The theory by Charles Darwin which states that due to the phenotypic differences and changes in traits whether they are heritable or not, individuals are reproducing generations with variations in characters. These variations are existing in all types of organisms. Darwin stated that natural selection in which individuals have heritable traits are better with survival because of the environment suitability. Variations will result in new species whenever isolation will happen.
Diagrams
Originated from/Coined by
Charles Darwin
Further Reading/ References
https://en.wikipedia.org/wiki/Natural_selection