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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.
 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 .
Very simply, there are five basic principles to this theory:  Variation has a genetic origin (i.e.
is heritable) and arises through processes such as mutation and recombination.
I have integrated this web page on human evolution into an electronic text, " Nature's Holism - Holism.The evolution of a species is linked to the environment.Adaptation is a continuous response to the environment by changeable species.Between the mid-1930s and mid 1940s geneticists, systematists and palaeontologists collaborated to create a united approach to evolution, the "Modern Synthesis".Ernst Mayr reduced all the hominid (human ancestor: (see hominid books)) fossil records to three species, A.