Aside from natural selection, darwin was the first biologist to propose:

Charles Darwin also was the first biologist to propose the use of the phylogenetic tree or the evolutionary tree. The phylogenetic tree illustrates how organisms are related, usually by traits. It also represents how organisms are related by common ancestors.  Other scientists refer to the phylogenetic tree or parts of the tree as a cladogram. 

Attached is an example of a simplified phylogenetic tree.