendation was well intended but rejected because the functional (fitness-related) significance of the large number of alleles at each of the many MHC genes is unknown. Managing them as a single linkage group would require very large populations or the inevitable loss of variation at other potentially important loci.

Interest in the concept of minimum viable populations (Gilpin and Soulé, 1986 ) spawned the development of new methods of pedigree analysis and population viability analysis. Populations have both a census size (N) and a genetic effective population size (N e ); the latter is one of the most important concepts in theoretical conservation biology and is defined in Section III,G and by Lande and Barrowclough (1987) . The characterization of genetic variation within and among popula-tions enables geneticists to help set conservation priorities. Comparative levels of variation and gene flow (or lack of either) provide clues to a population's viability and extinction proneness. Data on genetic relationships among populations guide translocation decisions and identify well-defined clusters of populations for management as separate entities.

Pedigree analysis refers to a suite of genetic models for understanding processes in small populations. First developed for assessing management practices in captive populations, pedigree analysis is also applicable to wild populations of individually monitored organisms. It is used to establish kinship and individual founder contributions, to identify genetically desirable and undesirable individuals and their descendents, to minimize inbreeding, to describe population structure and mating system, and to choose individuals for reintroduction or translocation. Examples of pedigree analysis with the Gene Drop computer program include Haig's (1998) study of the red-cockaded woodpecker, Picoides borealis. Pedigree management programs based on mean kinship or equalizing founder contributions seek to minimize inbreeding in local subpopulations and in the metapopulation as a whole. Captive breeding programs have been successful in slowing the loss of genetic variation and preventing inbreeding depression.

Population viability analysis (PVA) is the methodology used to assess the ecological and genetic risks facing a wild or captive population and to develop a conservation management plan. PVA refers to a suite of mathematical models that seek to predict the probability of a population's extinction by some time in the future, e.g., 20 or 100 years. Early models considered demography (growth rate, present population size, and birth rate) and environmental stochasticity, but Gilpin and Soulé (1986) broadened the definition to include genetic factors. Genetic factors, including genetic drift and fixation of deleterious mutations, are expressed through demographic factors that affect population dynamics. The genetic factors thus contribute to extinction probabilities through a very complex and little understood series of interactions affecting fitness. Geneticists can use population variation to provide estimates of the various parameters of interest to modelers and managers. Unfortunately, genetic models with linkages to ecological factors are still insufficiently developed to yield the type of statistically powerful PVAs managers seek. Most PVAs, in fact, have not included genetic parameters, but this is changing as the significance of genetics in the long-term survival of small populations becomes more widely appreciated (Beissinger and McCullough, 2000) .

Metapopulations are populations of subpopulations within some defined area, in which dispersal from one local population (subpopulation) to at least some other habitat patches is possible. There is significant turnover of local populations, local extinction, and recolonization by dispersal. The metapopulation concept is central to much ecology and conservation theory and singleand multiple-species metapopulation dynamics are reviewed by Hanski and Gilpin (1996) . The genetic effective size of a metapopulation is affected by the carrying capacity of the habitat patches, the rates of extirpation and recolonization, the number and source of the founders, the number of local populations, and the rate of gene flow between patches. It is difficult to establish metapopulation effective size using genetic data because it is strongly affected by extirpation and recolonization dynamics. As in the case of genetic effective population size of single populations, the metapopulation effective size is 10-100 times less than the census size in many species. Metapopulation dynamics, with frequent local extinction and recolonization of habitat patches by few founders, can reduce N e to a small fraction of N, with a resulting loss of genetic variability like that associated with a demographic bottleneck. Detailed studies of metapopulations are reviewed by Hanski and Gilpin (1997) and include the Glanville fritillary butterfly (Melitaea cinxia) and the red bladder campion (Silene dioica), both in Sweden; the checkerspot butterfly (Euphydryas editha) restricted to serpentine outcrops in California; and the pikas (Ochotona principes), a small mammal restricted to isolated talus slopes in alpine areas. Although the theory of metapopulations is well developed, and the relevance of metapopulation theory to the management of small, semi-isolated populations of threatened species is clear, the empirical testing of the ecological and genetic predictions is only just beginning.

Using molecular markers to characterize mating system, population structure, and phylogeography permits recognition of management units (MUs; sets of populations with shared distinctive alleles frequencies) and evolutionarily significant units (ESUs; sets of populations distinguished by strong phylogenetic structure based on multilocus mtDNA or nDNA variation). Moritz (in Smith and Wayne, 1996, pp. 442-456) describes Australian examples of MUs (the yellow-footed wallaby, Petrogale xanthopus, which is patchily distributed on isolated rock outcrops in southwest Queensland) and ESUs (four ghost bat ESUs previously treated as a single widespread species, Macroderma gigas, whose evolutionary divergence was not apparent until genetic methods were applied). Although the criteria used to define these two categories are not agreed upon, ESUs repre-sent deep phylogenetic subdivisions typically within a species or occasionally, as in the case of local endemics, the entire species, and MUs reflect shallower subdivisions within a species which for practical reasons become the focus of management activities. Translocations between ESUs are typically undesirable.

Subspecies or local geographic races have been the focus of intense conservation efforts. Genetics is useful in establishing whether such groups of populations are sufficiently different to warrant separate conservation efforts. The conservation of every local race or subspecies is difficult to justify if they are genetically almost identical. Although we may recommend trying to preserve every variety of wild tomato or maize in a seed bank, it is difficult to use the same justification to try to conserve all local variants of geographically widespread organisms.

Many biologists have abandoned the subspecies concept and the associated trinomial nomenclature. To them, the subspecies is an evolutionarily insignificant artificial taxon typically based on a few superficial morphological features. There are numerous cases in which genetic studies provide no support for the traditional subspecific taxonomy. Nevertheless, there is much to be learned from the geographic patterns of variation in nature. Subspecies were typically defined as geographical races with allopatric or parapatric distributions. The observation of significant hybridization without introgression, in the latter cases, led to the development of the semispecies concept which is of relevance to conservation. Semispecies are typically parapatrically distributed and hybridize where their ranges meet, but they show very limited introgression. They are effectively isolated groups of populations evolving independently of one another. Collectively, a group of semispecies comprise a superspecies and each semispecies is treated taxonomically as a full species. In this case, geographically defined taxa that used to be treated as subspecies are actually independent evolutionary lineages and therefore as worthy of conservation as other ''good'' species.

Conservation geneticists are often asked to advise on proposals to pool individuals from allopatric populations on the argument that it is better to save a generic species than no species. Although poorly differentiated subspecies may often be mixed without genetic harm, the pooling of individuals of well-differentiated semispecies or species is likely to have negative genetic consequences. Unfortunately, management decisions are of-ten made before the appropriate genetic data are available.

Geneticists are often involved in two other situations involving subspecies and local variants. The first involves populations on either side of national or state boundaries that may be assigned to different subspecies and can receive radically different levels of protection; a species-wide conservation plan may allocate resources differently and be biologically preferable. The second situation occurs when peripheral populations of a widespread species become the focus of conservation activities. Although such peripheral populations may be at high risk of local extinction, their conservation may not be warranted, especially in cases in which reintroduction is practical. On the other hand, some peripheral populations may be critical to a species' long-term survival. Some peripheral populations may be better adapted to changing climatic conditions than central populations even though the latter may be more genetically variable. At a time of global warming, poleward peripheral populations of a species may be more important than those closest to the equator.

As it is thought that half the species of larger vertebrates are at risk of extinction in the next 100 years, most discussions of the conservation of biodiversity focus on species. Species are fundamental units of evolution and classification (taxonomy). However, despite their centrality to the field, conservation geneticists rarely work at the level of whole species and concentrate instead on infraspecific levels of organization.

Ernst Mayr introduced the biological species concept in 1942; species were defined as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. This concept stimulated an enormous amount of research in the second half of the twentieth century. Despite its impact, problems with Mayr's original definition, with its overemphasis on reproductive isolation and its focus on sexually reproducing outcrossing populations over short time spans, led to the development of at least 18 alternative concepts by the end of the century. Although some of these have limited operational utility, three newer biological species concepts are of particular relevance to conservation geneticists: Wiley's evolutionary species concept, Templeton's cohesion species concept, and Mallet's genetic cluster species concept. All seek to recognize discrete groups of populations with a shared evolutionary future. All three concepts, like Mayr's, seek to capture the essential genetic relatedness within, and the genetic distance between, biological species. All allow that the absolute values of observed relatedness and distance will vary with the geological age of a species and will vary in different groups of plants and animals.

Mayr's biological species concept forced researchers to search for reproductive isolating mechanisms between species in nature and to investigate their potential significance in laboratory and greenhouse hybridization experiments. Such work typically took years and the results were often compromised by methodological limitations. It became clear that morphology was not always a good indicator of species boundaries and not surprisingly that traditional taxonomy was often a poor guide for conservation decision making. The introduction of protein electrophoresis and molecular genetic methods of measuring genetic variation has dramatically changed the approach to defining species. It is now possible to quickly establish whether populations exchange genes or whether they are effectively reproductively isolated. It is possible to estimate genetic distances between groups of populations and gauge their significance in comparison to within-population variation. It is possible to estimate the time since a speciation event and the historical patterns of gene flow within and between taxa. Our newly found ability to characterize and recognize species genetically does not diminish the value of field and laboratory studies of behavioral ecology, but it does permit geneticists to make powerful contributions to the practice of conservation.

Although it is fortunately still unusual for geneticists to work on the conservation of entire species, there are an increasing number of cases in which every individual in a species has been genotyped to some degree for management purposes. Such cases include Przewalski's horse (Equus przewalski), San Clemente loggerhead shrike (Lanius ludovicianus mearnsi), whooping crane (Grus americana), and Catalina mahogany (Cerocarpus traskiae).

Although it is unusual to think of geneticists working at higher levels of organization than species, regional multispecies phylogeographic surveys are useful in defining the historical interactions of whole communities of organisms. Studies of regional phylogeographic structure, as for example in the southeastern United States or savanna ecosystems of Tanzania, are relevant to the design and maintenance of biodiversity sanctuaries (Avise, 2000) . Elsewhere, populations in ecotonal regions have been found to have higher gene diversity and are thus recommended for higher conservation priority (Smith and Wayne, 1996) .

Mutations, the occurrence of heritable changes in the genetic material, are typically very rare processes that ultimately provide the raw material of genetic variation upon which the other agents of evolution operate. Mutations span a wide array of phenomena; from single base pair changes in the genetic code to accidental doublings of the number of chromosomes in a gamete. Many mutations are deleterious or lethal, some are near neutral, and a few may be beneficial to the carrier. The vast majority of mutations are completely invisible in the phenotype and can only be detected with an array of genetic techniques.

Mutations become of concern to conservationists in a couple of circumstances. First, the presence of a normally very rare allele of major effect in a remnant or closed captive population can have serious consequences. Deleterious traits discussed by Ryder and Fleischer (1996) include hairlessness in red-ruffed lemurs (Varecia variegata ruber), funnel chest in blackand-white ruffed lemurs (Varecia variegata variegata), and congenital diaphragmatic hernia in golden lion tamarin (Leontopithecus rosalia). Second, artificial selection for rare alleles is sometimes the goal of captive breeding programs. White tigers (Panthera tigris), homozygous for a recessive allele, are beautiful but suffer genetic disease with metabolic, nervous, and developmental consequences. Selection for such traits in the context of the conservation of an endangered species is unjustified.

As most populations of conservation concern lose genetic variability, the question arises as to whether new mutations will replace variability lost by population extirpation and genetic erosion. The answer is yes and no. Given that mutation rates are typically on the order of one per 10 5 cell divisions, the time for the accumulation of new variants in a population is on the order of tens of thousands of years. Therefore, conservation geneticists are more concerned with the deleterious effects of mutations in small populations than with their very long-term benefits.

Extinction due to genetic causes is almost unknown, but their contribution to the process should not be ignored. Although natural selection purges deleterious alleles from populations almost immediately, mildly deleterious, near-neutral mutations will gradually increase in frequency and become serious problems when their frequencies exceed 0.05 or 1/(2N e ). Fortunately, this process takes hundreds of generations in all but the smallest isolated populations. Therefore, although such mildly deleterious mutations are rarely considered by wildlife conservationists, they will ultimately diminish the long-term viability of many threatened taxa. On the other hand, managers of captive populations have to recognize this threat from the outset. If the goal of a breeding program is to return a captive population to the wild, then managers should maximize genetic variation including mildly deleterious mutants. Alternatively, if a population cannot be returned to the wild and must be sustained in captivity for many generations, then managers will need to purge deleterious mutations as they are identified.

Mutation rates at near-neutral genes controlling quantitative characters set a lower limit on the population size necessary for future evolution. Harmful mutation rates set lower limits for population sizes for avoiding inbreeding depression and for preventing genetic erosion of fitness by the accumulation of mildly deleterious mutations. The suggestion that small populations (N e Ͻ 100) may decline in fitness with the accumulation of mildly deleterious mutations, termed mutational meltdown, is under theoretical and experimental study.

Although much population genetic theory is premised on ''random mating,'' such behavior is rarely observed in nature. Even related species may have very different mating systems with very different genetic consequences. Self-fertilization in hermaphroditic organisms and obligate outbreeding in dioecious organisms are the two extreme modes. Conservation managers have to be aware of these differences if they are to mimic a species' natural history.

The most extreme examples of conservation problems involving mating systems involve cases in which the last surviving individuals in a sexually reproducing population all belong to the same sex. The last passenger pigeon (Ectopistes migratorius) was a female; the last member of one of the 11 surviving subspecies of Galapagos tortoise (Geochelone elephantopsus) is a male, Lonesome George. It is likely that cloning technologies currently under development will be applicable to saving such lineages in the future.

Inbreeding refers to the mating of close relatives in species that are normally outbreeding. Matings between father and daughter, brother and sister, or first cousins are examples of inbreeding. Many species of plants and animals have evolved devices to minimize close inbreeding. Species vary greatly in their tolerance of inbreeding; some trees and dioecious plants are obligate outcrossers. The genetic underpinnings of inbreeding depression are best understood in Drosophila, in which recessive lethal mutations and mildly deleterious mutations are major causes. Gradual inbreeding permits natural selection to purge the former but the partially recessive near-neutral mutations continue to increase in frequency and significance. Outcrossing populations that suddenly decline in numbers usually experience reduced viability and fecundity known as inbreeding depression. Inbreeding produces increased homozygosity of recessive partially deleterious mutants and by chance in small populations these alleles become fixed. In the simplest genetic situation of a trait under the control of a lethal recessive allele, there is an increased risk that the offspring of two related healthy but heterozygous individuals will inherit the harmful allele from each parent and die. Although the risk in this case is only one in four, this amounts to a powerfully strong fitness differential on which natural selection will act. Generalizing from this simplest single-locus case, geneticists speak of inbreeding depression as the manifestation of the whole genomic effects of mating of close relatives. These effects may involve outright genetic disease (congenital abnormalities) but are more often subtle and appear as decreased growth rate, behavioral abnormalities, and reduced fertility and fecundity. Therefore, inbreeding is rare in typically outbreeding populations but becomes a serious problem in small isolated populations. In small fragmented populations in nature and in captive populations, inbreeding depression can threaten population viability. Animal breeders learned these lessons from centuries of experience with artificial selection, and they limit inbreeding rates to less than 2% per generation. The consequences of very close inbreeding are well illustrated by experience with establishing ''inbred'' strains of laboratory mice; the majority of inbred lines die out within 10 generations.

There is abundant evidence that captive wildlife populations suffer inbreeding depression. Ralls (in Schonewald-Cox et al., 1983, pp. 164-184) was the first to show that even well-intended captive breeding programs subjected small populations to inbreeding depression. She reviewed empirical records for 40 species, mainly ungulates, in zoos and found in-breeding to be a problem in most cases. Inbreeding is also associated with decreased growth rate and blindness in a captive wolf population in Sweden. In the wild, the final decline of the heath hen (Tympanuchus cupido) on Martha's Vineyard island in 1932 involved inbreeding effects. Other better documented cases (Hedrick in Smith and Wayne, 1996, pp. 459-477; Lacy, 1997) involving declining or threatened wild populations involve the middle spotted woodpecker (Dendrocopos medius) in Sweden, the Florida panther (Puma concolor coryi), Barrow Island populations of the black-footed rock wallaby (Petrogale lateralis), common shrews (Sorex ananeus) in England, deer mice (Peromyscus polionotus), and Glanville fritillary butterfly (Melitaea cinxia) in the Aland Islands, Finland (Saccheri et al., 1998) .

In 1980, Franklin and Soulé independently showed, based on theory and experiments, that inbreeding depression can be avoided in the short term if N e Ͼ 50. The inbreeding coefficient F increases by 1/2N e per generation and centuries of animal breeding experience shows that a 1% increase in F per generation is tolerable; thus, an N e ϭ 50 is necessary to avoid inbreeding depression. Franklin and Soulé further concluded that an N e Ͼ 500 was necessary to enable a population to continue to evolve in the long term. Although this 500 number has been revised upwards, the simplicity of the Franklin-Soulé numbers caught the attention of managers and legitimized the role of genetics in conservation. The theory behind the 50 number is still accepted (Lande, 1999) , but it is important to realize that its derivation was based on controlled laboratory experiments; larger N e s are required in nature, where environmental fluctuations are more severe and stressful.

Outbreeding, or the crossing of unrelated individuals, is widespread in nature. It is widely believed that sexual reproduction evolved in part because chromosomal crossing over and recombination facilitated by outbreeding produces more genetic variability than do other mating systems. Many species of plants and animals have effective immunological and behavioral mechanisms to favor outbreeding. The latter include sex-biased dispersal of young adults from their natal population and elaborate courtship behaviors. Even in plants with both male and female flowers, outbreeding is ensured by asynchronous maturation of male and female gametes and the evolution of various self-incompatibility systems.

Outbreeding depression occurs when very distantly related conspecific individuals are mated or when members of two different but related species hybridize. The male and female genomes are sufficiently different to produce a hybrid with genetic disorders. Conservation geneticists encounter outbreeding depression in inadvertently mixed captive populations. Sterility, or partial sterility in one sex, and high neonate mortality are commonly observed manifestations.

Outbreeding depression occurs in nature in some hybrid zones between semispecies and species of plants and animals. Hybrids are interesting because they show that the evolution of many groups of plant and animal species involves both lineage splitting and lineage anastamosis. Fertile interspecific hybrids permit gene flow between species (introgression). Hybrids call into question species definitions that overemphasize reproductive isolation. The notion that ''species'' are somehow ''purebred'' and always reproductively isolated from their close relatives is not borne out by observations of some animal groups and many plants, in which low rates of hybridization between congeners often occur in nature.

In the past, it was argued that hybrid populations did not qualify for legal protection under the U.S. Endangered Species Act. However, hybrids are very much a normal part of nature. Rare or very rare in some groups, and more common in others, they present conservationists with a dilemma because their occurrence appears to diminish the value of a taxon. Should one save Florida panthers if they are known to harbor genes of introduced South American panthers, a different subspecies? Do Texas red wolves (Canis rufus) merit conservation if they are gray wolf-coyote hybrids? Should one save the remaining Przewalski's horses if it is shown that historical mismanagement resulted in a large fraction of the surviving animals being tainted by the genes of domestic horses, a karyologically distinct species? Whether hybrids should be afforded the same priority as nonintrogressed populations or species will remain controversial; in the previous cases, the answer was yes and geneticists contributed to pedigree management.

Habitat disturbance can result in increased opportunities for hybridization between species that would not normally interbreed. Fragmentation of the recently continuous Pacific Northwest old-growth forest has led to hybridization between the declining northern spotted owl (Strix occidentalis) and the barred owl (S. varia), which favors disturbed sites.

Hybridization is more common in plants than in animals; therefore, not surprisingly it is in plants that there are numerous examples of rare species being hybridized into extinction (genetic assimilation) by hybridization with a more common sympatric congener (Soltis and Gitzendanner, 1999) . This is the case for the Catalina Island mahogany, in which 5 of the remaining 11 adult trees are actually hybrids with the more common mountain mahogany (Rieseberg and Swensen in Avise and Hamrick, 1996, p. 305-334) .

Other cases involve plants (Asteracea: Argyranthemum) in the Canary Islands. The Simien jackal (Canis simensis) of Ethiopia is at risk of being introgressed into extinction by hybridization with domestic dogs. It is now recognized that restocking rivers with genetically uniform hatchery-bred salmon has contributed to the collapse of the Pacific Northwest salmon runs. Hatchery fish show reduced fitness in the wild (they are not locally adapted) and compete and hybridize harmfully with the remaining wild salmon (Lande, 1999) .

Gene flow is a fundamental agent of evolution based on the dispersal of genes between populations of a species. It involves the active or passive movement of individual plants, animals, gametes, or seeds. Gene flow involves not just dispersal but also the successful establishment of the immigrant genotypes in the new population. Gene flow is often confusingly referred to as migration, but the latter term is best reserved to describe dispersal behaviors involving a seasonal or longer term round-trip. Gene flow tends to homogenize linked populations and lack of gene flow permits interpopulation differentiation. It is of interest to geneticists and managers in that to conserve a population one needs to establish the historical patterns and rates of gene flow. This is typically estimated from allele frequency data and reported in terms of the number of ''migrants'' per generation. In theory, one migrant per generation between two populations will ensure that they remain genetically homogeneous. Inbreeding depression can be ameliorated by the artificial translocation of one reproducing migrant per generation between populations. Gene flow is often gender biased and limited to certain phases of the life cycle. It may be accelerated under certain climatic conditions that occur at frequencies of many years or at irregular intervals many years apart. Interspecific gene flow results in introgressive hybridization (discussed previously). The translocation of individual organisms results in gene flow if they reproduce at the release site. In the future, genetically depauperate populations will be enhanced by transloca-tion of individuals from more secure areas. Unfortunately, such genetic enhancement carries risks associated with the introduction of pathogens that could harm the target population or completely unrelated species. Furthermore, the introduction of individuals from genetically well-differentiated source populations may result in outbreeding depression in the threatened population of conservation concern (discussed previously). Gene flow can thus erode the genetic basis of adaptation to local conditions. If previously continuous populations become fragmented, historical patterns of dispersal and gene flow may be disrupted with potentially serious consequences for population viability. For example, if young female chimpanzees can no longer emigrate from their natal social group because of habitat destruction in the surrounding countryside, their isolated natal population will experience increased inbreeding.

Genetic drift involves the loss of alleles from a population by chance. Random fluctuations in allele frequencies in small populations reduce genetic variation, leading to increased homozygosity and loss of evolutionary adaptability to change.

The rate at which alleles are lost from a sexually reproducing population by genetic drift can be predicted. Sewall Wright (1969) developed the basic theoretical model in 1931 and showed analytically how the rate varies with population size. Actually, it is not the census size (N) that is important but rather the genetic effective population size (N e ). This parameter takes into account the fact that closely related individuals will share alleles by common descent. Monozygotic twins are genetically identical and therefore should be counted as one individual rather than two. Sibs share half their genes with each other and half with each of their parents and are therefore not equivalent to two genetically unrelated individuals. The genetic effective number of individuals in a population is therefore almost always less than the number of individuals counted by an ecologist. N e can, under some breeding systems, be one or two orders of magnitude less than N. Consider, for example, the number of adults in a sexually reproducing population: In a monogamous species the census count of adults is useful, but in a harem species only 1 of the 10 males may be contributing to N e . N e can be variously defined in terms of unequal sex ratios among breeders, fluctuations in population size over several generations, and variance in family size (Lande and Barrowclough, 1987) . Wright (1969) defined the variance effective population size (N e ) as the number of individuals in an ideal population that would experience genetic drift at the same rate as the actual population. N e can be defined and estimated in various ways using temporal ecological data, DNA sequences, and various methods of estimating migration rate. Some methods of estimation have theoretical value but little operational utility-it is almost impossible to determine the values that some algorithms require. Nevertheless, by estimating N e one can assess the effects of different population management strategies. Unequal numbers of males and females, increased variance in family size, and temporal fluctuations in N all cause N e to be much less than the census size, N. In many endangered populations N e is only 10-30, and at such levels genetic variation becomes significant for a population's viability.

Sudden population declines followed by recovery in numbers are referred to as population or demographic bottlenecks. They can have an immediate impact on variability at molecular genetic loci as genetic drift robs the population of its innate variation. The evidence of a bottleneck may persist for hundreds of thousands to millions of generations in low levels of variation at allozyme and molecular genetic marker loci. However, large populations that are almost isogenic at such loci may maintain high heritable variance in QTL, low inbreeding depression, and high heterozygosity for simple repetitive microsatellite DNA because variation at QTL may return to outbred levels in 10 3 to 10 4 generations. Furthermore, bottlenecks can actually result in a short-term increase in population variation because epistatic variation (due to interactions among genes controlling a trait) is converted into additive variation. Whether such release of previously hidden variation is beneficial or harmful to population viability is unknown.

Sudden reduction in N results in a loss of fitness unless there is a rapid and sustained recovery. Gradual reduction, on the other hand, permits natural selection to purge recessive lethal mutations and avoid a substantial part of inbreeding depression. The best advice a geneticist can give the manager of a collapsed population is to increase N as fast as possible and then worry about genetics.

Very low variability is known for many sexually reproducing species whose currently large populations have recovered from one or repeated brushes with extinction. If a large variable population collapses, then the few individuals that survive the catastrophe carry only a fraction of the original population's genetic variability through the demographic bottleneck. By chance, some individuals and the alleles they carried are lost to genetic drift. Only one of six mtDNA haplotypes survived the severe bottleneck (N ϭ 14) in the whooping crane in 1938. Genetic drift becomes a significant agent of evolutionary change in small populations. Drift may account for the very low levels of variability observed in African cheetahs and northern elephant seals. Cheetahs were not known to be genetically less variable than other cats or at genetic risk until half of a large captive breeding colony died soon after being exposed to a common domestic cat coronavirus (feline infectious peritonitis virus). Although the northern elephant seals are known to have recovered after having been overhunted to near extinction in the late nineteenth century, the low levels of variation in the cheetah may be attributable to metapopulation dynamics rather than a classic population collapse. Metapopulation structure, with frequent extirpation and recolonization of subpopulations, can reduce metapopulation N e orders of magnitude below the census populationsize and mimics the genetic effects of a demographic bottleneck.

Genetic erosion, the decrease in population variation due to random genetic drift and inbreeding, is both a symptom and a cause of endangerment of small isolated populations. Population genetic theory shows that variation will be lost by genetic drift with an almost clocklike regularity (Wright, 1969) . In closed populations, in the absence of factors promoting genetic variation (mutation and gene flow) the expected rate of loss of heterozygosity, or rate of loss of genetic variance in quantitative characters or selectively neutral variation, is 1/2N e per generation. Little variation is lost in any one generation but small N sustained for several generations can severely deplete variability. Most variability is lost within 2N e generations. An effective population of 10 is predicted to lose heterozygotes five times faster than a population of effective size 100; 50% of its heterozygosity will be lost in approximately 20 generations. Therefore, in theory, small isolated populations have a higher rate of loss of heterozygosity and are expected to have lower levels of genetic variation than large continuously distributed populations. Because variability is inherently related to evolvability, genetic erosion in small, recently fragmented populations may contribute to their endangerment. Barrett and Kohn (1991) and Young et al. (1996) review the growing literature of the population genetic consequences of habitat fragmentation for plants. There are numerous examples of a positive relationship between N e and population genetic variation at allozyme loci for remnant populations. Ouberg and colleagues conducted experimental investigations of genetic erosion with Scabiosa columbaria and Salvia pratensis plants under common garden conditions and found positive correlations between variance for adaptive traits related to growth rates and population size. Recently, others studied Clarkia pulchella and found an increased probability of extinction associated with decreased N e . Such experimental studies indicate the potential significance of genetic erosion in natural populations.

The phenomenon of genetic erosion has long been understood in terms of population genetic theory, but the critical early stages of the process in nature have gone undocumented because the changes are rapid and difficult to monitor. I developed a new molecular genetic method for monitoring genetic erosion and provided evidence for its commencement in mammal populations isolated recently in small forest fragments (Srikwan and Woodruff, 2000) . An opportunity to study genetic erosion empirically arose when 165 km 2 of lowland rain forest were flooded in 1987 following construction of a hydroelectric dam on Khlong Saeng, southern Thailand. Former hilltops became permanent islands in Chiew Larn reservoir and retained their original fauna of 12 species of small mammals. During years 5-8 postfragmentation, the demographic collapse of these communities and genetic erosion in three common species [a forest rat (Maxomys surifer), a tree mouse (Chiropodomys gliroides), and a tree shrew (Tupia glis)] whose populations were effectively isolated on some islands were monitored. Nuclear microsatellite markers are sufficiently variable to be used to monitor the process of genetic erosion in nature. As expected, small populations lost variability by genetic drift faster than large populations, and allelic variability is a better indicator of the onset of genetic erosion than heterozygosity. Interestingly, in one of the three species studied, genetic erosion commenced before detectable demographic decline.

This demonstration that the process of genetic erosion can be monitored in free-ranging natural populations provides numerous research opportunities because habitat fragmentation is a very ubiquitous phenomenon. Furthermore, the method can easily be upscaled to larger mammals of conventional concern to conservationists. Although monitoring genetic ero-sion in long-lived species may not be practical, much can be learned immediately by comparing isolated populations to those still more continuously distributed.

The importance of such rapid genetic erosion on population viability remains unclear because there are so few studies of the process in nature. Two larger questions remain to be answered: At what point (in terms of N and N e ) does genetic erosion threaten a population's viability? and What level of natural or artificial gene flow can protect a population from the negative effects of genetic erosion? Answers to such questions may emerge from the 35-year study of the decline and assisted recovery of an isolated population of greater prairie chicken, Tympanuchus cupido (Westemeier et al., 1998) . Unfortunately, there are very few studies of this duration.

Genetic enhancement, the introduction of selected individuals into a threatened population with the intent of maintaining or increasing its genetic variability and hence viability, is a conservation method in its infancy.

Although the genetics may seem straightforward, the translocation of new individuals carries a significant risk of introducing diseases into the threatened population. Furthermore, ill-planned genetic enhancement may lead to a breakdown of local adaptation (outbreeding depression) and actually decrease a threatened population's viability.

Natural selection, the differential survival and reproduction of some genotypes over others, is the major agent of microevolutionary change. It is of interest to conservation geneticists for two reasons. First, human activities can radically alter selection coefficients in both natural populations and managed ones. Such humaninfluenced evolutionary change is termed artificial selection whether or not it is intentional. Intense harvesting based on size or gender can cause rapid changes in behavior and natural history and reduce fitness. Examples include reduced body size in game fish and the impact of hunting only male horned or tusked mammals on social behavior. Second, one of the major challenges facing geneticists this century will be assisting species to adapt to ongoing global climatic changes. In the past, in the absence of humans, natural selection favored individuals adapted to change and many species shifted their ranges to accommodate major changes. Unfortunately, in the twenty-first century, the pace of environmental alteration and destruction is too fast for many species to respond. Conservation managers will have to intervene on behalf of many species if they are to survive.

Quantitative characters are typically under stabilizing selection and show some optimum phenotype from which they can evolve in response to environmental changes. This optimum balances current fitness with the need for future flexibility or adaptability. The maintenance of this variability imposes a fitness cost or genetic load on a population-the price for long-term evolvability.

The rate of directional selection that a population can manage, in response to some environmental change, is determined in part by its innate variability. Rapid anthropogenic changes, such as those associated with global warming, place a premium on genetic variation and adaptability, especially in fragmented populations. To maintain variability in quantitative characters (longterm adaptability), the Franklin-Soulé number for an effective population size of at least N e ϭ 500 is often cited. Revisionary work by Lande (1999) has shown, however, that an upward revision to N e ϭ 5000 is required. Such numbers are larger than those found in many endangered and threatened populations and underscore the need for genetic vigilance in their management and the importance of keeping numbers as high as possible.

The risk of extinction due to fixation of mildly deleterious mutations is comparable in importance to environmental stochasticity and could substantially decrease the long-term viability of populations with N e of less than a few thousand. The current recovery goals for many threatened species are inadequate to ensure their long-term viability if this requires N ϭ 10,000 individuals. Genetic and demographic factors, acting synergistically, require that minimum viable populations be Ͼ10 4 .

Until a textbook on conservation genetics is written, the reader must consult conference proceedings and the primary literature for examples of the application of genetics to conservation management. The journals Conservation Biology and Molecular Ecology are especially useful in this regard. Without making reference to the specific methods employed (because these continuously change), the following examples, in addition to those mentioned previously, are illustrative of the type of contributions conservation geneticists have made. Details may be found in Loeschcke et al. (1994) , Olney et al. (1994) , Frankham (1995) , Avise and Hamrick (1996) , Smith and Wayne (1996) , Lacy (1997) , Landweber and Dobson (1999) , and the specific references cited in the following sections. Olney et al. (1994) Geneticists have identified low genetic variability as a concern in wild and captive populations of many species, including cheetah, Californian Channel Island fox (Urocyon littoralis), Newfoundland black bear (Ursus americanus), Gir Forest Asian lions (Panthera leo), southern koalas (Phascolarctus cinereus), European bison (Bison bonasus), Arabian oryx (Oryx leucoryx), Père David's deer, and Torrey pine (Pinus torreyana). A loss of self-incompatibility alleles may pose a threat to reproduction in plants with genetically determined selfincompatibility systems such as the rare lakeside daisy (Hymenoxys acaulis) in Illinois.

Geneticists identified inbreeding as a probable cause of reproductive failures in populations of Ngorongoro lions (Panthera leo), Florida panther (Puma concolor coryi), Barrow Island black-footed rock wallaby (Petrogale lateralis), bighorn sheep (Ovis canadensis), Puerto Rican parrot (Amazon vittata), and the Isle Royale gray wolf (Canis lupus).

Geneticists have discovered that the mating systems of many species differ from expectations based on direct observations, often with profound implications for captive management and for management of ''wild'' populations. In some species, females preferentially mate with males outside their social group, e.g., Atlantic salmon (Salmo salar), blue tits (Parus caeruleus), and pilot whales (Globicephala sp.). Preferential interpod mating in long-finned pilot whales indicates the importance of conserving as many pods as possible. Genetics shows that highly gregarious black vultures (Coragyps atratus) are in fact monogamous and that Galapagos hawks (Buteo galapagoensis) are polyandrous. Despite behavioral observations suggesting a high frequency of matings between sibs in Australian splendid fairy wrens (Malurus splendens), genetics shows that outcrossing is the norm. In other birds, including stripe-backed wrens (Campylorhynchus nuchalis), geneticists found that subordinate males reproduce. The mating system of wild gray wolf, chimpanzees, and other species have been established and used to improve captive breeding programs or management in reserves. Establishing the mating system of the red-cockaded woodpecker led to improved estimates of N e and changes in the recovery program for a small population in South Carolina.

Geneticists have elucidated the hybridity of some taxa in both the wild and in captivity, including Przewalski's horses (many of which are domestic horse hybrids), Asian lions (most animals in Western zoos were hybrids between Asian and African lions and were removed from the breeding program), and the red wolf of Texas [shown to be primarily a natural coyote (Canis latrans) ϫ gray wolf hybrid]. Genetic data revealed the threat by hybridization to indigenous Scottish deer (Cervus elaphus scoticus) by introduced Japanese sika deer (C. nippon). Genetics was used to identify hybrids among the remaining Catalina Island mahogany and establish seedlings of pure Cercocarpus traskiae at several sites to protect the species from extinction by introgression. Genetic markers are being used to conserve remnant cutthroat trout populations by identifying and removing populations introgressed by hybridization with nonnative species.

Because every individual in most species is genetically distinct, it is possible to census populations by counting unique multilocus genotypes in an area. Geneticists have censused very difficult to count animals such as African forest elephants and the largely fossorial northern hairy-nosed wombats (Lasiorhinus krefftii) by individually genotyping dung samples. Fecal genotyping (including sexing) has also been used to provide more accurate census data of animals such as coyotes and bears than could be obtained from long-term ecological surveys. The genetic data also provided information on home ranges and pedigree relatedness without requiring that the animals be seen or disturbed.

Population Structure Avise (2000) reviews many examples of phylogeographic studies that provide managers with essential data on population structure and on the characterization of MUs and ESUs. Geneticists have also provided estimates of historical gene flow between populations that would be impossible to obtain by direct observation (e.g., chimpanzees, humpback whales, and green turtles). Comparative phylogeographic studies of four sympatric species of East African bovids underscore the dangers of extrapolating results from one species to another and support conservation efforts that take species-specific differences into account.

Karyological differences distinguish Bornean and Sumatran orangutans (Pongo pygmaeus) and captive breeding programs are now managed to prevent hybridization of the two. Dik-dik species (Madoqua) can also be distinguished karyologically and neonate mortality in captive populations has been reduced following the sorting of animals by karyotype. MtDNA sequences have been used to sort sibling species and subspecies of gibbons (Hylobates sp.) of unknown geographic origin into correct ESUs, to distinguish sibling species of chimpanzees (Pan troglodytes and P. verus) that have been mixed in captivity, and to identify subspecies of black rhinoceros, Diceros bicornis. Genetics was used to show that the conservation of the living fossil tuatara (Sphenodon) of New Zealand depends on the management of not one but two genetically recognizable species. There are many cases in which genetic data justify conservation efforts for isolated subspecies or varieties that are shown to be genetically well differentiated-for example, Darwin's fox (Dusicyon fulvipes) of Chile, Kemp's ridley turtle (Lepidochelys kempi), San Clemente Island loggerhead shrike, and several ''subspecies'' of Hawaiian Amakilu honeycreeper (Hemignathus virens). The Mexi-can wolf (Canis lupus baileyi) was found to be a genetically distinct ESU, untainted by hybridization with gray wolf, coyote, or dog. The endangered San Clemente Island shrike is illustrative of several of the points made previously because it is technically a subspecies of a widespread mainland bird and might accordingly be written off as a peripheral population, local variant, or ''just a subspecies.'' Taxonomic practice and field observations notwithstanding, my genetic survey revealed no evidence that it has hybridized with the neighboring mainland subspecies since 1915 despite repeated opportunities. It is genetically differentiated and apparently reproductively isolated and merits management as a separate ESU (Mundy et al., 1997) .

Geneticists have also provided data questioning the justification of other taxon-focused conservation efforts. The 27 original subspecies of leopard (Panthera pardus) were found to be referable to only 8 genetically defined subspecies or ESUs. Similarly, genetic variation provides no justification for conserving all 30 described subspecies of puma (Puma concolor). The dusky seaside sparrow (Ammodramus maritimus nigriscens), after considerable and unsuccessful efforts to save it, was shown to be only a marginally distinct local race of a common widespread species.

Taxonomic status (species or subspecies) does not automatically lead to the justification of conservation efforts. Proposals to reduce the eastern Pacific black sea turtle, Chelonia agassizii, to a subspecies of the widespread green turtle, C. mydas, are supported genetically but do not justify the abandonment of this taxon as a high conservation priority.

A phylogeographic survey of 14 subspecies of the songbird bananaquit (Ceoreba sp.) from 15 West Indian islands and the mainland of South and Central America showed that if it were necessary to restock the small and vulnerable populations on the northern Lesser Antilles Islands, the birds from nearby Puerto Rico or from Jamaica would be genetically inappropriate. Similar studies of orioles (Icterus sp.) are relevant to the conservation of the Montserrat oriole, I. oberi, which is under threat of catastrophic (volcanic) extinction.

Phylogeographic analyses have also helped to define natal homing patterns in marine turtles on foraging grounds. It was found that both green turtle and loggerhead turtle rookeries are demographically autonomous and that low levels of interrookery matrilineal exchange suggest that extirpated colonies are unlikely to recover by natural recruitment of nonindigenous females. Similar methods have been used to define stocks in whales and dolphin and elucidate the migratory strategies of different groups of humpback whales intermingled at the breeding ground near Hawaii.

Hierarchical phylogenetic analyses have been used to suggest conservation priorities among species of cranes. Within a group of related species the evolutionarily oldest monophyletic clades are considered to represent a greater genetic heritage than recently originated clades (Forey et al., 1994) . Similarly, it has been argued that areas with a disproportionate number of evolutionary ancient genotypes are more valuable than areas populated by recent colonists.

Two genetic generalizations complicate the prospects for successfully moving organisms around in the wild or for returning them to the wild after a period in captivity. First, the chances for successful reintroduction are diminished by evidence for rapid genetic adaptation to captivity in fish, plants, and Drosophila. The same problem applies to wildlife brought into captivity for reintroduction at some future time. Second, in plants, cryptic local adaptation results in the fitness of transplants being about half that of residents even when environments are apparently similar. This makes it difficult to reestablish populations once they are extirpated. Genetic criteria were used to chose founders for a new population of the extirpated Guam rail on the island of Rota, for sea otters (Enhydra lutris), and for a Western Australian shrub (Corrigan grevillea) reduced to 27 plants. Genetic data were also used to influence the choice of source population for a Gila topminnow reintroduction program (Vrijenhoek in Loeschcke et al., 1994, pp. 37-53) . Fish from a population with high allozymic diversity were selected to successfully replenish the diversity and viability of a declining and nearly monomorphic population rather than fish from a less variable but adjacent population.

Genetic study of red-cockaded woodpeckers in the southeastern United States led to the recommendation that translocations be made between nearby populations rather moving birds over great distances. In another study, it was shown to be genetically appropriate to use gray wolves from British Columbia as a source of animals for reintroduction into Yellowstone National Park. Genetic variation and multilocus genetic similarity were used to justify the introduction of panthers from Texas (traditionally regarded as a different subspecies) to counter severe inbreeding depression (low het-erozygosity, poor sperm quality, and cryptorchidism) in the remnant Florida panther population.

Genetic criteria have also been used to argue against certain types of translocations. For example, it was found that Tasmanian eastern barred bandicoots (Parmeles gunnii) should not be used as a source population for enhancing the endangered mainland Australian population. South African wild dogs (Lycaon pictus) were genetically inappropriate for reintroduction into Kenya. Isolated northern and southern populations of Brazilian muriqui (Brachyteles arachnoides) are well differentiated genetically, so threatened northern populations should perhaps not be translocated to larger southern reserves.

An example of the hazards of implementing a transplantation program without first considering genetic factors involves the endangered Hawaiian silversword. In this case, the outplanted individuals were all descendents of only one or two maternal plants and therefore retained only a small fraction of the genetic diversity of the remaining populations of the species they represented. Furthermore, because they were so closely related to one another, they had significant reproductive problems associated with self-incompatibility and a seed set of Ͻ20% (Rieseberg and Swensen in Avise and Hamrick, 1996, pp. 305-334) .

Genetic methods are used in forensic identification of tissues of endangered species in illegal or misrepresented trade (e.g., abalone, ''caviar,'' cage birds, and primates including chimpanzees). Misrepresentation occurs when a wild-caught parrot or falcon is claimed to be legally captive bred. Geneticists showed that some whale meat legally on sale in Japan was actually meat of endangered and allegedly protected species including humpback whale. Sequence data revealed that loggerhead turtles from Caribbean nesting beaches are threatened by a Mediterranean fishery off Spain.

Geneticists are playing an increasingly important role in the management of mixed stocks of threatened and commercially harvested fish. Consider the conservation management of salmon (Oncorhynchus) in the U.S. Pacific Northwest. Salmon with their precise homing behavior present a major problem because each local spawning population should be managed as a genetically distinct taxon. Should managers give every natal stream-adapted salmon stock equal priority? Geneticists have developed allelic frequency marker systems for stock identification that are sufficiently sensitive to permit real-time regulation of mixed stock fisheries involving both hatchery and wild salmon. The latter can be partially protected because they return from the sea to the rivers a few weeks later than the former and the fishery can be terminated when they are detected.

Using the same approach, geneticists developed markers to identify endangered and protected winterrun chinook salmon in the Sacramento River in California. Fall-(hatchery) and spring-run chinook salmon do not enjoy protection, but the fishery could not be managed from a conservation perspective until the different races could be identified during the downstream migration of smolts to the ocean.

Genetic tracking of movements of migratory birds permits sorting arctic and Mexican falcons (Falco perigrinus) that mix when the former reach their wintering grounds. Similar genetic tracking permitted the identification of the wintering grounds of several species of declining arctic shorebirds and led to changes in conservation focus from the breeding grounds to the wintering grounds. Finally, geneticists have been able to monitor the loss of variability in translocated populations of wild turkey (Meleagris gallopavo), white-tail deer (Odocoileus virginianus), and alpine ibex (Capra ibex).

Space does not permit presentation of all the caveats, corrections, and revised recommendations made in most of the previous cases as more data became available and as various tests were repeated. Many of the classic examples are less clear-cut than originally proposed. Space does also not permit mention of all the examples in which genetics showed populations were not genetically depauperate or sufficiently distinct to warrant priority conservation efforts. Such contributions are of equal importance to biodiversity conservation because they free up limited resources for other investigations.

Although most of the previous examples involved captive or wild populations of threatened species, much useful conservation genetics can be done using laboratory animals as model organisms. Valuable experimental tests of conservation genetic principles have been completed using Drosophila, Tribolium, mosquitofish (Gambusia holbrooki), the butterfly (Bicyclus anynana), Mus, and Peromyscus (Frankham, 1995; Leberg in Smith and Wayne, 1996, pp. 87-103) . For example, using laboratory populations of Drosophila it has been shown that equalizing family size can double N e and more intensive pedigree management can increase the N e /N ratio 40-fold. Similarly, in most of the examples dis-cussed previously, investigators also examined the genetics of a close relative of the taxon of interest. It is standard practice to develop genetic methods using a common relative as a surrogate before commencing work on a highly endangered taxon. This approach also has the advantage of providing comparative data useful in interpreting the results of a study of an endangered taxon.

The magnitude of the task facing conservation geneticists is daunting. There are on the order of 10 million species living on the planet today and about 2 million of these are recognized and named in a formal taxonomic sense. Describing a new species requires little more than that a scientist know what it looks like and where it is found; unfortunately, this constitutes our state of knowledge for most of biodiversity. Closer to 10 4 than to 10 5 species have been characterized ecobehaviorally, and only on the order of 10 3 species have been examined by geneticists. If sound conservation is based on a knowledge of ecology, behavior, and genetics, then we must admit that we are currently capable of scientifically managing the evolution of less than 1000 species. However, the number of species requiring individual management to prevent their extinction in the next 100 years is in excess of 10,000. Conservation geneticists have devoted their efforts disproportionately toward the charismatic megavertebrates. Whales and cats have received more attention than bats and rats. Given the enormous number of species requiring attention, one might inquire as to how priorities are set. First, most research has gone into species that were favored for utilitarian reasons; they provide us with food, clothing, medicines, recreation, or companionship. Most of this research has been aimed at stock improvement rather than whole genome or species conservation. Second, as already noted, the charismatic megavertebrates and a few groups of flowering plants have received inordinate attention. Perhaps not surprisingly, rare species have also been studied more than common ones. The same applies to phylogenetically unique species, living fossils, and evolutionary relics. However, because the real goal is to save functional ecosystems, conservation geneticists are rethinking their priorities. Although some rare species clearly merit genetic management, it would be better to focus more attention on ecological keystone species whose activities are critical to the maintenance of entire communities. We also need to know more about the genetics of ecologically successful colonizing species and of clades of species that have evolved very recently (the cichlid flocks of Rift Valley lakes) because their study may teach us how to manage apparently less successful taxa.

Conservation geneticists rarely advocate bringing plants and animals into captivity ''to save them.'' Species are typically better managed in their natural communities than in isolation. Existing institutions concerned with conservation, however, are not equipped to deal with the magnitude of the task they face (Woodruff, 1989) . Parks and wildlife reserves are the preferred approach to both species and community conservation. Zoos and botanic gardens are extremely limited in what they can accomplish and can at best serve only to shelter a few critical cases that require intensive care. Germplasm frozen storage systems are valuable adjuncts for researchers, but no credible geneticist has yet proposed that we will be able to awaken these frozen tissues and recreate the animals from which they were derived. Although frozen tissue banks are extremely valuable for geneticists, the revitalization of mammoths, quaggas, thylocenes, and dodos is still science fiction.

In its first two decades, conservation genetics was perceived by some wildlife ecologists as an unnecessary intrusion into their field. It was argued that demography and behavior are far more important than genetics in saving endangered species. Others have argued that the genetic threat to population viability has been overstated (Lande, 1988) . Genetics was viewed as too theoretical and contributing too little and too slowly to the day-to-day efforts to save populations in nature. Furthermore, molecular genetics studies, which are relatively expensive, compete for the limited funds available to the traditional conservationists. Some of the criticisms were justified and some were not. It is easy to disparage the potential contribution of genetics to saving a particular population or species if genetics is defined very narrowly as, for example, the determination of heterozygosity in a remnant population. In the case of cheetahs in Africa, it is clear that predation by lions and humans is more significant today than low variability. Similarly, in captivity, different husbandry practices in different zoos are more significant than poor sperm quality. However, if one takes a longer term view, the answer is different: Genetics is and will be increasingly important. As this review shows, geneticists have a great deal to offer managers. It is incorrect to suggest that ecology and genetics are alternative approaches. Although there are clearly times when genetic studies will be lower priority in a multifaceted conservation strategy, it is undeniable that increasingly more populations will need genetic management. Genetics, ecology, and behavior are all necessary parts of biodiversity conservation.

Although conservation geneticists focus on populations and species, their ultimate goal is the conservation not of things but of a process, evolution, that produced them. The ultimate goal of conservation biology is to preserve the processes of organic evolution-to maintain the ability of populations and species to evolve and communities to function and provide ecosystem services. The basic science is still not equal to the task conservation geneticists are expected to perform. The relationship between genetic variation and ''genetic health'' is illusive and needs clarification. Society's expectations of conservation geneticists also need to be specified or we will forever be accused of treating the symptoms and not the causes of the biodiversity crisis. Typically, species are not afforded legal protection until their populations have fallen into the hundreds, 10-100 times below the level at which their genetic integrity and viability are reasonably secure. Geneticists need to point out that current standards of endangerment are far too low, that recovery from previous mass extinctions took on the order of 10 million years, and that we have not thought through the global implications of a 50% decrease in the number of remaining larger plant and vertebrate species (Myers, 1996) . 

Conservation Genetics: Case Histories from Nature

Genetic and evolutionary consequences of small population size in plants: Implications for conservation

Population Viability Analysis

Mammalian cytogenetics and conservation of species

Systematics and Conservation Evaluation

Conservation and Evolution

Conservation genetics

Minimum viable populations: Processes of species extinction

Molecular contributions to conservation

Metapopulation Biology

Importance of genetic variation to the viability of mammalian populations

Genetics and demography in biological conservation

Extinction risks from anthropogenic, ecological, and genetic factors

Effective population size, genetic variation, and their use in population management

Conservation Genetics

Non-invasive genotyping for vertebrate conservation

Microsatellite variation and microevolution in the critically endangered San Clemente loggerhead shrike (Lanius ludovicianus mearnsi)

The biodiversity crisis and the future of evolution

Genetic and phylogenetic analyses of endangered species

Creative Conservation. Interactive Management of Wild and Captive Animals

Genetic research and its application in zoos

Inbreeding and extinction in a butterfly metapopulation

Genetics and Conservation: A Reference for Managing Wild Animal and Plant Populations

Molecular Genetic Approaches in Conservation

Molecular systematics and the conservation of rare species

Genetic erosion in isolated small mammal populations following rain forest fragmentation

Quantitative genetics: A promising approach for the assessment of genetic variation in endangered species

Tracking the long-term decline and recovery of an isolated population

The problems of conserving genes and species

Evolution and the Genetics of Populations

The population genetic consequences of habitat fragmentation for plants