Charles Darwin was motivated to provide a scientific explanation for these three major findings more than a century and a half ago. Darwin ushered in a scientific revolution—the age of evolutionary biology—when he presented his concept in his book The Origin of Species.

For the time being, we shall define evolution as descent with modification, a phrase used by Darwin to propose that Earth's numerous species are descended from ancestral species that differed from the present-day species. Evolution may also be described as a shift in a population's genetic composition from generation to generation.

• The term evolution refers to descent with modification, a phrase Darwin used in proposing that Earth’s many species are descendants of ancestral species that were different from the present-day species

Evolution may also be seen in two distinct but related ways: as a pattern and as a process. Data from numerous scientific fields, including biology, geology, physics, and chemistry, illustrate the pattern of evolutionary development. These data are facts—observations about the natural world—and they demonstrate how life has developed over time. The mechanisms that generate the observable pattern of change comprise the evolutionary process.

These processes describe the natural causes of the observed natural events. Indeed, the ability of evolution as a unifying theory to explain and link a wide range of observations about the living world is essential to its power as a unifying theory.

Carolus Linnaeus (1707–1778), a Swedish physician and botanist who wanted to catalog life's diversity "for the greater glory of God," was one such scientist. Linnaeus created the two-part, or binomial, system for naming species (such as Homo sapiens for humans) in the 1750s, which is still used today. In contrast to the scala nature’s linear hierarchy, Linnaeus used a layered categorization system, putting related species into progressively broad groups.

Comparable species, for example, are placed in the same genus, and similar genera (plural of genus) are grouped in the same family, and so on. Carolus Linnaeus (1707–1778), a Swedish physician and botanist, was one such scientist. Lamarck was reviled throughout his lifetime, particularly by Cuvier, who disputed that species ever develop.

Organisms share traits due to shared descent (homology) or natural selection affecting independently developing species in comparable settings in similar ways (convergent evolution).

Fossils demonstrate that previous organisms varied from present animals, that many species were extinct, and that species developed over vast periods of time; fossils also chronicle the emergence of new groupings of organisms. Some biogeographic patterns can be explained by evolutionary theory.

Many studies, like those on soapberry bug populations and MRSA, have directly seen natural selection leading to adaptive evolution.

An overwhelming body of scientific data supports evolution. a variety of experimental methods and genetic studies to evaluate natural selection and other evolutionary principles

Despite the fact that Darwin's theory assigns life's diversity to natural processes, the various results of evolution are exquisite and inspirational. “There is grandeur in this vision of existence... [in which] countless forms most beautiful and most marvelous have arisen,” Darwin wrote in the concluding phrase of The Origin of Species.

When ancient and modern specimens of the astragalus (a kind of ankle bone) are compared, it is discovered that cetaceans are closely linked to even-toed ungulates. (a) The astragalus is formed like a dog in most animals, with a double hump on one end (red arrows) but not the opposite end (blue arrow). (b) Fossil evidence suggests that the early cetacean Pakicetus possessed an astragalus with twin humps at both ends, a form found exclusively in pigs (c), deer (d), and all other even-toed ungulates.

Fossils provide a third form of evidence for evolution. The fossil record chronicles the path of evolution, demonstrating that ancient creatures varied from modern organisms and that many species were extinct.

Fossils also provide evidence of evolutionary changes that have happened in diverse groups of animals.

Some homologous traits, such as the genetic code, are shared by all species because they date back to a distant ancestor. Homologous traits that developed recently, on the other hand, are shared exclusively by smaller groups of species.

Consider the tetrapods (from the Greek tetra, four, and pod, foot), a group of vertebrates that includes amphibians, mammals, and reptiles. Tetrapods, like other vertebrates, have a backbone.

However, unlike other vertebrates, tetrapods have limbs with digits. As this example shows, homologous features form a nested pattern: The deepest layer is shared by all life (in this example, all vertebrates have a backbone).

Evolutionary trees are theories that synthesize our present understanding of ancestral patterns. Our belief in these correlations, like any other hypothesis, is based on the strength of the supporting facts. Scientists may utilize well-supported evolutionary trees to make specific and, at times, startling predictions about species.