Evolution is the process through which life on Earth has diversified and adapted. A primary force driving this change is natural selection, a mechanism that shapes populations by favoring traits that enhance survival and reproduction. This article explores how natural selection influences characteristics determined by a single gene, leading to observable shifts within populations.
Defining Natural Selection and Single-Gene Traits
Natural selection is a process in which organisms better suited to their environment are more likely to survive and produce offspring. Individuals with advantageous characteristics tend to pass those traits on to the next generation with greater success. Over time, this selective pressure can lead to changes in the genetic makeup of a population.
Single-gene traits are characteristics determined by the alleles of a single genetic locus. For instance, the ability to taste phenylthiocarbamide (PTC) is a single-gene trait, as is the presence of attached or unattached earlobes in humans. Many common human traits, such as eye color or height, are influenced by multiple genes and are not considered single-gene traits.
The Process of Selection Acting on Single Genes
Natural selection influences the frequency of specific alleles for a single-gene trait within a population. First, individuals within a population display variation in a particular trait. This variation arises from different alleles present in the gene pool.
These varying traits are heritable. Genes, the basic units of heredity, contain the instructions for these traits and are transmitted across generations. This ensures that any advantageous traits can be perpetuated in the population.
When certain alleles lead to a trait that provides a survival or reproductive advantage, individuals carrying these alleles are more likely to survive and produce more offspring. This differential survival and reproduction means that the beneficial alleles are passed on more frequently. For example, a trait that helps an organism find food or avoid predators would be advantageous.
Over many generations, this consistent advantage causes the frequency of the beneficial allele to increase within the population. Simultaneously, less advantageous alleles become rarer. This shift in allele frequencies is a measurable form of evolution, demonstrating how natural selection can fine-tune a population’s characteristics to its environment.
Observable Changes in Populations
The impact of natural selection on single-gene traits is evident in various real-world scenarios. Antibiotic resistance in bacteria is a clear example. Bacteria can acquire resistance through single gene mutations. When antibiotics are present, they act as a strong selective pressure, favoring resistant bacteria, leading to a rapid increase in the frequency of resistance-conferring alleles within the bacterial population.
Another compelling illustration is the relationship between sickle cell trait and malaria. Sickle cell trait, caused by a single gene mutation affecting hemoglobin, provides a protective effect against severe malaria. Individuals who inherit one copy of the mutated gene (heterozygotes) are more resistant to malaria, giving them a survival advantage in malaria-endemic areas.
The peppered moth (Biston betularia) offers a classic example of industrial melanism, where a single gene controls its coloration. During Britain’s Industrial Revolution, soot darkened tree trunks, making light-colored moths vulnerable to predators. Dark-colored moths, carrying a dominant allele for melanism, became camouflaged. As pollution decreased later, the selective pressure reversed, and the lighter form regained its advantage, demonstrating the dynamic interplay between environment and single-gene traits.