Genetic diversity is considered to be the baseline of biodiversity. It is the cumulative sum of all the genetic traits or characters present in the genome of any given population or species.
Africa exhibits higher genetic diversity levels than most other areas of the world.
The term ‘biodiversity’ refers to the amount of variation in all biological entities within ecosystems. It is often defined as being the cumulative sum of all genes, species, and ecosystems of a particular region. Hence, it can be studied on three interrelated levels: genetic, species, and ecosystem. However, the genetic level forms the baseline. Genetic diversity in organisms forms the root of any genetic adaptation or variability. It is the basis for evolution and natural selection. The genetic diversity of a species paves the way for the species to be able to adapt to its changing environment. It refers to the possession of various genetic traits in a species, that may or may not be expressed depending on the prevalent environmental conditions. However, this term should not be confused with that of genetic variation.
Genetic variation refers to the prevalence of two or more allelic forms of a particular gene in a given population, whereas, genetic diversity refers to the prevalence of a diverse range of genes in the gene pool of organisms. Hence, variation deals with individuals, whereas, diversity deals with entire populations or species. Genetic diversity can be considered to be a combination of variation as well as variability (tendency for variation to occur in a species). This combination helps biological species to be adaptable to changes in its environment. These changes could be a result of change in population sizes, weather conditions, natural disasters, arrival or departure of competing species, etc.
Sources of Genetic Diversity
In germ-line mutations, the alteration in the DNA sequence is passed onto the organism’s offspring. In time, these mutations may accumulate and cause defects in the genome, which could ultimately prove fatal, causing the line to die out. On the other hand, some mutations may be conserved by the genome, and ultimately, over time, lead to the development of a new trait, i.e., cause the evolution of a gene. This would increase the genetic diversity of that organism. In some cases, the evolution of new genes may have a drastic result, causing the formation of an entirely new species. However, this process is incredibly slow, taking place over a long period of time, although, it is the way true diversity manifests in nature and induces evolution.
It refers to the creation of a new species. The differentiating factor between the old and new species is that, they are unable to breed with each other. Speciation takes place via multiple processes, which include geographical isolation, competition, and polyploidy.
◆ Geographical isolation occurs when natural phenomenon cause a habitat to be split in such a way that a part of the population is separated from the main population, with no means of rejoining it. Eventually, the separated individuals adapt to their new surroundings, and evolve at a different rate and in a different direction than the parent population, causing them to diverge to an extent where they are no longer sexually compatible with the main population. In essence, they have evolved to form a new species. This same effect is observed when a species migrates from one habitat to a new one (immigration).
◆ Speciation by competition occurs when a particular food source becomes limited, or by the arrival of a different species in the habitat that survives on the same food source. In such a scenario, certain parts of the population will undergo changes and adapt themselves to become better at acquiring the food, and protecting it from being taken away. Hence, in terms of species propagation, the better adapted individuals have a better chance at reproducing, due to their enhanced ability to secure food in comparison to the other individuals who have diverted to other food sources. Therefore, the selective breeding of the enhanced part of the species would eventually lead to them becoming a separate species altogether
◆ Polyploidy is observed more in case of plants than in animal species. This is so, because, animal reproductive processes are much more sensitive to gross chromosomal changes as compared to plants. In plants, however, polyploidy is quite a common phenomenon. Polyploidy refers to the presence of more than two copies of a chromosome in the genetic material of the organism. In plants, instances of self-fertilization are quite common, and there are also cases where similar species have been successfully crossed with each other. Due to this, the polyploid effect can be observed instantaneously in the subsequent offspring. The most common example of this is that of common wheat (Triticum aestivum), which contains six sets of chromosomes. It is the result of the crossing between a diploid Triticum urartu and a diploid Aegilops speltoides, which resulted in Triticum turgidum, that possess 4 sets of chromosomes. Finally, Triticum aestivum was obtained by the crossing of Triticum turgidum with a diploid Aegilops tauschii.
Errors in Meiosis
During prophase of meiosis, in plants and animals, crossing over of chromatids occurs. This causes an exchange of genetic material between the two homologous sister chromatids. The intermixing of genes from the parental chromatids causes an increase in the genetic diversity of the resulting offspring. In case the crossed over chromatids do not separate and remain joined during chromosomal segregation, one daughter cell gains an additional chromosome, while the other is devoid of one. This increases the diversity in one cell while, at the same time decreases it in the other.
During meiotic divisions, translocations and chromosomal structural changes may lead to the formation of dissimilar sister chromatids. In such an event, the load of genes being distributed to each daughter cell will vary, causing the induction of diversity in the genetic material of each daughter cell. However, in some cases, these errors might also prove fatal to the daughter cell/offspring.
Factors Affecting Genetic Diversity
Any event or gradual process that culminates in the extinction of a species causes the loss of all the genetic diversity represented by that species of plant or animal. Entire species may be wiped out due to mass extinction events like the Ice Age or the meteor and volcanic eruptions that wiped out the dinosaurs. Species may also go extinct individually due to their excessive hunting as a source of food or valuable commodity. For example, the dodo birds were hunted as a convenient food source since they were unable to engage in evasive tactics. Also, the rhino has been driven to the very brink of extinction by hunting it for its horns. Alternatively, species may experience extinction due to alterations to their ecosystem. An example would be deforestation in order to utilize land to set up cities or factories. This would lead to the destruction of that ecosystem and all the indigenous specious therein. In some cases, certain genes may also go extinct in an organism due to its inability to be transferred to the next generation.
As mentioned previously, competition for food resources may cause the adaptation and evolution of a species. However, this is the positive effect. If the negative effect is considered, there is chance that the newly arrived competing species is stronger and better adapted than the indigenous species. This would lead the new species to out-compete the old species, and alienate them from the food source. The unavailability of a food source would cause the old species to die out, and hence, become extinct. Although, this outcome can be averted by the migration of the old species to a new environment which lacks a competing species.
These events refer to situations where a considerable amount of the population die due to natural phenomena or human activities. The surviving population just represents a fraction of the original genetic diversity of the species, and hence, if the population of the species rebuilds, the overall genetic diversity will also be low. The lost diversity will require a considerable amount of time to be recovered. This is coupled with the side effects of inbreeding and genetic homogeneity, leading to an effect called the bottleneck effect.
Significance of Genetic Diversity
A high diversity is vital, since genetic diversity is directly related to biodiversity. Reduced diversity would eventually reduce the overall biodiversity of an ecosystem, whereas, greater diversity would lead to increased biodiversity due to the adapting and speciation of the species when faced with an adverse environment. If we consider an organism with higher diversity of genes, it implies that it has a higher chance of utilizing its diverse gene pool and adapting itself to change along with its changing environment. This allows it to evolve to be better suited to survive. One can equate this to possessing a fully-kitted-out tool box while carrying out repair work. In case some repair fails, one has the necessary tools to deal with it and overcome the setback. However, if the tool box contains a minimal amount of tools, the repairs becomes difficult to manage, and may even lead to further complications. Similarly, if an organism is not genetically diverse, but is homogeneous and uniform, then in the event of a change in circumstances, it will be ill-equipped to adapt to and overcome the adversity, causing it to die out. Even if such an event does not occur, the uniform gene pool will lead to the side effects of inbreeding, such as accumulation of genetic defects, loss of genetic health, and eventual death of species.
◾ The domestication of wolves into various breeds of dogs via selective breeding has greatly increased the genetic diversity of the canine genome as compared to its lupine origins.
◾ In general, woody plants like trees have a higher genetic diversity than vascular plants like grasses. This is due to the size of their geographic range, and also due to the ability to scatter their seeds over a wide area by the use of various seed dispersal methods.
◾ The cheetah population in the world has a very low genetic diversity, as a consequence of a bottleneck event that occurred 10,000 years ago. This low diversity has led to an increase in lethal genetic disorders and low reproductive success.
◾ In potato cultivation, new plants are formed as a result of asexual reproduction, and hence, are clones of the parent plant, implying that there is very low genetic diversity. This proved to be disadvantageous for the potato plant during the potato famine in Ireland in the 1840s, when potato fields were ravaged by a rot-causing oomycete called Phytophthora infests.
◾ In oceanic plankton, ocean viruses help maintain good levels of genetic diversity by a process called genetic shifting. The viruses carry genes of other organisms in addition to their own genome, and when this viral particle infects a cell, these genes are transferred into the infected cell as well, where the host genome changes and incorporates these genes, thereby gaining genes and diversity.
◾ Numerous varieties of corn have emerged as a result of hybridization between multiple related genera, and also due to the presence of mobile genetic elements called transposes. These factors have resulted in a marked increase in its genetic diversity, which can be observed in the form of the diverse colors exhibited in its kernels.
A high level of genetic diversity of a species is essential for its prolonged survival, despite the changing environment. Hence, in order to preserve and maintain the current biodiversity of the various ecosystems on the planet, appropriate conservation measures must be undertaken. Additionally, research could be carried out regarding ways to increase the genetic diversity of the species that lack it.