Natural selection is the process where organisms better adapted to their environment tend to survive and produce more offspring. This differential survival and reproduction is the driving force of evolution, acting directly on the observable characteristics, or phenotypes, of individuals. A single-gene trait, sometimes called a Mendelian trait, is controlled by a single gene locus with two or more variations, called alleles. This leads to distinct, non-continuous phenotypes like having a widow’s peak or attached earlobes. Analyzing the effects of selection on these traits allows scientists to precisely track changes in the genetic makeup of a population over generations.
The Mechanism of Allele Frequency Change
Natural selection’s impact is measured by changes in allele frequencies within the gene pool. Alleles are the different versions of a gene, and their frequency is the proportion of a specific allele relative to all alleles for that gene in a population. Selection acts not on the gene directly, but on the physical trait (phenotype) it codes for, which determines an organism’s fitness. Fitness is a relative measure of reproductive success, describing how well an individual passes its genes to the next generation.
If a single-gene trait, such as a particular coat color, provides a survival advantage, individuals possessing that trait are more likely to survive and reproduce. The alleles responsible for that advantageous phenotype are passed on more frequently. Over time, this consistent differential reproductive success causes the frequency of the beneficial allele to increase, while less favorable alleles decrease. This shift in allele ratios represents evolution occurring at the genetic level.
Directional Selection
Directional selection occurs when the environment favors individuals expressing one extreme of the phenotypic range for a trait. This selection pushes the average phenotype of the population to shift in one direction over time. The result is an increase in the frequency of the alleles that code for the favored extreme trait.
A classic example is the evolution of antibiotic resistance in bacteria, often controlled by a single gene. When an antibiotic is introduced, it acts as a strong selective pressure, killing most non-resistant bacteria. Only those with the resistance allele survive and reproduce, rapidly increasing its frequency. Similarly, the peppered moth’s color shift during the Industrial Revolution demonstrated this process. As industrial soot darkened tree trunks, dark-colored moths, controlled by a single gene, gained a camouflage advantage and rapidly increased in frequency.
Stabilizing Selection
Stabilizing selection operates by favoring the intermediate phenotype and selecting against individuals with extreme variations of the trait. This process does not change the average value of the trait but reduces the amount of genetic variation within the population. By eliminating the extremes, stabilizing selection keeps the population clustered around the optimal, average trait value.
The most widely cited example is human birth weight. Babies with very low birth weights struggle to survive, while those with very high birth weights often face complications during delivery. Infants with an average birth weight of about seven pounds have the highest survival and reproductive rates. This pressure against both extremes reduces the frequency of alleles associated with very small or very large size, maintaining a narrow, optimal range for the trait.
Disruptive Selection
Disruptive selection acts by favoring individuals at both extremes of the phenotypic distribution. This selection pressure works against the intermediate phenotype, which is less successful than either extreme. The result is a population that splits into two distinct groups, which can eventually lead to the formation of new species.
A biological example is the African finch known as the black-bellied seedcracker (Pyrenestes ostrinus), where bill size is controlled by a single gene. These birds feed on sedge seeds that come in two sizes: very hard, large seeds and softer, small seeds. Birds with very large bills can crack the hard seeds, and birds with very small bills are efficient at handling the small seeds. Medium-billed birds are inefficient at handling either size, giving them a lower survival rate. This selection against the intermediate bill size maintains two separate, high-frequency alleles in the population, creating a two-peaked distribution for the single-gene trait.