Only after this, did God breathe his Spirit into Adam.There are thus two aspects of the human personality, the physical constitution and the spiritual constitution. 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 .
Biologists included principles from genetics in evolutionary theory during the 1920s and 1930s.Archaeological geology as a science had to precede the proposal of evolution, as an understanding of the immense age of the earth is necessary to understand evolution.Cuvier established amongst the scientific community the fact of extinction.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.Scientists now have accurate methods (see below ) for dating fossils.methods rely on characteristic faunal and geological patterns to bracket the period when the fossil existed.This environment has a living (other animals) and non-living component (e.g. Widgets A unique adaptation usually precedes the entry into a new niche .Tool use by our early ancestors is one such adaptation.( Adaptive radiation is the evolution from an unspecialised ancestral stock of many different species that adapt to a variety of ecological roles (see books)).Mammals and subsequently humanity may not have become dominant on earth had it not been for the global catastrophe that led to the extinction of the dinosaurs 65 million years ago.