The history of evolutionary theory examines the application of the scientific method toward the study of the origin and nature of living organisms. In the process, scientists gradually learned to give up their presuppositions and look to nature for the answers. Charles Darwin faithfully adhered to the spirit of the scientific method, synthesized his observations and thoughts, and with many others formulated groundbreaking theory for our modern understanding of biological evolution. Mendel first outlined basic genetics, providing an explanation of the origin of phenotypic variation and the nature of inheritance. The discovery of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) has greatly enhanced our understanding of human genetics.

The concepts of ecology and the adaptation of species to their habitats forms the basis of the evolution of organisms. Environments are always changing, and it stands to reason that changes in species' adaptations can account for evolutionary change. Natural selection is the basic process that brings this about, maintaining a species' adaptive relationship with its environment. If there is sufficient variation, it alters a species' adaptations in response to changed environmental circumstances.

Change also occurs within a species' gene pool. Mutations supply new genetic variation. Gene flow and genetic drift affect the distribution of genetic variation within a species. Not only do environments change over time, but also species themselves—all this constantly providing natural selection with new and variable sets of relationships between species and environments.

When a portion of a species becomes isolated from the rest of the species, the stage is set for speciation, the evolution of a new species. If the isolated portion accumulates enough genetic and therefore physical differences over time, it may become reproductively isolated from the original species. The result is that it may no longer be able to produce offspring with members of the original group and a new species has evolved.

The diversity of life on earth is the result of countless speciation events. This can be depicted as a huge, incredibly complex bush. The main stem represents the single origin of life, but it then begins branching, producing millions upon millions of twigs, each standing for a new species, a new natural experiment in adaptation.