In contact with nature the organisms acquire capacities to become more and more complex.
Nature generates circumstances that forces organisms to change to be adapted to their environment (these circumstances are called the Lamarckian factors).
The transformism or the assertion of a principle of change.
The genetic drift leads to a homogenisation of populations and therefore not to the maintenance of the genetic polymorphism.
The differential selection between phenotypes (and thus between genotypes) leads to the fixation of an advantaged allele, if the selective value of the homozygous for this allele is higher than any of the others genotypes or it leads to the maintenance of a genetic polymorphism, if the selective value of the heterozygous is higher than the others genotypes.
It was elaborated in the 40-50s and it modifies and improves Darwin theory.
It is Theodosius Dobzhansky (1900-1975), (naturalist then geneticist) who revised the evolutionism. In his book "Genetics and the Origin of Species", he considers that under the action of natural selection, all the evolutionary phenomena are the result of change in the gene frequency within the line.
Gradual evolution is explained by the interactions between mutations and recombination through the screen of natural selection.
Atlas of Genetics and Cytogenetics in Oncology and Haematology 2008-03-01
Online version: http://atlasgeneticsoncology.org/teaching/30026/evolution