"Two genes are said to be paralogous if they are derived from a duplication event, but orthologous if they are derived from a speciation event." W-H Li   The differences between orthologous and paralogous genes may be made clearer by studying the illustration opposite. A gene called A in species w is duplicated producing initially two copies of A. With time the two copies diverge by evolution forming related genes A1 and A2. These two genes are said to be paralogous to one another. Paralogy typically involves comparisons within a species. Two species, x and y evolve from species w, their common ancestor. The descendants of the A1 and A2 genes are now called A1x, A1y, and A2x, A2y to reflect which species they now occupy. A1x is orthologous to A1y and A2x is orthologous to A2y. The comparison is between two species. A1x and A2y are paralogously related as are A2x and A1y.