The evolutionary synthesis is the term coined by Ernst Mayr and William B Provine for the period between 1930-1950 that saw the fusion of Darwinian selection theory with Mendelian genetics. Historians of science such as Vassiliki Betty Smocovitis, also argue that it saw the emergence of the discipline of evolutionary biology that was accompanied by the creation of a new journal, Evolution, and the first international Society for the Study of Evolution.
All evolutionary phenomena can be explained in a way that is consistent with known genetic mechanisms and the observational evidence of naturalists.
Evolution is gradual and is caused by small genetic changes, recombination and natural selection. Discontinuities among species are explained as originating gradually, through geographical separation and extinction rather than saltation. However, this does not mean that the rate of evolutionary change is constant. Several mass extinctions in the evolutionary history of life give us evidence that massive extinctions leave niches vacant, allowing other species to achieve rapid diversification and speciation.
Selection is the main driver of change, even if variations in fitness are slight. The object of selection is the phenotype, the externally observable traits of an organism. This is produced by the interaction of the genotype, the heritable information in the form of DNA with its surrounding environment. As the genotype codes for the phenotype, and the genotype is passed through reproduction, the gene is the unit for selection. But as the phenotype are only the external traits, it is the genotype that create them which are selected for, specifically in the form of genes, packets of genetic information within the chromosome. The role of genetic drift is equivocal.
The genetic diversity of natural populations is a key factor in evolution. But as evolution proceeds through natural selection, several genetic variations are selected against. Thus, natural selection destroys variation which it feeds upon. Also are barriers to gene flow, which are key infuences in genetic drift and speciation.
The fossil record can be explained by extrapolating micro evolutionary observations to macro evolutionary events.