Selection drives evolutionary change by determining which traits persist and increase in frequency within a population over generations. This process is broadly categorized into natural selection, where the environment is the filter, and artificial selection, where human preference dictates the outcome. While the goal of selection is to shift a group’s characteristics, a key question is whether artificial selection increases the genetic variation available for future change. Answering this requires looking at how human-guided evolution interacts with the differences encoded in an organism’s DNA.
Understanding Artificial Selection and Genetic Variation
Artificial selection, often known as selective breeding, is the practice of intentionally choosing organisms with specific desirable traits to reproduce. This human-imposed pressure is applied across agriculture and animal husbandry, ranging from breeding dairy cows for higher milk yields to selecting crops for greater pest resistance. The aim is to accelerate the concentration of a preferred characteristic, rapidly changing the population’s makeup over relatively few generations.
Genetic variation is the raw material that makes selection possible, representing the differences in DNA sequences, or alleles, among individuals within a population. Without this inherent diversity, no trait could be selected for or against. Selection acts by leveraging these existing differences, increasing the frequency of beneficial alleles while decreasing the frequency of less desirable ones.
The Process of Selective Breeding
The methodology of artificial selection is straightforward: a breeder identifies an individual exhibiting a valuable trait and ensures that organism reproduces. For instance, a farmer selects cattle that produce the most milk or a horticulturalist chooses the pepper plant that yields the largest fruit. The key step is preventing individuals without the desired traits from reproducing or reducing their reproductive success.
This process effectively filters the existing gene pool, concentrating specific alleles within the lineage. A farmer who selects for increased oil content in corn is not creating new genes; they are simply choosing the kernels that already possess the best combination of genes for oil production to parent the next generation. This repeated culling and preferential mating shifts the average trait value over time, confirming that genetic variation existed in the original population.
The Net Effect on Existing Genetic Diversity
The consequence of intense artificial selection is a reduction in the overall genetic variation within the selected population. Selection acts as a directional filter, systematically removing alleles that do not contribute to the desired phenotype, leading to a population that is increasingly genetically uniform for the selected trait. This homogenization occurs because the breeder uses only a small subset of the original gene pool for reproduction, creating a genetic bottleneck.
Widespread selective breeding in agriculture has resulted in highly productive, uniform strains that have replaced diverse, locally adapted varieties. The commercial Cavendish banana, for example, is now almost genetically identical worldwide. This uniformity makes the entire crop vulnerable to a single disease, such as a fungal blight, because no individual possesses the genetic variation needed for resistance. Similarly, intense selection for specific traits in purebred dogs often requires breeding closely related individuals, which compounds the loss of variation and increases the frequency of harmful recessive genetic conditions.
Artificial selection does not increase the amount of existing genetic variation; instead, it rapidly utilizes and depletes it to achieve the desired outcome. The process can quickly reach a limit when the available genetic variants for a particular trait are exhausted, halting further progress. While new varieties are created, increasing phenotypic diversity (the variety of observable traits), the genetic diversity within the breeding line is systematically narrowed.
Sources That Introduce New Variation
Since artificial selection is primarily a filtering mechanism that reduces variation, any truly new genetic material must come from external processes. The ultimate source of all new alleles is mutation, which involves random changes in the nucleotide sequence of DNA. These changes are the only way to introduce novel genes or new versions of existing genes into a population’s gene pool.
Another significant mechanism for introducing variation, particularly in agricultural breeding, is gene flow. This occurs when new alleles are transferred into the selected population through hybridization or cross-breeding with different varieties or wild relatives. Plant breeders often seek out wild germplasm—the collection of genetic resources—to reintroduce traits like disease resistance lost during generations of intense selection for productivity. These external introductions provide the raw material upon which selection can then act.