Natural selection is the primary mechanism driving evolutionary change, where environmental pressures selectively favor certain traits within a population. This process is a predictable outcome of how organisms interact with their surroundings. The consequence is a measurable shift in the frequency and distribution of observable characteristics, or phenotypes, across successive generations. Understanding the conditions necessary for this process and the pattern of change it creates helps explain how populations adapt and how their collective traits are altered over time.
Prerequisites for Evolutionary Change
Evolutionary change through natural selection requires certain conditions within a population. The first is variation in traits among individual organisms. Members of the same species must exhibit differences in characteristics like size, color, or resistance to disease. Without this inherent diversity, the environment has no raw material to act upon.
The second necessary component is inheritance, meaning the variation must be heritable—passed down genetically from parent to offspring. If a trait that provides an advantage is not encoded in an organism’s genes, it cannot become more common in the next generation. This genetic link ensures that successful traits are propagated through the lineage.
The final requirement is differential survival and reproduction. Some individuals possess traits that allow them to survive better or produce more offspring than others. This difference in reproductive success is the engine of natural selection; organisms with advantageous heritable traits contribute disproportionately to the next generation’s gene pool. When these three factors are present, the distribution of traits within the population will change.
Directional Selection and the Trait Shift
One pattern of natural selection is directional selection. This mode occurs when individuals exhibiting one extreme of a phenotypic range have higher biological fitness than those with intermediate or opposite traits. The environmental pressure consistently favors one end of the trait spectrum. This sustained pressure forces the population to shift its average trait value.
Over time, the continuous advantage given to the extreme phenotype causes the entire frequency distribution of the trait to move in that direction. For example, if larger body size is favored, the population’s average size will steadily increase across generations. This is visually represented as the peak of the distribution curve shifting along the trait axis.
The population’s mean trait value changes, moving toward the formerly extreme phenotype. This selective process can lead to relatively rapid evolutionary changes, particularly when an environment changes abruptly. The frequency of beneficial alleles increases, while less-fit alleles are gradually reduced, leading to a new, skewed distribution of the trait.
Case Study: The Peppered Moth
The evolution of the Peppered Moth (\(Biston\) \(betularia\)) in England provides a classic instance of directional selection altering trait distribution. Before the Industrial Revolution, the vast majority of these moths were the light-colored, or typica, form. Their pale, speckled wings provided excellent camouflage against light, lichen-covered tree trunks, hiding them from predatory birds. The darker, melanic form (carbonaria) existed but was rare, making up less than 2% of the population due to its high visibility on light bark.
The Industrial Revolution introduced coal-burning factories, releasing massive amounts of soot and sulfur dioxide into the atmosphere. This pollution killed the lichens and blackened the tree trunks across industrial regions, dramatically changing the moths’ habitat. The light-colored moths became suddenly conspicuous against the darkened bark, making them easy targets for visual predators. The once-rare dark moths were now perfectly camouflaged against the soot-covered trees.
This environmental shift reversed the selective advantage, favoring the dark phenotype. Dark moths survived and reproduced more successfully than light moths, causing the frequency of the dark trait to surge. In Manchester, the frequency of the melanic form rose from nearly zero in 1848 to approximately 98% by 1895, a shift that occurred in fewer than fifty generations. This rapid change illustrates the power of directional selection acting on existing, heritable variation.
The moth’s adaptation continued when legislation, such as the Clean Air Acts, reduced air pollution significantly in the mid-twentieth century. As air quality improved, soot disappeared, allowing lichens to return and the bark to lighten. The selective pressure shifted back; dark moths became more visible, and light moths regained their advantage. This environmental reversal caused the trait distribution to shift once again, with the light form becoming the predominant phenotype in urban areas, mirroring the pattern seen in the cleaner countryside.