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Genetic variation is the difference in DNA among individuals [1] or the differences between populations[2]. There are multiple sources that gives maintenance in populations, which are mutation and genetic recombination.[3] Inside these sources there are others that contribute to genetic variation, like sexual reproduction and genetic drift.[2] Additionally, genetic variation has different forms according to the size and type of the small set of genomic variants. [4]


Without genetic variation, some of the three primary basic sources of mechanisms and evolutionary change cannot operate[5]. This is why individuals of one generation are qualitatively different from one another and in Darwin's theory it explains how the mechanism of evolution begins with the variation that exists among organisms within a species. [6]

Variabilities from populations

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Among individuals within a population

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Genetic variation can be identified by many levels of observation traits.[7] There is the phenotypic variation in either quantitative traits (which vary continuously and are coded for by many genes), or the discrete traits (which fall into discrete categories and are coded by one or a few genes, like the different colors of petals from certain flowers). If a trait is advantageous and helps the individual survive and reproduce, the genetic variation will be passed to the next generation (a process known as natural selection).[8]

It can also be identified by examining variations at the level of enzymes using the process of protein electrophoresis. [7] Polymorphic genes have more than one allele at each locus. Half of the genes that code for enzymes in insects and plants may be polymorphic, whereas polymorphisms are less common among vertebrates. Random mutations are the ultimate source of genetic variation. Mutations are rare and most of them are neutral or deleterious, but in some instances, the new alleles can be favored by natural selection. [9]

It is caused by phenotypic variations in the order of bases in the nucleotides in genes. [10] The DNA sequence results to be different from the order of amino acids in proteins coded by it, and if the resultant differences in amino acid sequence influence the shape, thus the function of the enzyme.[11] Some causes are mutation, random mating between organisms, random fertilization, and crossing over (or recombination) between chromatids of homologous chromosomes during meiosis.[12] New technology now allows scientists to directly sequence DNA that has identified even more genetic variation than was previously detected by protein electrophoresis. The examination of DNA has shown genetic variation in both coding regions and the non-coding intron region of genes.[7]

Between populations

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Genetic variation is advantageous because it enables some individuals and, therefore, a population, to survive despite a changing environment.[12] Genetic threats to population viability will be expressed through their effects on demographic and ecological processes. [13] There are genetic differences in populations from different locations. An investigation was done by Okinawa Institute of Science and Technology Graduate University, Ecology and Evolution unit, Okinawa, Jap were looking for genetic diversity in bees from Islands. There were a cytoplasmic disequilibrium and genetic structure characteristic of a population that was experimenting with hybridization, in this case, between A. m. unicolor and A. m. ligustica, A. m. carnica, and A. m. mellifera-like individuals. [14] Another similar investigation was done from Escuela de Biología, Universidad de Costa Rica, Ciudad Universitaria Rodrigo Facio, San José, Costa Rica. The studies revealed that there were high levels of genetic variability at the loci in another bee (African and European alleles) population. [15] Their adaptability allows some individuals to give generations maintenance.[16] Genetic variability in a population is important for biodiversity. [17]


The amounts of variation vary from species to species, depending on history and environment. But also variations can be inherited by reproduction or be the result of what the individual does and how is their surrounding, this is environmental variation.[18] For example, humans have inherited variations like gene frequencies (those for skin color or hair form). It is a characteristic for individuals between a population of the same species (like the major geographical groups' so-called geographical races). [19] On the other hand, environmental variation can be caused by the physical environment (light, temperature, minerals, exposure, etc.) and biological factors (parasites or any other species that interact with the species under study).[20] A study taken from British Columbia explains that in springtime the sea blush (Plectritis congesta) grows profusely into very large plants but on Mill Hill near Victoria, the sea blush is found to be much smaller. Yet when seeds from both these locations were planted in one growth chamber at the University of British Columbia, the plants were indistinguishable. The soil on the small island (they have named it Plectritis Island) is very rich in nutrients, possibly a result of an accumulation of sea-bird droppings[21].[22][20]

Maintenance in populations

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Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits to their offspring.

The source of all variations must be a mutation. However, that process does not itself drive evolution. Its rates are so low that mutation alone cannot account for the rapid evolution of populations and species.[23] A variety of factors maintain genetic variation in populations, like some recessive alleles that are damaging inside a diploid population, these alleles hide from the selection in heterozygous individuals.[24] Organisms that carry these new mutations will die out.[16] Another factor is natural selection because it maintains some genetic variations in balanced polymorphisms. They may occur when heterozygotes are favored or when selection is frequency-dependent. In other words, the organisms reproduce and pass on their genes (there is a chemical and mechanism conservation). This way there is insurance that the allele between the population will survive. [25] Natural selection and genetic drift also act directly on phenotypes to enable phenotypic variation. [16] both are the main forces that drive evolution.[26]

Sources

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Some important sources of genetic variation are gene flow and sex. The gene flow is any movement of genes from one population to another and sex can introduce new gene combinations, because it is a genetic shuffling, into a population.[5] This is because sexual reproduction cannot produce new alleles, so meiosis and fertilization shuffle alleles from past mutations into new combinations or [27] crossing over (genetic recombination). Variation and recombination can be facilitated by transposable genetic elements, endogenous retroviruses, LINEs, SINEs, etc.[28][29] Mutations are likely to be rare and most of them are neutral or deleterious, but in some instances, the new alleles can be favored by natural selection.[30] Polyploidy is an example of a chromosomal mutation. It is a condition where the organisms have three or more sets of genetic variations. [31]

Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation. However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction. Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction.[12]

Ladybugs in Mill Valley
Ladybugs in Mill Valley

Structure in genomes

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Measurement

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The measurement is important for the evolutionary factors for its many roles in genetic variations.[32] The reflection must be temporal as well as spatial changes. Two classes of measures will be considered: those describing the genetic variability within populations, and genetic variability between populations. For each of these we have to deal with their pattern of changes over time under different evolutionary forces so that the genetic affinities between populations can be studied with the same principles that dictate genetic variations among individuals within a population. Genetic variation within a population is commonly measured as the percentage of polymorphic gene loci or the percentage of gene loci in heterozygous individuals.[33]

Several investigations explains the significance about polyploidy for the evolution of vertebrates and other eukaryotes. The main category polypoids are is allopolyploids, where the genomes are hybrid and differentiated.[34] The origin of the polyploid comes from the three ancestral genomes, with the help of triplications in the genetic information, they create SNP and haplotype studies in polyploid species more challenging than comparable studies in diploids. The haplotype studies are a group of genes within an organism that inherited together from a single parent. The SNP (or SNP´s) is a variation which is in one specific position in the DNA sequence.[35]

Forms

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Genetic variation can be divided into different forms according to the size and type of genomic variation underpinning genetic change. Small-scale sequence variation (<1 kilobase, kb) includes base-pair substitution and indels.[36] Large-scale structural variation (>1 kb) can be either copy number variation (loss or gain), or chromosomal rearrangement (translocation, inversion, or Segmental acquired uniparental disomy). The forms have an important role to balance selection into varieties so the adaptive genetic variation and evoulationary potential of species change.[37] However, studies indicate that this balance won't last long to maintain the adaptive polymorphism (Fijarczyk & Babik, 2015), so it will affect few genes. Also, environmental changes causes the switch of alleles in frequencies and polymorphisms get unstable. There is a high amount of studies that report the transient nature of balancing selection. (Fijarczyk & Babik, 2015).


Genetic variation and recombination by transposable elements and endogenous retroviruses sometimes is supplemented by a variety of persistent viruses and their defectives which generate genetic novelty in host genomes. Numerical variation in whole chromosomes or genomes can be either polyploidy or aneuploidy.[38] About the ploidy level, the higher it is, the larger the size [31]

Populations and examples of variation

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An enormous amount of phenotypic variation exists in the shells of Donax varabilis, otherwise known as the coquina mollusc. This phenotypic variation is due at least partly to genetic variation within the coquina population.
A range of variability in the mussel Donax variabilis

Genetic Variation and Evolution

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Organisms adapt to their surroundings by biological mechanisms. For example, humans, more specifically Tibetans, breathe thin air normally because their bodies evolved and adapt by living at high altitudes. Most people would be sick, others could survive a short time because their bodies raise levels of hemoglobin, which is a protein that transports oxygen in the blood. But continuously high levels of hemoglobin are dangerous, so increased hemoglobin levels are a bad solution to high-altitude survival in the long term. On the other side, Tibetans evolved genetic mutations that allow them to use oxygen far more efficiently without the need for extra hemoglobin. Other organisms can also exhibit behavioral adaptation, like emperor penguins in Antarctica share warmth in the middle of the winter by clumping together.[40]

An adaptation is a usual characteristic in a population and is produced by natural selection. A form of adaptation is behavior, which allows better evasion of predators, a protein that works better at body temperature, or an anatomical trait that allows the organism to access an important new resource. An example of what is not considered adaptation is vestigial structures. It is a feature that was an adaptation for the organism's ancestor, but that evolved to be non-functional because the organism's environment changed. [41] When it is not needed this adaptability, then natural selection gets rid of it.

Charles Robert Darwin and Alfred Russel Wallace developed the theory of natural selection after there were many discussions about adaptation. Wallace believed that the evolution of organisms was connected in some way with the adaptation of organisms to changing environmental conditions[40]. After many discoveries, Wallace proposed a Theory of Evolution which matched the unpublished ideas of Darwin. This encouraged Darwin to collect his scientific ideas and collaborate with Wallace, both published their scientific ideas jointly in 1858.[42]


Charles Robert Darwin

Charles R. Darwin

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Since his childhood, he showed signs of appreciation for natural history, especially, for collecting things. Darwin began studying medicine to continue the family saga. He went to Cambridge and learned scientific dissertations that motivated him. He was taught the appliance of a scientific methodology, analysis and to take detailed information from his observations. [43] Already with a lot of curiosity and information, Charles Darwin began to write in his notebook, firstly about ¨Transmutation of species¨. On his travels, he collected plants, animals, and fossils, and took copious field notes. These collections and records provided the evidence he needed to develop his remarkable theory.[44] His few investigations were already very close to the idea of selection. When he read the book of Thomas Robert Malthus, he found the answers he was looking for. However, publishing his notes and research would have repercussions and not everyone would accept the publication. Meanwhile, he kept collecting new information until he received a letter from Alfred Russel Wallace, which made him change his mind. On November 24, 1859, his first copies On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life were sold (1,250) that same day.[45]

The theory of Darwin

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Charles Darwin was the first person to appreciate clearly that evolution depends on the existence of heritable variability within a species to generate the differences between ancestral and descendant populations. Darwin’s theory of the mechanism of evolution begins with the variation that exists among organisms within a species. Individuals of one generation are qualitatively different from one another. The evolution of the species as a whole result from the differential rates of survival and reproduction of the various types, so the relative frequencies of the types change over time. For Darwin, the evolution of the group resulted from the differential survival and reproduction of individual variants already existing in the group—variants arising in a way unrelated to the environment but whose survival and reproduction do depend on the environment.[46] As this was expressed in the first coherent draft of the theory, a 39-page manuscript written in 1842, this discussion transferred the concept of selection of forms by human agency in the creation of the varieties of domestic animals and plants, to the active selection in the natural world by an almost conscious agency. This agency selects out those traits most beneficial to organisms with conditions of life, analogous in its action to the selection by a man on domestic forms in the production of different breeds. [47]

The power of Selection, whether exercised by man or brought into play under nature through the struggle for existence and the consequent survival of the fittest, absolutely depends on the variability of organic beings. Without variability, nothing can be effected; slight individual differences, however, suffice for the work and are probably the chief or sole means in the production of new species. Charles Darwin (1868)[48]



See also

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References

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Further reading

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