Life on Earth is characterized by vast diversity and remarkable adaptations. These arise from two fundamental biological processes: evolution and natural selection. Understanding these concepts is central to comprehending how life has changed and continues to change over time. This article clarifies the relationship between evolution, a broader pattern of change, and natural selection, a significant driving force behind it.
What is Evolution?
Evolution describes the change in the heritable characteristics of biological populations across successive generations. It is a continuous process that has shaped all life forms, resulting in the vast diversity observed today. This transformation occurs at the population level; individuals do not evolve during their lifetime. Instead, the genetic makeup of a population shifts over time. These changes can be small-scale, such as variations in gene frequencies, or large-scale, involving the divergence of new species from a common ancestor.
What is Natural Selection?
Natural selection is a primary mechanism through which evolution occurs, acting as a filtering process within populations. This process operates on several core principles. Individuals within a population exhibit variations in their traits, such as differences in size, color, or speed. Many of these variations are heritable, meaning they can be passed down from parents to their offspring.
Due to environmental pressures, not all individuals survive and reproduce equally. Those with certain advantageous traits are more likely to thrive and pass on their beneficial characteristics. This differential survival and reproduction leads to populations becoming better suited, or “adapted,” to their specific environment over time.
The Dynamic Relationship: Natural Selection as an Evolutionary Force
Natural selection serves as one of the main drivers of the broader evolutionary process. While evolution encompasses any change in heritable traits, natural selection specifically explains how these changes lead to organisms becoming better suited to their surroundings. It works by favoring individuals whose inherited traits provide an advantage in a particular environment, allowing them to produce more offspring.
This process acts on the observable characteristics, or phenotypes, of individuals. Its long-term effects are seen in the genetic composition of entire populations. Over many generations, the advantageous traits become more common, gradually altering the characteristics of the species. This channels evolutionary change towards adaptations that increase an organism’s fitness, which is its ability to survive and reproduce.
Evidence in Action: Examples of Natural Selection
Numerous examples illustrate natural selection actively shaping life forms. One clear instance is the development of antibiotic resistance in bacteria. Within a bacterial population, some individuals may possess random genetic mutations that allow them to survive exposure to antibiotics. When an antibiotic is introduced, it eliminates the susceptible bacteria, leaving the resistant ones to multiply rapidly and pass on their resistance genes. This leads to a population dominated by resistant strains.
Another compelling case involves the peppered moth (Biston betularia) in England during the Industrial Revolution. Before industrialization, light-colored moths were camouflaged against lichen-covered trees. As soot from factories darkened tree trunks, the dark-colored, or melanic, moths became better camouflaged, while the light-colored moths became more visible to predators. This environmental shift led to a rapid increase in the dark moth population.
The Galapagos finches also provide a classic example. These birds exhibit variations in beak size and shape, which are heritable. During droughts, when only large, hard seeds are available, finches with larger beaks are better able to crack them and survive. This leads to a subsequent generation having a larger average beak size.