Orthologous genes are formed by speciation, while paralogous genes are formed by duplication, contributing to gene evolution.
Paralogous genes can be useful, allowing tandem repeats of some genes. This allows highly expressed genes to be produced, with non-transcribed spacers found between the highly conserved coding sequences. alpha and beta globin are an example of orthologous genes, where each protein has a slightly different function, developed through subfunctionalisation.