Natural selection, gene flow and random genetic drift (the random change in gene combinations with each generation) drive evolution, resulting in a change in gene frequencies within populations .
 Adaptation is genetically based and so leads to phenotypic changes that accumulate over time;  Speciation (species formation) evolves through reproductive isolation and (genetic) divergence of populations;  Genetic changes through natural selection lead to new species and eventually new taxa .
Once multicellular organisation became possible as atmospheric oxygen levels rose, the early multicellular organisms rapidly diverged into many adaptive forms.
Mass extinction events and an organism's unique adaptations mark the history of evolution, both of which may lead to adaptive radiation.
Evolution only gained significant momentum after the theory of evolution, published by Charles Darwin in November 1859, implied that man was merely another product of life on earth, with origins shared by the other creatures and not its ultimate purpose. Wallace proposed the same theory at a joint presentation to the Linnaean Society in London .
Without the universal acceptance of the principle of evolution, there is no chance for the serious proposal of holism.
Between the mid-1930s and mid 1940s geneticists, systematists and palaeontologists collaborated to create a united approach to evolution, the "Modern Synthesis".