Genetic variability refers to the differences in DNA sequences among individuals in a population, which is the foundation for uniqueness in the living world. For example, the vast assortment of dog breeds, from a Great Dane to a Chihuahua, all belong to the same species (Canis lupus familiaris). This diversity is a direct result of the genetic information present within that single species.
The Sources of Genetic Variation
The origin of all new genetic information is mutation, a permanent alteration in the DNA sequence. These changes can arise from errors during DNA replication or from exposure to environmental factors like UV radiation. While many mutations have no effect, some can introduce a new version of a gene, called an allele, into a population. For instance, a single mutation in the MC1R gene is responsible for red hair in humans.
Gene flow is the transfer of genetic material from one population to another. When individuals migrate, they bring their unique alleles, increasing the genetic diversity of the new population and reducing differences between the two groups. An example is plant pollen being transported by wind or insects over long distances, which fertilizes plants in a different area and mixes the gene pools.
Sexual reproduction shuffles existing genes into new combinations, creating unique offspring. During meiosis, the process that forms sperm and egg cells, two events create this variation. The first is crossing over, where homologous chromosomes exchange segments to create new allele combinations along a single chromosome. This is analogous to shuffling a deck of cards; the same cards are present, but each shuffle creates a new, unique order.
The second process is independent assortment, which describes the random way chromosomes sort themselves into sex cells. This further multiplies the potential genetic combinations. Together, these recombination events ensure that, with the exception of identical twins, every individual has a distinct genetic makeup.
The Role of Genetic Variability in Evolution
Genetic variability is the raw material for natural selection, which drives evolutionary change. A population with high genetic variation is more likely to possess individuals with traits that are advantageous in changing environmental conditions. When the environment shifts, some individuals will have pre-existing traits that allow them to survive and reproduce more successfully than others.
The peppered moths (Biston betularia) in England during the Industrial Revolution illustrate this principle. Before this period, light-colored moths were abundant because they were well-camouflaged against lichen-covered trees, while a dark-colored variation existed at a low frequency. As industrial pollution blackened the trees with soot, the light-colored moths became conspicuous to predators, while the dark-colored moths became better camouflaged.
This environmental shift conferred a survival advantage on the dark moths. Consequently, they survived longer, reproduced more, and passed their dark-color alleles to their offspring. Over generations, the frequency of the dark-colored moths increased in industrial areas, demonstrating how natural selection acts on existing genetic variation to cause adaptation.
Consequences of Limited Genetic Variability
When a population has low genetic variability, its ability to adapt to environmental changes is restricted, making it more vulnerable to extinction. If all individuals are genetically similar, a single threat like a new disease could wipe out the entire population, as no individuals may have the genes to resist it.
The cheetah (Acinonyx jubatus) is an example of a species with low genetic variability. Scientists believe they experienced a population bottleneck—a sharp reduction in population size—thousands of years ago, which reduced their genetic diversity. As a result, modern cheetahs are so genetically uniform that skin grafts between unrelated individuals are not rejected. This uniformity makes the species highly susceptible to diseases.
The danger of genetic uniformity is also evident in agriculture, as shown by the Irish Potato Famine of the 1840s. The Irish population relied on a single variety of potato, the “Lumper,” which was grown clonally. Because the potatoes were genetically identical, they were all susceptible to the same pathogen. When a water mold, Phytophthora infestans, arrived, it decimated the crop, leading to widespread famine.
Genetic Variability in Humans
Genetic variability is responsible for the diversity of human traits. Physical characteristics like height, skin tone, and eye color are influenced by DNA variations. This diversity also extends to blood type, metabolic rates, and susceptibility to various health conditions.
This variation affects how individuals respond to diseases. For example, variations in immune system genes can affect a person’s ability to fight infections. A well-known example is the sickle cell allele; individuals with one copy of this allele have increased resistance to malaria, a survival advantage where malaria is common.
The field of pharmacogenomics studies how a person’s genetic makeup influences their response to drugs. Variations in genes that control how medications are metabolized can determine if a drug is effective or causes side effects. This understanding allows for personalized medicine, where treatments are tailored to an individual’s genetic profile.