Natural selection is a fundamental mechanism of evolution, explaining how populations of organisms change over generations. It describes a process where individuals better suited to their environment tend to survive and produce more offspring. This process is a driving force behind the diversity of life observed on Earth, shaping species to fit their surroundings. Natural selection operates through several interconnected principles that explain how these adaptations arise and become more common within a population.
Variation Within Populations
Individuals within any given population exhibit differences in their traits. These variations can manifest in numerous ways, such as differences in size, color, speed, or disease resistance. For instance, some birds might have longer beaks, while others have shorter ones. This inherent diversity is a prerequisite for natural selection, providing the raw material upon which selection can act. Without these differences, there would be no traits for the environment to favor or disfavor.
Heritability of Traits
Many observed variations are heritable, meaning they can be passed from parents to their offspring. Offspring tend to resemble their parents in various characteristics. For example, a bird born to parents with longer beaks is likely to inherit the genetic information that contributes to longer beak length. While precise genetic mechanisms were unknown to early natural selection theorists, the concept of characteristics being passed down through generations is central. Heritability ensures that beneficial adaptations can be transmitted to subsequent generations, influencing the population’s composition over time.
Overproduction of Offspring
Most species produce more offspring than can realistically survive to adulthood and reproduce. This leads to a natural “struggle for existence” within a population. Resources such as food, water, and shelter are often limited, creating competition among individuals. For example, a single pair of fish might lay thousands of eggs, but only a fraction will survive to maturity due to predation, disease, or lack of resources. This overproduction ensures a surplus of individuals, making competition for survival and reproduction an ongoing process.
Differential Survival and Reproduction
Due to overproduction of offspring and competition for limited resources, individuals with traits better suited to their environment are more likely to survive, find mates, and reproduce. Those with advantageous characteristics have a higher chance of passing traits to the next generation. For instance, if the environment favors birds with longer beaks for accessing specific food, they will likely survive longer and produce more offspring than shorter-beaked birds. Over many generations, these beneficial traits become more prevalent, leading to changes in the species. This process, where traits are “selected” by nature for their contribution to survival and reproductive success, is the essence of natural selection.