Genetic variation is the difference in deoxyribonucleic acid (DNA) among individuals within a population or species. It is the foundation of biological diversity, providing the raw material for differences in traits, appearance, and internal functions. A gene is a specific segment of DNA containing instructions for a functional molecule that influences a characteristic. Different versions of that gene are called alleles, and the combination of these alleles defines an organism’s unique genetic makeup.
The Core Sources of Genetic Variation
New genetic differences primarily arise through mutation and genetic recombination.
Mutation involves a random, heritable change in the sequence of DNA nucleotides. These changes can range from a single misplaced base pair to the duplication or deletion of large chromosome sections. Mutation provides the ultimate source of all novelty in a gene pool.
Genetic recombination generates new combinations of existing alleles, often occurring during sexual reproduction. During the formation of sperm and egg cells, homologous chromosomes exchange segments of DNA through crossing over. This shuffling ensures the offspring receives a unique blend of parental genes.
A third mechanism influencing variation is gene flow, which involves the migration of individuals between populations. When individuals move, they introduce their unique alleles into the new population’s gene pool, increasing diversity. This movement counteracts the tendency for isolated populations to become genetically distinct.
Relatable Examples of Human Variation
The ABO blood type system is an example of human genetic variation controlled by a single gene. This gene has three common alleles—\(I^A\), \(I^B\), and \(i\)—which determine the presence or absence of specific carbohydrate molecules on red blood cells. The combination of two of these alleles dictates whether a person has type A, B, AB, or O blood.
Another example involves the ability to taste phenylthiocarbamide (PTC), governed by the TAS2R38 gene. Individuals who inherit the “taster” allele can detect the bitter compound, while those with two copies of the “non-taster” allele cannot perceive the chemical. Variation also exists in human disease susceptibility, such as the sickle cell trait. A single amino acid change in the hemoglobin protein provides resistance to malaria in prevalent regions.
Variation Driving Survival in the Wild
In non-human species, genetic variation is directly linked to a population’s ability to respond to environmental pressures. A classic illustration is the rapid evolution of the peppered moth (Biston betularia) in response to industrial pollution. Before the Industrial Revolution, most moths were light-colored, camouflaging them against lichen-covered trees; however, a rare, darker allele existed.
As soot blackened the tree trunks, light moths became visible to predators, while the dark moths were camouflaged and survived at a higher rate. This shift quickly drove the frequency of the dark allele to nearly 90% in polluted areas, demonstrating how existing variation is leveraged for survival.
A similar mechanism is observed in the development of antibiotic resistance in bacteria. When a bacterial colony is exposed to an antibiotic, most cells are killed. However, a few individuals may possess a random mutation that makes them resistant to the drug. These resistant bacteria survive the treatment and rapidly reproduce, passing on their protective allele. This process of selection quickly leads to a predominantly resistant population.
The Essential Role of Diversity in Evolution
Genetic variation serves as the resource, acting as the raw material for natural selection. Without a pool of diverse alleles, a population cannot adapt when its environment changes due to climate shifts, new predators, or emerging diseases. The presence of multiple alleles means some individuals will possess traits that allow them to survive conditions lethal to others.
This diversity creates population resilience, the capacity of a species to withstand and recover from disturbances. Populations with low genetic diversity, such as endangered species, are vulnerable because a single threat can wipe out the entire group. Maintaining high genetic variation ensures a species retains the evolutionary potential to meet unknown challenges.