Natural selection is a fundamental biological process explaining how life on Earth has changed and diversified over millions of years. It is a primary mechanism driving evolution, leading to organisms becoming better suited to their environments. This process results in the gradual accumulation of traits that enhance survival and reproduction, shaping the diversity of species observed today.
The Four Principles of Natural Selection
Natural selection operates through several interconnected principles, each contributing to adaptive changes in populations. These principles highlight how certain traits become more common over generations, allowing species to thrive in their habitats.
One principle is variation: individuals within any population exhibit differences in their traits. These variations can manifest in physical characteristics, physiological functions, or behaviors. They are the raw material upon which natural selection acts.
Following variation is inheritance: many varying traits are heritable, passed down from parents to offspring. Genetic information, encoded in DNA, carries these characteristics across generations. This transfer ensures advantageous variations in parents can be present in their descendants.
Another principle is overproduction and competition: most species produce more offspring than the environment can sustain. Limited resources, such as food, water, and shelter, create a struggle for existence. Not all offspring will survive to reproduce, leading to selection based on an individual’s ability to acquire resources and avoid threats.
The final principle is differential survival and reproduction: individuals with traits better suited to their environment are more likely to survive, reproduce, and pass on those beneficial traits. These advantageous characteristics increase an individual’s fitness, their ability to contribute offspring to the next generation. Over generations, these successful traits become more prevalent, leading to evolutionary change.
Observing Natural Selection
Natural selection’s effects are observable in numerous real-world scenarios, demonstrating how these principles drive evolutionary change. These instances provide evidence of how populations adapt over time in response to environmental pressures. Examining specific examples clarifies the dynamic interplay of variation, inheritance, competition, and differential reproduction.
Antibiotic Resistance in Bacteria
One example is the rapid evolution of antibiotic resistance in bacteria. Some bacteria possess genetic mutations conferring resistance to certain antibiotics. When exposed to an antibiotic, susceptible bacteria are eliminated, but resistant ones survive and reproduce. Because bacteria reproduce quickly, resistant strains rapidly increase, passing on their resistance. This leads to the widespread presence of “superbugs” difficult to treat with conventional medications.
Galápagos Finches
The Galápagos finches offer an illustration of natural selection through their beak shapes. During drought, small, soft seeds decrease, leaving only larger, harder seeds. Finches with larger, stronger beaks are better equipped to crack these, increasing their survival and reproduction. Consequently, average beak size increases in subsequent generations, as offspring inherit advantageous beak traits.
Peppered Moth
The peppered moth in England provides another historical example, illustrating industrial melanism. Before the Industrial Revolution, light-colored moths blended with lichen-covered trees, camouflaged from predators. As industrial pollution darkened tree trunks with soot, light-colored moths became more visible, while rare dark-colored variants gained an advantage. Birds preyed more easily on conspicuous light moths, increasing the dark moth population. When pollution controls were implemented, tree trunks lightened, reversing the selective pressure and making light-colored moths more prevalent again.