A theory of evolution is an attempt to explain how evolution occurs. These theories started by describing evolution as an aspect of living, biological beings, with scientists including Lamarck and Charles Darwin. During the 20th century, the notion of evolution was extended to universe, i.e. all existing organisms, from subatomic particles to human society, by scientists including Pierre Teilhard de Chardin, Julian Huxley and James Lovelock (Gaia theory (biology)).
There are several theories of biological evolution, many of which are compatible with one another. Others, such as Lamarck's theory of inheritance of acquired characteristics, have been proven false.
The commonly accepted scientific theory today is known as the modern synthesis (also known as the Neo-Darwinian synthesis), based primarily on Charles Darwin's theory of natural selection, but updated with newer discoveries in biology and genetics, in particular Mendelian inheritance. Population genetics is the branch of biology that provides the mathematical structure for the modern synthesis.
In popular usage, "the" theory of evolution refers to this or other Darwinian theories. However, within this framework there are still differences of opinion, for example between punctuated equilibrium and strict gradualism or regarding the relative importance of natural selection and genetic drift.
Important concepts in biological evolution:
Pierre Teilhard de Chardin and Julian Huxley formulated theories describing the gradual development of the Universe from subatomic particles to human society, considered by Teilhard as the last stage. (see Gaia theory). Nine levels are described (scheme (http://noosphere.cc/evolutiontendencies)), the "classical" biological stages being levels 6, 7 & 8 of the universal evolution. Stages 1 to 5 are grouped into the Lithosphere, also called Geosphere[?] or Physiosphere[?], where (the progress of) the structure of the organisms is ruled by structure, mechanical laws and coincidence. Stages 6 to 8 are grouped into the Biosphere, where (the progress of) the structure of the organisms is ruled by genetical mechanisms. The actual stage, stage 9, is called the Noosphere, where (the progress of) the structure of human society (socialization) is ruled by psychological, informational and communicative processes.