Origins of Genetic Variation

Mechanisms Generating Genetic Variation

Mutation: A change in the DNA sequence. This can occur spontaneously or can be induced by environmental factors (mutagens). Mutation introduces new alleles into the population and is the only source of new genetic material.
Recombination: Occurs during meiosis in sexual reproduction. Homologous chromosomes line up in pairs and exchange segments of DNA in a process known as crossing over. This allows the alleles from two parents to be shuffled, creating unique combinations of alleles in the offspring.
Independent assortment: Also occurs during meiosis. The maternal and paternal copies of each chromosome line up randomly on the metaphase plate before they are pulled apart. This means each gamete (egg or sperm) gets a different combination of the parent’s chromosomes.
Fertilisation: The combination of the male and female gamete also generates variation as it brings together two different sets of alleles.

Sexual reproduction increases genetic variation because it combines alleles from two parents. This leads to genetic diversity within a population, providing material for natural selection to act upon and increasing the chances of adaptation to changing environments.
The genetic variation produced by sexual reproduction also contributes to the uniqueness of each individual. Except for identical twins, no two individuals have the same genetic makeup.

Genetic drift refers to random changes in allele frequencies that can lead to changes in genetic variation. This process is particularly potent in small populations.
Two specific scenarios where genetic drift can have a significant effect are called the bottleneck effect and the founder effect. Both can lead to reduced genetic diversity and increased likelihood of inbreeding.

Gene flow, or migration, is another mechanism that can increase genetic variation within a population. When individuals from one population move to and breed in another population, they bring their unique genetic material with them.
This not only increases the overall genetic diversity of the receiving population but also decreases the genetic differences between the two populations.

Natural selection acts on genetic variation within populations. Individuals with beneficial traits, conferred by their specific set of alleles, are more likely to survive and reproduce.
Over generations, the alleles associated with these beneficial traits increase in frequency within the population. However, if the environment changes, different traits may become advantageous, leading to a change in allele frequencies.
Natural selection does not create genetic variation but changes the distribution and frequency of alleles within populations.