In the 1920s and 1930s the so-called modern synthesis connected natural selection and population genetics , based on Mendelian inheritance, into a unified theory that applied generally to any branch of biology. The modern synthesis explained patterns observed across species in populations, through fossil transitions in palaeontology, and complex cellular mechanisms in developmental biology .   The publication of the structure of DNA by James Watson and Francis Crick in 1953 demonstrated a physical mechanism for inheritance.  Molecular biology improved our understanding of the relationship between genotype and phenotype . Advancements were also made in phylogenetic systematics , mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees .   In 1973, evolutionary biologist Theodosius Dobzhansky penned that " nothing in biology makes sense except in the light of evolution ," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent explanatory body of knowledge that describes and predicts many observable facts about life on this planet.