By Ashley Seehorn
Updated Mar 24, 2022
English naturalist Charles Darwin harnessed meticulous observation and rigorous logic to formulate a comprehensive theory that delineates how species change over time. While debates persist about the application of evolution to human populations, Darwin’s principles hold universally across all living organisms.
Darwin’s theory hinges on four interrelated ideas: 1) no two individuals of a species are identical; 2) heritable traits are transmitted from parents to offspring; 3) populations produce more offspring than the environment can sustain; and 4) only those individuals who survive the ensuing competition reproduce, passing advantageous traits to the next generation.
Variation is the lifeblood of evolution. Even closely related individuals—siblings, for instance—display differences in coloration, height, weight, and other attributes. While some features, such as the number of limbs or eyes, remain largely constant, others fluctuate markedly. Populations isolated by geography, such as those on Australia, the Galápagos, or Madagascar, often exhibit distinct variations that reflect their unique environmental pressures.
Genetic inheritance dictates the traits passed from parents to progeny. Traits that enhance survival are more likely to be transmitted to subsequent generations. Environmental factors—like nutrition—can influence the expression of certain characteristics (e.g., muscle mass), but only genes are inherited. For example, an organism may inherit genes for a larger skeletal mass; if limited nutrition hampers growth, the organism may still survive and reproduce, thereby passing on the genetic potential for a larger skeleton.
Most species generate more offspring annually than their habitats can support. This surplus births a natural competition for finite resources. Those unable to secure enough sustenance perish, while survivors continue to the next stage of life.
Survival in this competitive arena leads to reproduction, embedding the successful traits within the gene pool. This mechanism—commonly referred to as natural selection—ensures that traits beneficial to survival become more prevalent over time. Darwin famously adopted the phrase “survival of the fittest,” crediting Herbert Spencer for its origin.
