Evolutionary theory is the premise that the inherited characteristics of all living organisms change over successive generations within each biological population. Evolution is a slow and gradual process that gives rise to biological diversity and higher rates of survival.
The idea of evolution also suggests that all living organisms share a common ancestor. It explains why most all species share fundamentally similar physical characteristics while still varying widely. Over billions of years, mutations in the genetic code of simple organisms gave rise to various other, more complex, organisms. The mutations that proved beneficial to survival were the ones that were passed on to successive generations, while the less-beneficial or disadvantageous mutations that didn't aid in the organisms' survival eventually died off. Over generations, these small changes added up and contributed to entirely new organisms. Using current species and fossil records, scientists are able to determine which species are related.
Charles Darwin, the father of evolutionary theory, coined the term "natural selection" for this process of advantageous mutation preservation. Darwin, a 19th-century English naturalist, spent his time studying the geographical distribution of species and fossils. His work eventually led him to developing evolutionary theory about species adaptation. In the early 20th century, advances in technology allowed for genetics to be integrated into the theory. This led to the confirmation that, in addition to adaptation, mutation and genetic drift also contribute to evolution.