Genetic Diversity: Sources, Threats, and Conservation
Genetic diversity is a fundamental source of biodiversity which has been defined by different authors as “any measure that quantifies the magnitude of genetic variability within a population” (Hughes et al. 2008) or “the very makeup of the variation of organisms and species on Earth” (Elliott 2002). According to Ennos et al. (2000), genetic diversity presents “the range and sum of genetic variation within a population or populations,” where the term diversity, which simply means the state of displaying dissimilarities, differences, or variety, has acquired an extended meaning which signifies the sum of the variation.
Each individual species is made up of individuals that possess their genes, which are the source of its own unique features (genes are responsible for both the similarities and the differences between organisms). A species may have different populations, each having different genetic compositions. For example, the huge variety of people’s faces reflects each person’s genetic individuality, but the term genetic diversity also covers distinct populations of a single species, such as the thousands of breeds of different dogs or the numerous rose’s varieties (GD 2018). Genetic diversity could be defined as a wealth of different alleles in the genetic makeup within individuals, populations, or species.
Genetic variation and genetic variability are terms which are closely related to genetic diversity.
Genetic variation of a deme (deme is a part of a population) is defined as “the occurrence of different genetic variants,” where the basis of variation is “the number of polymorphic gene loci in conjunction with the numbers of alleles and their frequency distributions in a given population” (Hattemer 1991). Genetic variation can be quantified at different levels (among species, among major types within a species, among breeds within a major type, between breeders’ lines within a breed, and among individuals), and the term genetic diversity is used to express the degree of this variation (NRC 1993).
Genetic variability could be defined as “the potential of the population to produce individuals possessing different genotypes” (Hattemer 1991).
In 1989, the World Wildlife Fund (WWF) defined biodiversity as “the richness of life on Earth – millions of plants, animals and microorganisms, including the genes which they carry, and complex ecosystems that create the environment” (Caliskan 2012).
Different genes and their combinations are found in individuals (genotypes), made up of populations of different organic species (trees, birds, insects, fish, animals, etc.) which develop in certain ecosystems (forests, rivers, seas, etc.). Each individual, the population and the whole species, inseparably belongs to a particular ecosystem, whose conditions are adapted. All three levels of biological diversity are necessary for the survival of life, and all are important for people (Levin 2001).
Diversity is an important and inseparable part of every ecosystem. It is the result of the functioning of evolutionary changes. Mutations, genetic recombination, and natural selection, which occur in each ecosystem, produce variability, new quality, and differentiation between living organisms. Once created, diversity tends to be maintained and multiplied.
Each organic species is characterized by a certain number of chromosomes that is unchanged and represents a specific feature of the species. All genetic information as the only material connection between parents and offspring are contained in the chromosomes. Gene is a physical and functional inheritance unit that transmits hereditary messages from generation to generation. It primarily refers to the sequence of nucleotides in the chain of deoxyribonucleic acid (DNA) that is packaged in chromosomes. The complex structure of hereditary (genetic) material enables an unexpectedly large combinatorial of genotypes, which is the basis for natural selection, that is, the basis of biological diversity (Dražić 2015).
Each individual, in addition to the features that characterize the species, also has its own individual characteristics, leading to the diversity within a single species. This variability is the result of the interaction of the genotype and the environment in which the individual develops. All this results in the fact that the living world consists of a huge number of species, which were created by three main factors of evolution: inheritance, variability, and selection (Isajev and Šijačić-Nikolić 2003).
The species consists of one or more populations. The individuals within the population can genetically differ one from the other – intrapopulation variability. Variability is also present among different populations of the same species – interpopulation variability.
The population may consist of only a few individuals or millions of individuals, provided that some of them produce a common offspring. The degree of genetic differences between individuals within the population can be very variable. This genetic diversity arises because individuals have slightly different forms of DNA sequences that form genes. Different forms of a single gene are called alleles, and these differences originate from mutations that alter the DNA sequences. Different alleles affect the development and physiology of the individual organisms and may affect their adaptive ability to reproduce and survive in specific environmental conditions. The totality of genes and alleles within the population or species is their genepool, while the individual combination of alleles that an individual possesses is its genotype (Primack et al. 2015).
All hereditary possibilities of an organism are not expressed in its features. Two organisms can be identical in their outward appearance and differ in their genetic constitution. An organism with all its properties, which develop based on the expression of its genotype, as well as the influence of environmental factors and the interactions of genotype and environment, is designated as a phenotype. Thus, the phenotype represents an organism with all morphological, biochemical, physiological, and other characteristics. The phenotype of the organism is not stabilized during the lifetime of an individual (Tucović 1990). Genotype generally remains constant in different environments, although occasional spontaneous mutations may occur and cause it to change. However, the same genotype can produce a wide range of phenotypes in different environments, due to the effect of the environment on the expression and function of genes influencing the trait (Baye et al. 2011).
Importance of Genetic Diversity
Genetic diversity among species is a necessity for a plethora of reasons. Genetic diversity is essential to the long-term survival of species since it allows different species to adapt to environmental changes as well as to be less susceptible to extinction. The presence of high genetic diversity is advantageous to a population; it is vital to the survival, adaptability, and evolution of populations.
Genetic diversity is crucial as a way for populations to adapt to changing environments and tolerate stress from any given environmental factor; having a diverse gene pool ensures that a species will be able to survive. If there are more different genetic variants in populations, it is more likely that some individuals in a population will possess alleles which are suited for the environment and are responsible for their capacity to adapt to new conditions, survive, and produce offspring bearing that allele.
In addition to adapting to environmental changes, genetic diversity is also required for populations to evolve and cope with new diseases and pest epidemics; it also provides the opportunity for tracing the history of populations, species, and their ancestors. On the other hand, one of the consequences of low genetic diversity could be the inability of the population to cope with biotic or abiotic stresses (Caliskan 2012). Plant and animal populations which have a less diverse genepool have more susceptibility to extreme stresses (e.g., drought, disease epidemics).
The long-term survival of the species depends on its ability to adapt to changing environmental conditions, and the loss of genetic diversity would mean a reduction in the long-term adaptability of populations or species. Some individuals from the population might be able to tolerate an increased load of pollutants in their environment or climate changes and will continue to live in those conditions, while the others, carrying different genes, do not adapt successfully, or are unable to reproduce and survive, under the same environmental conditions and have to leave it. The future ability of populations to respond genetically to environmental change is dependent on the adaptive genetic variation, which determines directly the rate of response to natural selection (Ennos et al. 2000).
Higher species diversity leads to a greater diversity of habitats and increased production, which in return allows an even greater species diversity. Diversity plays an important role in maintaining the structure and functions of the ecosystem. High ecosystem diversity enables faster recovery from the changes that have occurred under the influence of sudden disorders and faster establishment of basic functions. In ecosystems with low diversity, disorders can more easily lead to a permanent shift of functions, which causes a loss of resources and changes in species composition.
Genetic diversity within species is very important, as it provides for the rate at which evolution occurs. Genetic variation and diversity can be considered as the “key to evolution on Earth” and “the ‘raw material’ permitting species to adjust to a changing world” (Elliott 2002). The slowing down of evolution could potentially kill off a species which cannot readily adapt to an environmental changing; without evolution all species would die, including humans, who would be without resources essential for survival. Stable conditions stimulate the evolution of unique species, enriching the development of humanity (Fjeldsa and Lovett 1997; Elliott 2002). Humans need genetic diversity within the environment in order to themselves survive; as the world is dependent upon it in order to continue physical processes, it could be stated that “genetic diversity is, in fact, the basis, and greatly valued asset of life on Earth” (Elliott 2002).
Genetic diversity of species is also one of the most important considerations in landscape restoration. This is true both for agricultural species as well as native species used in restoration activities; it provides greater disease protection and helps ensure that the ecosystem services present on the landscape are resistant to environmental changes (Beatty et al. 2018).
The importance of plant genetic diversity is, according to Govindaraj et al. (2015), now being recognized as “a specific area since exploding population with urbanization and decreasing cultivable lands are the critical factors contributing to food insecurity in developing world.” Plant genetic diversity can be conserved and stored in the form of plant genetic resources, such as gene banks and DNA libraries – repository which preserves genetic material for a long period (Govindaraj et al. 2015).
The fitness of organisms is also dependent on heterozygosity, which presents the diversity of genes that are not always present in populations which cannot adapt. Finally, with the higher genetic diversity present in populations, there is a lower risk of extinction of species. According to Markert et al. (2010): “Decreased population genetic diversity can be associated with declines in population fitness (e.g., Maehr et al. 2006; Westemeier et al. 1998). These declines are thought to involve components of the so-called genetic ‘extinction vortex’, which directly ties losses in population genetic diversity to increased extinction risk (Gilpin and Soule 1986).”
Oliver (2018) emphasizes that “genetic diversity is a primary cause of phenotypic (physiological, morphological, and behavioral) variations among individuals, which in turn affect ecological functions and the contribution to ecosystem services”; hence, the erosion of genetic diversity in species populations that remain can affect ecosystem services and human well-being (Oliver 2018).
Sources of Genetic Diversity
There are two basic types of reproduction of organisms: asexual reproduction and sexual reproduction. Genetic diversity occurs in both types of reproduction, but in asexual reproduction, variations are so less due to small errors in DNA copying, through mutations at DNA level, while in sexual reproduction, variations occur through mate selection and the possibility of recombination.
Asexual reproduction presents the process of creating offspring which are genetically identical to the parent; no new genetic variation is introduced into the population. Only if a mutation occurs during cell division will the offspring differ. It is common in single-celled organisms and also occurs among fungi, plants, and animals. Asexually reproducing populations may consist entirely of clones and thus have very little or no genetic diversity. In asexual reproduction, cells divide via mitosis, the cell division that ensures that the two daughter cells receive the same number and type of chromosomes as the parental cell (Eriksson and Ekberg 2001). If there is a mistake in mitosis or the copying of the DNA, then that mistake will be passed down to the offspring possibly changing its traits, but not all mutations in asexual reproduction result in variations in the offspring. Mitotic recombination may occur in somatic cells during their preparation for mitosis. In asexual organisms, the study of mitotic recombination is one way to understand genetic linkage. Additionally, mitotic recombination can result in the expression of recessive genes in an otherwise heterozygous individual (Hartl and Ruvolo 2012). Mitotic crossing-over occurs only in diploid cells, such as somatic cells of diploid organisms. It is rare, but important, in some organisms; for example, some fungi, which do not have a sexual cycle, use mitotic crossing-over as a source of variability. Moreover, when examining cancer in humans, mitotic crossing-over is thought to be important in causing recessive mutations that cause cancer expression (Bošković and Isajev 2007).
Sexual reproduction brings together two haploid cells from different parents to produce a diploid zygote, which contains a unique combination of genes from both parents and will grow into an individual that has their features. Through sexual reproduction, a population maintains its genetic diversity and creates unique individuals with each new generation. Sexual reproduction relies on a special type of cell division called meiosis, where the final result is the formation of four haploid daughter cells (Eriksson and Ekberg 2001). In meiosis I (prophase I) chromosomes pair up with each other and exchange different segments of their genetic material forming new combinations of existing genes. This process is known as crossing over (or recombination) between chromatids of homologous chromosomes. Each haploid cell produced by a parent organism contains half of the parent’s genetic material. Due to recombination, the offspring have a different set of alleles and genes than their parents do.
Recombination that occurs during sexual propagation presents a significant source of genetic variability as well as phenotypic variability. A new genetic combination is created when the chromosome sets, originating from both parents, combine to create a genetically unique offspring. Although mutations provide basic, raw material, the random recombination of alleles in different combinations, characterized by species with sexual reproduction, dramatically increases the potential for genetic variability (Primack et al. 2015).
Recombination changes the frequency of genotypes, but not the frequency of genes, which means that recombination does not directly contribute to evolution. The influence of recombination is indirect because changes in genotype frequency can lead to the changes in population adaptability.
Chromosome combinations occur during the formation of the sex cells (gametes) during meiosis and when mating the male and female sex cells (fertilization). In the formation of gametes, and fertilization (the formation of a zygote), a great variety of combinations of cellular chromosomes, which carry entire groups of genes, is realized. The number of combinations depends on the number of chromosomes in the cell. The higher number of chromosomes, the number of their combinations is higher.
Separation of homologous chromosomes in meiosis (anaphase I) and in the formation of gametes (in spermatogenesis and oogenesis). For example, 23 pairs of human chromosomes are combined on the principle of a coincidence so that they can give 223 = 8,000,000 genotypically different gametes.
Fertilization, when the complete genomes contained in the gametes of the mother and father are combined. For example, the number of possible combinations of chromosomes, when a human zygote is created, is 223 (of the mother) × 223 (of the father) = 246 =>70 trillion.
Mutations are defined as changes in the structure of genetic material whose occurrence cannot be attributed to a gene or chromosome recombination. The term mutation originates from the Latin word mutatio which means change, replacement. Mutations can be defined as changes in phenotypes that are not the result of combinations, recombination, and gene interaction, but are determined by changes in chromosomes and genes (Tucović 1990; Sijačić-Nikolić and Milovanović 2010).
Mutations are heritable changes in genetic constitutions of individuals (White et al. 2007) or changes to one or more nucleotides in the DNA sequence.
Mutations affect the change in the genetic structure of populations by creating new alleles. Mutations are the source of variation, but the process of mutation does not itself drive evolution, and the rate of change in gene frequency from the mutation process is very low because spontaneous mutation rates are low (Freeman 2000). Mutations are rare and in a shorter period, they have a very weak effect on the change in the genetic composition of the population. However, in the long run, mutations represent an indispensable source of genetic diversity in populations and, in combination with other evolutionary adaptation factors, an important factor in changing the genetic composition of the population (Morić 2016).
Mutations are classified in several different ways. In multicellular organisms, mutations can occur in somatic cells or in vegetative cells (leaf, liver, etc.), and in generative or gametophytic cells, which subsequently produce haploid gametes. Somatic mutations are distinguished as a result of changes in hereditary material in somatic cells. These changes are transmitted by successive cycles of mitotic divisions, so they can cause changes in the individual organs and lead to damage to the organism (tumors), but they are not transmitted to the offspring. Only changes in the generative tissue, which are interconnected into gametes and over them to the next generation, represent real mutations, those that increase the genetic diversity (Bošković and Isajev 2007).
Mutations can occur spontaneously in natural conditions (spontaneous mutations) or may be caused in laboratory conditions (induced mutations). In natural conditions, mutations are subject to natural selection, whereby newly born individuals, if they have reduced vitality, are most often eliminated by the competition. Otherwise, if they have positive characteristics in relation to the average of the individuals in the population, such individuals enrich the genome. Mutations that have value for humans and action-oriented selection are expanding and accumulating in certain species, thus creating the basis for the synthesis of new varieties. Spontaneous mutations, as a source of genetic diversity, are limited because they occur relatively rarely and their occurrence will need to be awaited for many years. Therefore, it now resorts to induced or artificially induced mutations in laboratory conditions that, in the function of a man, increase genetic variability and enrich the genepool (Isajev and Šijačić-Nikolić 2003). Induced mutations are caused by mutagens, such as physical (radiation, temperature), chemical (medicines, pesticides, additives), and biological (viruses, bacteria) mutagens.
According to the effect the mutation causes in the carrier, mutations can be beneficial, harmful (more often), or neutral. Harmful mutations can often be lethal (lead to death), sublethal (reduce the adaptive ability of the organism, e.g., hemophilia), and conditionally lethal (when the organism needs to enter what is missing from external conditions).
According to the size of the phenotypic changes, macromutations and micromutations can occur. The macromutations lead to large, noticeable changes in the phenotype, while the small changes of the micromutations are more difficult to detect and are not noticeable.
All mutation changes are classified accordingly into various types and subtypes of mutation, depending on the place of origin: mutations of the genome, chromosome structure mutations, gene mutations, and out chromosomal mutations.
Genetic diversity also implies the existence of harmful alleles in populations, which are called genetic loads. Studies of gene frequencies in populations have provided an approach to some problems in human genetics, such as one concerned with the prediction of the whereabouts and expected expressions of recessive genes in a population, many of which are known as genetic loads and segregate freely in the human populations (Gardner 1964). Two categories of genetic load have been distinguished: segregational load which presents “the amount of fitness or the number of offspring left by individuals in a population, as compared with the expected if there were no changes through Mendelian segregation” and mutational load, which presents “the amount of fitness in a population as compared with that which would be expected if no mutations occurred” (Gardner 1964).
Random Genetic Drift
Genetic drift could be defined as “the process whereby the frequency of alleles in a population changes from one generation to the next as a consequence of chance sampling events”; it leads to the loss of genetic variation from populations and is most pronounced in small populations (Ennos et al. 2000).
Genetic drift is a phenomenon of random changes in allelic frequency that affects small populations due to the random selection of parental alleles that are transmitted to offspring. Due to the fixation of one and simultaneous loss of other alleles in a locus, genetic diversity decreases, and due to the decline in polymorphism, heterozygosity is reduced, leading to increased homozygosity loci. However, at the same time, by a random change in allele frequencies, significant genetic differences between populations occur. To this must be added the significant impact of inbreeding within small populations (Morić 2016).
White et al. (2007) emphasize that in small populations, allele frequencies can change randomly and unpredictably from generation to generation, due to the vagaries of sampling, which is called genetic drift.
Genetic drift is a powerful evolutionary factor in small-size populations. For example, a heterozygous individual produces two types of gametes, approximately in equal amounts, by crossing such a heterozygous organism with homozygous with recessive qualities, a progeny of which only one offspring is heterozygous and the other homozygous. However, this is only approximately, on average, true, and it is increasingly probable, if the parent plant produces enough offspring. If the number of offspring produced is smaller, the lower the probability that will be the same number of homozygous and heterozygous individuals among them. If the number of offspring is only three, it is possible that all of them are homozygous or heterozygous, and this significantly changes the frequency of the gene. The random genetic drift in the population in this way can lead to the accumulation of semilethal gene and the elimination of useful genes for the adaptive ability of the population (Tucović 1990).
Gene Flow (Migration)
Gene flow or migration can be defined as “the movement of alleles among populations” (White et al. 2007).
According to Eriksson and Ekberg (2001), the meaning of term gene flow is that “individuals from one population participate in the procreation of a new generation in the recipient population and that the donor and recipient populations have different allele frequencies.” For example, in plants, there are gene flows via pollen (e.g., wind-pollinated species may spread pollen hundreds of kilometers), seed, or fruit dispersal.
Gene flow is a very powerful mechanism of evolution that implies gene migration between two populations of different frequency alleles. This factor can be the source of genetic novelties (new alleles or chromosomal rearrangements) if immigrants have alleles that the population had not previously owned.
Introduction of new alleles leads to increasing of genetic variability intrapopulation.
Continued gene flow over generations leads to reduced interpopulation genetic divergence.
The effects of gene flow depend on the number of individuals that the population exchanges by generation and the differences in the genetic structure among the populations (Šijačić-Nikolić and Milovanović 2010).
Natural selection is a process in which the genetic composition of the population changes over time due to the greater reproductive contribution of individuals that are better suited to the living environmental conditions. It is the fundamental mechanism of evolutionary adaptation of populations and species to environmental changes. Individuals can be acclimatized in the short term by physiological changes, while in the long term, they can be adapted only by changes in the genetic composition of the population.
The natural selection presents “differential transfer of alleles to the next generation, which results in increased adaptability” (Eriksson and Ekberg 2001). Darwin’s theory of evolution refers to natural selection and selection which destroys individuals less adapted to living conditions supports those better adapted. Starting from Darwin’s theory, the next generations of scientists have proven that natural selection and spontaneous change in genetic material are the basis of biological evolution and the emergence of new species. Eliminating less-adapted organisms in a population, the selection eliminates those genes, under whose influence the phenotypic trait is manifested, which is insufficiently aligned with the environmental conditions and more susceptible to the effects of natural selection. Accordingly, natural selection favors or discriminates against phenotypes, but also their genotypes, by making a crucial influence on the frequency of certain genes in the population. Via natural selection, certain individuals contribute more to the next generation than others and cause changes in gene frequencies (Eriksson and Ekberg 2001). It implies the survival of the most adapted and reproductively successful individual in any given population.
According to Panhuis et al. (2001): “Sexual selection results from differential mating success among individuals within a population. Competition for fertilization occurs through direct competition between members of the same sex (e.g. male-male competition and sperm competition) or the attraction of one sex to the other (e.g. female choice). Although long recognized as important in intrapopulation evolution, sexual selection has more recently been invoked as a driving force behind speciation.”
Corl and Ellegren (2012) emphasized that “one way in which sexual selection can influence genetic evolution is by affecting levels of genetic diversity.” Sexual selection can increase the variance in reproductive success, which will lower the effective population size of a species that could result in decreased levels of genetic variation; it often results in males having higher variance in reproductive success than females which will lower the effective population size of DNA predominantly transmitted by males (Nunney 1993; Charlesworth 2001, 2009; Corl and Ellegren 2012). Sexual selection could erode genetic diversity and hence reduce the potential for evolutionary change, because it, generally, increases mating skew, thus reducing effective population size (Kirkpatrick and Ryan 1991; Candolin and Heuschele 2008). The maintenance of genetic diversity across generations depends on both the number of reproducing females and males, so both sexes should transmit a high proportion of the genetic diversity they contain to help maintain diversity; on the other side, variance in reproductive success, litter size or multiple paternity, etc. can affect the relative contributions of female and male parents to genetic variation of progeny, while some mating systems might reduce the amount of genetic diversity transmitted by females or males (Pérez-González et al. 2014).
Todd and Miller (1997) noted that “the most successful, complex, and numerous species on earth are composed of sexually-reproducing animals and flowering plants,” which are “typically undergo a form of sexual selection through mate choice,” meaning that the biodiversity evolution may be driven “not simply by natural-selective adaptation to ecological niches, but by subtle interactions between natural selection and sexual selection.”
In spite of natural selection and adaptation of populations to specific environmental conditions, migration of genes greatly affects the reduction of differences between populations. Genetic differentiation is also caused by selection, which can contribute to the creation of specific genetic spatial patterns and enhance or reduce genetic differentiation. It is important to emphasize that the selection, due to the diversity of microecological conditions in the stands, can encourage the creation of spatial patterns. Otherwise, in inbreeding depression, the selection encourages the extinction of individuals formed by self-fertilization or inbreeding and leads to a decrease in the genetic structure of the population (Morić 2016).
Mutations, genetic drift, and natural selection increase genetic variation among populations (interpopulation diversity), while gene flow (migration) decreases this variation (Eriksson and Ekberg 2001). The same evolutionary factors operate within populations (Fig. 2). Mutations and gene flow increase the genetic variation within populations (intrapopulation diversity), while natural selection and genetic drift reduce it (Eriksson and Ekberg 2001).
In addition to these four factors, inbreeding also reduces genetic variation within the population.
Inbreeding is the process of reproducing/copulating with closely related individuals of the same species. It is quite controversial as to whether or not inbreeding is directly linked with extinction of organisms and/or species, but in either case, inbreeding has adverse effects on the local gene pool of a population (Brook et al. 2002). An extreme example of inbreeding is self-fertilization, i.e., crossing at the level of one tree. However, inbreeding includes the occurrence of crossings of relatives, further relatives, and members of small populations. The consequences of inbreeding can be different. The first and obvious consequence is the uniformity of the offspring and the reduction of the degree of variability between individuals. The second consequence is a decline in the vitality of the population. During self-pollination, often there is a lack of fertilization, or the embryo is discarded (coming to dying) even during the embryonic phase, which results in the creation of an empty seed. Another consequence of inbreeding is the decline in adaptation capacity. Due to the cross-breeding in the population, there is not only a decline in vitality but also the ability of progeny to adapt to changing environmental conditions.
Genetic Diversity Determination
Determining the level, form, and causes of genetic diversity of some species is essential information for its cultivation, breeding, developing a genetic diversity conservation program, as well as evolutionary, taxonomic, or ecological research of species.
Genetic diversity analysis provides powerful data that helps for better understanding of genetic variation and improved conservation strategies (Caliskan 2012).
Quantitative traits, traits which vary continuous and are controlled by many gene loci (polygenic, metric traits, e.g., plant height)
Traits controlled at the single-gene level (qualitative traits, genetic markers)
A genetic marker is “any visible character or otherwise assayable phenotype, for which alleles at individual loci segregate in a Mendelian manner” (White et al. 2007). Determining genetic diversity can be based on different genetic markers: morphological, biochemical, and molecular markers. Molecular markers are superior to both morphological and biochemical markers because they show genetic differences on a more detailed level, independent of environmental conditions, are relatively simple to detect, and abundant throughout the genome (White et al. 2007; Abdel-Mawgood 2012). In order to precisely determine the level and cause of genetic diversity, it is desirable to use a combination of different methods (using different markers) because they are characterized by specific advantages and disadvantages.
The variation in phenotypic traits or allelic states may either be non-neutral or neutral concerning fitness consequences (Hughes et al. 2008). Polygene phenotypic properties exhibit different traits and indicate that differentiation is the result of adaptation to different environmental conditions, especially if the genetic composition of the population is influenced by the natural selection; on the other side, most DNA markers are neutral and are not affected by natural selection, so the ability to determine adaptive differences by its use is limited, but it increases with the increase in the number of markers used.
Genetic diversity might be determined by a range of techniques: by observation of inherited genetic traits, by studying the chromosomes and specific karyotype, or by analyzing the DNA information using molecular technology.
The measurement of genetic diversity by quantitative traits begins by estimating the variance in a phenotypic trait among individuals that is due to genetically inherited differences, which could involve detailed knowledge about the pedigree of a natural population, as well as separate experiment that replicates multiple genetic families (i.e., sib analysis) collected from a single population, etc. (Hughes et al. 2008).
The capacity of molecular markers to measure genetic diversity has improved greatly over the past half-century (Avise 2004).
DNA markers based on DNA-DNA hybridization (restriction fragment length polymorphism (RFLP) markers)
DNA markers based on the amplification of DNA sequences using the polymerase chain reaction – PCR (random amplified polymorphic DNA (RAPD) markers, amplified fragment length polymorphism (AFLP) markers, simple sequence repeat (SSR) markers or microsatellites, expressed sequence tagged polymorphisms (ESTPs) markers, etc.).
Various techniques have been developed for identifying genetic differences between organisms using molecular genetic markers. The choice of technique will depend upon the material being studied and the nature of the questions being addressed.
The quantification of diversity within and among populations and/or individuals
The quantification of genetic relationships
The display of relationships
Polymorphism or rate of polymorphism
The proportion of polymorphic loci
The richness of allelic variants
The average number of alleles per locus
The effective number of alleles
The average expected heterozygosity
Interpopulation differentiation for one locus
Interpopulation differentiation for several loci
Population’s contribution to total genetic diversity
Analysis of molecular variance
Diversity and differentiation at the nucleotide level
Classification or clustering
Molecular data can be usefully complemented with morphological or evaluation data.
Threats to Genetic Diversity
The genetic integrity of many plant and animal species is imperiled due to different natural and human threats. Some of the threats to genetic diversity are habitat degradation and loss, deforestation, fragmentation, pathogens, invasive and allochthonous species, environmental pollution, global climate change, etc. (White et al. 2007).
These and similar activities reduce the sum of available genes, thus leaving a population that is less capable of tolerating any further natural or anthropogenically caused changes. These threats could lead to the loss of whole species and also genetic diversity within a species, which can result in the loss of useful and desirable traits. Reduced genetic diversity may “eliminate options to use untapped resources for food production, industry and medicine” (GD 2018).
Habitat Loss, Deforestation, and Fragmentation
Habitat loss and deforestation are caused by urbanization, conversion of forests for agriculture land, overgrazing, overharvesting of fuel and industrial wood, etc. (White et al. 2007). It can be especially damaging to genetic resources; for example, logging may lead to dysgenic selection if the fastest-growing, most pest-resistant individuals are selectively harvested and less desirable trees are left to supply seed for regeneration (White et al. 2007).
White et al. (2007) emphasize that fragmentation alters patterns of gene flow by spatially separating individuals (directly) or by impacting the abundance and behavior of the animals responsible for seed dispersal and pollination (indirectly).
Pathogens, Invasive and Allochthonous Species
Pathogens may cause widespread death of their hosts or coevolution which enables both pathogen and host individual to survive. The accidental or intentional introduction of exotic insects into native populations, as well as the introduction of exotic plant species, can also impact genetic diversity.
Infestations of harmful insects can lead to drying of forests of wider proportions. Plantations, which are established of genetically homogeneous materials (using clones), are characterized by an increased risk of such occurrences.
The spread of allochthonous species, mainly, has been supported by previous ecosystem changes. Disorders of vegetation in ecosystems, such as those resulting from deforestation and forest fires, allow faster expansion of nonnative compared to native species, because they are more competitive, leading to a significant suppression or disappearance of indigenous species. Invasive species are also a greater threat where ecosystem disorders have previously occurred, and their impact is further enhanced by climate change. These species reduce the genetic diversity of forest species and displace indigenous species from natural habitats, altering the structure of populations and endangering the stability of ecosystems.
Environmental Pollution and Global Climate Change
Different human activities cause pollution of water, air, and soil, which significantly threatens genetic diversity. Air pollution is most often caused by traffic and industry and can threaten the plants directly or indirectly. Due to increased concentrations of harmful substances in the air, there may be a loss of chlorophyll, stopping the process of photosynthesis and growth, gradual extinction of plant organs, and drying of plants. Air pollution can cause drying of forests on large volumes.
Air pollution affects the gene pool of plant species, reducing the production of pollen and seed viability or development of underground and aboveground part of the plants. Harmful substances from the air, dissolved in water, reach the soil, and therefore the plants adopt the substances from the soil. The presence of heavy metals in the soil can significantly affect the vitality of certain species.
The negative effect of these factors is increasing, due to changes that occur at the global level and occur at a high rate, such as climate change. Forest genetic diversity is one of the most important tools in dealing with the problem of global climate change. Forests represent an important component of the global carbon cycle, together with soil, and have a great capacity to accumulate and release carbon.
However, in addition to numerous biotic and abiotic factors that threaten them, forest genetic resources are affected by climate change, which leads to a significant reduction in their vitality, resistance, adaptability, and gradual decline. Forests could be changed by composition, structure, and distribution patterns since some species would migrate, while others would disappear due to climate change.
Conservation of Genetic Diversity
While threats to genetic diversity continue to intensify, the international community is increasingly committed to conserving biodiversity at all levels, including genetic variation within species (White et al. 2007). The goal of conservation genetics is to maintain genetic diversity at many levels and to provide tools for population monitoring and assessment that can be used for conservation planning (Govindaraj et al. 2015).
Conservation genetics is primarily concerned with the research and conservation of different elements of genetic diversity (genotypes, populations, patterns of genetic variation, species). Short-term preservation of genetic diversity ensures reproductive capacity, while long-term conservation of genetic diversity ensures the maintenance of adaptive evolutionary potential of species (Galov 2007).
The process of conserving genetic diversity consists of three main components: defining objectives of conservation, knowledge of genetic diversity of populations, and application of conservation methods. The most appropriate gene conservation strategy for each species depends on the population genetic structure, the size of its geographic range, and local attributes (White et al. 2007). For example, the forest tree species are conserved by in situ methods (genetic diversity is maintained in populations growing in their place of origin) and ex situ methods (involve holding germplasm in cold storage or growing trees in plantations outside their place of origin).
Understanding genetic variation within and among populations is essential for the establishment of effective and efficient conservation practices for rare and endangered species. The maintenance of genetic variation is one of the major objectives for conserving endangered species (Avise and Hamrick 1996). Several aspects of conservation biology, such as loss of genetic diversity in conservation programs and restoration of threatened population, can only be addressed by detailed population genetic studies (Hamrick and Godt 1996; Shanshan et al. 2006). Genetic variation at the intraspecies level is a prerequisite for future adaptive change or evolution and has profound implications for species conservation (Schaal et al. 1991). Knowledge of genetic variation within and among populations provides essential information in the formulation of appropriate management strategies for their conservation (Milligan et al. 1994).
The principles of genetic diversity conservation can be regarded as identical for all living beings; however, the methods which are applied vary, depending on the specificity of the conservation goals, biological nature, and distribution of the material that is the object of conservation.
Knowledge of genetic variation within a species, as well as within or between populations, is the starting point for the breeding processes. The greater the genetic variability, the higher the phenotypic variability, and it is easier to select individuals, groups of individuals, or the entire population according to selection criteria.
Reliable information on the distribution of genetic diversity within species is a prerequisite for successful implementation of the objectives of genetic resources conservation. Knowledge and proper application of theoretical models, methods, and techniques for assessing the current diversity within and among populations or other hierarchical levels of the genetic organization is an essential part of conservation programs and improving the species.
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