Natural selection is the non-random process that causes populations of living things to change over time, representing a fundamental mechanism of evolution. This process acts directly on the physical traits of individual organisms, determining which ones are most likely to survive and reproduce in a given environment. The resulting differential success in passing on genetic material gradually shifts the characteristics of the entire population across generations. Understanding this mechanism offers a scientific explanation for the diversity of life on Earth and clarifies how organisms become suited to their surroundings.
The Necessary Conditions for Natural Selection
For natural selection to act upon a population, four specific conditions must be present. The first condition is Variation, meaning individuals within a population must exhibit differences in their traits. This genetic diversity arises randomly through mutation and genetic recombination.
The second requirement is Inheritance, where these varying traits must be reliably passed down from parents to offspring. Without heritability, any advantageous trait would be lost in the next generation, preventing the accumulation of beneficial changes. The third condition is Selection, which refers to differential survival and reproduction. Some individuals possess traits that give them an advantage in obtaining resources, avoiding predators, or finding mates, making them more likely to successfully produce offspring than others.
The fourth element is Time, which allows the cumulative effects of selection to become visible as an Adaptation. Over many generations, advantageous traits become more common in the population because the individuals carrying them reproduce more successfully. This gradual accumulation of favorable characteristics results in a population better suited to its environment.
Classic Example: Industrial Melanism in Peppered Moths
A classic example of natural selection is industrial melanism observed in the Peppered Moth (Biston betularia) in Great Britain. Before the Industrial Revolution, the majority of these moths were the light-colored form, typica, which featured white wings speckled with black. This coloration provided camouflage against the lichen-covered, light-colored tree trunks where they rested, hiding them from predatory birds.
The industrial era introduced a selective pressure, as coal-burning factories blanketed the landscape in soot, killing the lichens and blackening the tree bark. The light moths now stood out against the dark background, making them easy targets for visual predators. Simultaneously, a dark-colored variant, carbonaria, which was previously rare, suddenly gained a survival advantage.
This melanic form arose from a spontaneous mutation that caused the moth to be nearly black. In the polluted environment, the dark moths were effectively camouflaged, while the light moths suffered high rates of predation. The selective pressure of bird predation caused the frequency of the dark form to rise from less than 2% to over 98% in heavily industrialized areas by 1895.
The subsequent reversal of this trend offers confirmation of the mechanism. Following the implementation of Clean Air Acts, pollution levels dropped, and lichens began to return, lightening the tree bark. As the environment reverted, the selective advantage reversed, and the lighter typica form began to increase in frequency, demonstrating that the trait’s advantage depends entirely on the environmental conditions.
Contemporary Example: The Evolution of Drug Resistance
The rapid evolution of drug resistance in infectious microorganisms, particularly antibiotic resistance in bacteria, is a contemporary example of natural selection with public health implications. When a population of bacteria is exposed to an antibiotic, the drug acts as an intense selective pressure. Most bacteria in the population are susceptible and are quickly killed by the treatment.
Due to random genetic mutations, a few individual bacteria may possess a trait that allows them to survive the antibiotic, such as a mechanism to pump the drug out of the cell or an enzyme to chemically inactivate it. These resistant individuals are the only ones left to reproduce, as their susceptible neighbors have been eliminated. Bacteria have short generation times, sometimes dividing every 20 minutes, allowing this selective advantage to rapidly manifest in the population.
The resistant bacteria quickly multiply, passing their resistance genes to their offspring, and they can also transfer these genes horizontally to other bacterial species. This rapid proliferation results in a new population dominated by drug-resistant strains, often termed “superbugs,” such as Methicillin-resistant Staphylococcus aureus (MRSA). The medical community is challenged by this evolutionary arms race, where the widespread use of antimicrobial drugs selects for the survival and spread of resistant pathogens.