Genetic variation as a cause of evolution

What is genetic variation?

• A representation of the genetic differences that occur within a population is termed as genetic variation.
• The genetic variation of whole the species is usually termed as genetic diversity.
• The differences in genes or DNA segments are genetic variations and each variation of a gene is termed as an allele.
• For instance, there is a high amount of genetic variation in a population of several different alleles at a single chromosome locus.
• For natural selection, genetic variation is important, because natural selection can only increase or decrease the frequency of alleles that already exist in the population.
• Since conditions are unpredictable, genetically diverse populations may be able to adapt faster than those that do not involve genetic variation to evolving circumstances.
• All examples of genetic variations that can occur in a human population are the skin tone, hair color, dimples, freckles, and type of blood of an individual.
• The modified leaves of carnivorous plants and the development of flowers that mimic insects to attract plant pollinators are examples of genetic variation in plants.
• Albinism, cheetahs with markings, snakes flying, animals that play dead, and animals that resemble leaves are examples of genetic variation in animals.
• These variations allow the animals in their habitats to better adapt to conditions.

What are the causes of genetic variation?

• The main causes of genetic variation are:
  • Mutation
  • Gene flow
  • Sexual reproduction
  • Genetic drift
• However, there are other causes of genetic variation:
  • Random mating
  • Crossing over
  • Random fertilization
  • Non-random mating
  • Environmental variance

i. Mutation:

• In general, mutation is the primary source of genetic variation, which is the raw material for evolution through natural selection.
• A change in the DNA sequence is a mutation.
• Often these differences in gene sequences may be beneficial for an organism.
• By modifying genes and alleles in a population, mutations result in genetic variation.
• They may affect an individual gene or a whole chromosome.
• Though mutations alter the genotype of an organism (genetic makeup), they do not necessarily change the phenotype of an organism.
• They may affect an individual gene or a whole chromosome.
• Though mutations alter the genotype of an organism (genetic makeup), they do not necessarily change the phenotype of an organism.
• During replication, mutations occur in the DNA molecule, such that the daughter cells vary in sequence or DNA quantity from the parent cells.
• A mutation initially appears in a single cell of the body, but it is transferred on to all cells descended from the first.
• Mainly two types of mutation are discussed they are gene mutations and chromosomal mutations.

ii. Gene mutations:

• In the case where the nucleotide sequence is modified and is passed on to the offspring, gene mutation occurs.
• The alteration might be either insertion or deletion or substitution of one or a few nucleotides.
• The addition or deletion of nucleotides within the DNA sequence in the coded protein shifts the reading frame and hence results in highly altered sequence of amino acids.
• Substitutions of nucleotides, however, may or may not severely affect the biological function of the protein.
•  Those substitution that results in terminator codon are supposed to be harmful.
• Gene mutation take place spontaneously without being purposedly induced by humans.
• Ultraviolet light, X-rays, and other high-frequency electromagnetic radiation, as well as exposure to such mutagenic substances, such as mustard gas, can also trigger them.
• The effects of gene mutations can vary from negligible to lethal.
• Mutations that modify one or even many amino acids can have a minor or undetectable impact on the ability of the organism to live and replicate if the coded protein’s basic biological role is not hampered.
• But when an amino acid substitution influences an enzyme’s active site or changes a protein’s essential function in some other way, the effect can be severe.

iii. Chromosomal mutations:

• The mutation process produces several new genetic variants for each generation. The nucleus of each cell contains chromosomes that hold the inherited material, or DNA.
• Chromosomes occur in pairs and each of them is inherited from each parent.
• In the pair, the two members are termed as homologous chromosomes.
• As a rule, every cell of an organism and all people of the same species have the same number of chromosomes.
• However, the number, size, and organization of chromosomes differs between species.
• Alterations in the number, size, or organization of chromosomes within a species are referred as chromosomal mutations, chromosomal abnormalities, or chromosomal aberrations.
• By fusion of two chromosomes into one, by fission of one chromosome into two, or by addition or subtraction of one or more whole chromosomes or sets of chromosomes, changes in number can occur.
• Polyploidy is the condition in which one or more additional sets of chromosomes are acquired by an organism.
• Changes in the arrangement of chromosomes may occur by inversion when a chromosomal segment rotates 180 degrees in the same place; by duplication when a segment is added; by deletion when a segment is lost; or by translocation when a segment changes from one place to another.
• These are the mechanisms by which chromosomes evolve.
• The amount of DNA is not changed by inversions, translocations, fusions, and fissions.
• The significance of these mutations in evolution is that they alter the linkage relationships among genes.
• Genes that are closely related to each other are isolated and vice versa; this may impact their expression because genes are mostly sequentially transcribed, two or more at a time.

iv. Gene flow:

• It is also termed as gene migration.
• New genes are introduced into a population as organisms migrate into a new environment.
• The availability of new alleles in the gene pool makes it possible for new gene combinations.
•  The emigration, i.e., moving out of organisms from a population also alters the gene frequencies.
• The emigration results in the lack of genetic diversity.
• Immigration of new organisms into a population may favor organisms to adapt in changing environmental conditions.
• Sustained gene flow between two populations can also result to a combination of the two gene pools, decreasing the genetic variation between the two groups.
• Gene flow strongly plays role against speciation.
• It does so by the recombination of the gene pools of the groups, and hence, repairment of the developing differences in genetic variation that would have resulted to full speciation and formation of daughter species.
• For instance, if a species of grass grows on both sides of a road, pollen is likely to be migrated from one side to the other and vice versa.
• If this pollen is capable of fertilizing the plant where it ends up and producing viable offspring, then the pollen alleles have effectively connected the population to the other on one side of the road.

v. Sexual reproduction:

• Sexual reproduction enhances genetic variation by producing various gene combinations.
• The process by which gametes or sex cells are formed is known as meiosis.
• As alleles in the gametes are separated and haphazardly united upon fertilization, genetic variation occurs.
• During meiosis, the crossing over or swapping of homologous chromosomes also results in the genetic recombination of genes.

vi. Genetic drift:

• Genetic drift is the alteration of a population’s allele frequencies owing to occurrences of random chance, such as natural disasters.
• The converse of natural selection is genetic drift.
• The theory of natural selection maintains that certain individuals in a population have characteristics that allow more offspring to survive and produce, whereas other people have characteristics that are harmful and can cause them to die before reproducing.
• These selection pressures will alter the gene pool and the traits within the population over successive generations.
• A huge, strong male gorilla, for instance, will mate with more females than a small, weak male, so more of his genes will be passed on to the next generation.
• His descendants will continue to control the troops and also pass on their genes.
• Over time, in the gorilla population, the selection pressure would cause the allele frequencies to change towards big powerful males.
• Unlike natural selection, genetic drift explains the impact of chance on populations in the absence of positive or negative selection pressure.
• Allele frequencies within a population may change through random sampling, or the survival or reproduction of a random sample of individuals within a population.
• Instead of a male gorilla generating more offspring since he is stronger, he may be the only male accessible when a female is ready to mate.
• His genes are transferred on to future generation because of chance, not as he was the biggest or the strongest.
• Genetic drift is the transition of alleles within a population due to chance events that cause random samples of the population to produce offspring or not.
• Small populations are more prone to the powers of genetic drift.
• On the other hand, large populations are buffered against the consequences of chance.
• If one individual in a population of 10 happens to die at a young age before leaving any offspring to the next generation, all of their genes (1/10 of the gene pool of the population) will instantly be lost.
• The individual comprises only 1 percent of the total gene pool in a population of100; thus, genetic drift has much less effect on the genetic structure of the larger population.
• The Bottleneck effect:
  • Natural disasters, such as a natural catastrophe that destroys a large portion of the population at random, may also magnify genetic drift.
  • When only a few individuals survive, the bottleneck effect arises and reduces diversity in a population’s gene pool.
  • The survivors’ genetic structure becomes the whole population’s genetic structure, which could be very different from the pre-disaster population.
• The founder effect:
  • If any part of the population leaves to start a new population in a new location or if a population gets separated by a physical barrier of some sort, another scenario arises in which populations may experience a strong impact of genetic drift.
  • In this scenario, it is not likely that those individuals are representative of the total population, which leads to the founder effect.
  • When the genetic structure changes to match that of the founding fathers and mothers of the new population, the founder effect occurs.
  • The underlying effect is believed to have been a key factor in the genetic history of Dutch settlers in South Africa’s Afrikaner population, as evidenced by mutations that are prevalent in Afrikaners, but rare in most other populations.
  • This was likely due to the fact that these mutations were carried by a higher-than-normal proportion of the founding colonists.
  • As a result, Huntington’s disease (HD) and Fanconi anemia (FA), a genetic condition known to cause blood marrow and congenital defects, including cancer, have exceptionally high incidences in the population.

vii. Non-random mating:

• If individuals nonrandomly mate with the other individuals within a population, i.e., they select their mate, selections can drive evolution in a population.
• There are several explanations for the phenomenon of nonrandom mating.
• Simple mate choice or sexual selection is one reason; female peahens, for instance, may prefer peacocks with larger, brighter tails.
• Traits that contribute to more matches for an individual lead to more offspring and ultimately lead to a higher prevalence of that feature in the population through natural selection.
• An individual’s desire to mate with partners that are phenotypically similar to themselves is one common type of mate choice, called positive assortative mating.
• Physical location is another cause of nonrandom mating.
• This is particularly true in large populations distributed over large geographical distances where not all individuals would have equal access to each other.
• These may be miles apart across forests or over rugged terrain, while others may live nearby immediately.

viii. Environmental variance:

• Genes are not the only players involved in population variance determination.
• Other factors, such as the environment, also affect phenotypes.
• For instance, due to daily exposure to the sun, an environmental factor, a beachgoer is likely to have darker skin than a city dweller.
• For certain animals, some major features, such as gender, are determined by the environment.
• For instance, some turtles and other reptiles have temperature-dependent sex determination (TSD).
• TSD implies that individuals grow into males if their eggs are incubated over a certain temperature range, or females at a distinct temperature range.
• Geographic separation among populations can contribute to variation in the phenotypic variation between certain populations.
• In most populations, this geographical variation is seen and can be important.
• One sort of geographic variation, named a cline, could be seen as populations of a given species differ progressively across an ecological gradient.

Gene pool:

• The term gene pool in evolutionary science refers to the collection of all available genes that can be passed down in the population of a single species from parents to offspring.
• The more variation there is in the population, the greater the gene pool.
• The gene pool decides the phenotypes (visible features) are present at any given time in the population.

How do gene pools change?

• Due to the movement of individuals into or out of a population, the gene pool can shift within a geographic region.
• When people with population-specific characteristics move abroad, the gene pool in that population shrinks and the characteristics are no longer available to be passed on to the offspring.
• In the other hand, as new people with new special features immigrate into the population, they expand the gene pool.
• A new form of diversity is added into the population when these new individuals interbreed with individuals already present.
• The size of the gene pool directly impacts the population’s evolutionary trajectory.
• The theory of evolution claims that natural selection works on a population to benefit the favorable features of that environment while weeding out the undesirable characteristics at the same time.
• The gene pool changes since natural selection operates on a population.
• Within the gene pool, beneficial adaptations become more frequent, and the less desirable characteristics become less prevalent or may even vanish entirely from the gene pool.
• Populations with larger gene pools are more likely than those with smaller gene pools to survive as the local climate changes.
• This is due to the fact that a broader range of traits are available to larger populations with wider diversity, which gives them an advantage as the environment changes and demands new adaptations.
• If there are few to no individuals with the genetic diversity needed to survive changes, a smaller and more homogeneous gene pool puts the population at risk of extinction.
• The more diverse the population, the more likely they are to survive significant changes in the environment.

Examples of gene pool in evolution:

• Individuals who are antibiotic-resistant are more likely to withstand any form of medical intervention in bacteria populations and live long enough to reproduce.
• The gene pool evolves over time (rather quickly in the case of rapidly reproducing organisms such as bacteria) to comprise only bacteria resistant to antibiotics.
• In this way, new strains of virulent bacteria are produced.
• A large number of plants that farmers classify as weeds are so tenacious because they have a diverse gene pool that helps them to adapt to a variety of environmental conditions.
• On the other hand, advanced hybrids also need very precise, even ideal conditions, because they have been bred to have a very small gene pool that prefers certain features, such as beautiful flowers or large fruits.
• It can be said that dandelions are superior to hybrid roses, on basis of size of gene pools.