Natural selection is a fundamental process that drives evolution, leading to changes in the heritable traits of a population across generations. It serves as a primary mechanism explaining the vast diversity of life observed on Earth. Organisms that are better suited or “adapted” to their surroundings are more likely to survive and successfully pass on the genetic information contributing to their success. Charles Darwin, alongside Alfred Russel Wallace, is largely credited with developing and popularizing the theory of evolution by natural selection in the mid-19th century.
Variation Within Populations
A foundational principle of natural selection is variation among individuals within any population. Members of the same species are not identical; they exhibit differences in physical characteristics, behaviors, and physiological attributes. These variations can manifest in traits such as size, color, speed, or disease resistance.
Genetic mutation is the primary source of this variability, introducing new gene variants. Sexual reproduction also shuffles existing genetic material through recombination, creating new trait combinations.
This variability provides the raw material upon which natural selection acts. Without it, environmental pressures would have no distinct traits to favor or disfavor.
Inheritance of Traits
Many variations are heritable, meaning they can be passed from parents to offspring. This genetic inheritance ensures that traits contributing to an individual’s success can be transmitted to the next generation. While the precise mechanisms of heredity were not fully understood during Darwin’s time, the concept that offspring generally resemble their parents was clear. For natural selection to lead to evolutionary change, advantageous characteristics must have a genetic basis that allows them to be reliably passed down. Traits acquired during an organism’s lifetime, such as a scar, are not genetically encoded and cannot be inherited, thus not contributing to this evolutionary process.
Overproduction of Offspring
Organisms tend to produce more offspring than can realistically survive and reproduce to adulthood. This phenomenon is widespread across species, from bacteria to elephants. Reproductive capacity typically exceeds the availability of limited environmental resources.
Factors such as food, water, space, and mates are finite, creating a struggle for existence within populations. This overproduction leads to competition for essential resources. The concept of overproduction and the resulting competition was influenced by the economist Thomas Malthus, whose work highlighted the struggle for resources in human populations, inspiring both Darwin and Wallace.
Differential Survival and Reproduction
The culmination of these principles lies in differential survival and reproduction, often considered the “selection” aspect of natural selection. Because of the competition arising from overproduction and limited resources, not all individuals will survive to reproduce, or reproduce equally successfully.
Individuals possessing heritable traits that provide an advantage in their specific environment are more likely to endure, thrive, and produce offspring. This greater reproductive success means they pass their beneficial genes to the next generation at a higher rate.
Conversely, individuals with less favorable traits are less likely to survive and, consequently, produce fewer offspring. This process is termed “fitness” in evolutionary biology, defined by an organism’s ability to contribute genes to the next generation, rather than simply its physical strength.
Natural Selection in Action
These four principles—variation, inheritance, overproduction, and differential survival and reproduction—interplay to drive evolutionary change. Heritable differences within a population, coupled with the struggle for limited resources, create a scenario where certain individuals succeed more than others. Those with traits better suited to their environment are “selected” for, meaning they are more likely to survive and pass on advantageous characteristics. Over many generations, this favoring of beneficial traits increases their frequency within the population. This process results in populations becoming better adapted to their environments, illustrating how natural selection orchestrates gradual adaptation over time.